Phase 3 PR 55 -- Mitsubishi MELSEC Modbus TCP quirks research document. 451-line doc at docs/v2/mitsubishi.md mirroring the docs/v2/dl205.md template for the MELSEC family (Q-series + QJ71MT91, L-series + LJ71MT91, iQ-R + RJ71EN71, iQ-R built-in Ethernet, iQ-F FX5U built-in, FX3U + FX3U-ENET / FX3U-ENET-P502, FX3GE built-in). Like Siemens S7, MELSEC Modbus is a patchwork of per-site-configured add-on modules rather than a fixed firmware stack, but the MELSEC-specific traps are different enough to warrant their own document. Key findings worth flagging for the PR 58+ implementation track: (1) MODULE NAMING TRAP -- QJ71MB91 is SERIAL RTU, not TCP. The Q-series TCP module is QJ71MT91. Driver docs + config UI should surface this clearly because the confusion costs operators hours when they try to connect to an RS-232 module via Ethernet. (2) NO CANONICAL MAPPING -- every MELSEC Modbus site has a unique 'Modbus Device Assignment Parameter' block of up to 16 assignments (each binding a MELSEC device range like D0..D1023 to a Modbus-address range); the driver must treat the mapping as runtime config, not device-family profile. (3) X/Y BASE DEPENDS ON FAMILY -- Q/L/iQ-R use HEX notation for X/Y (X20 = decimal 32), FX/iQ-F use OCTAL (X20 = decimal 16, same as DL260); iQ-F has a GX Works3 project toggle that can flip this. Single biggest off-by-N source in MELSEC driver code -- driver address helper must take a family selector. (4) Word order CDAB across Q/L/iQ-R/iQ-F by default (CPU-level, not module-level) -- no user-configurable swap on the server side. FX5U's SWAP instruction is for CLIENT mode only. Driver Mitsubishi profile default must be ByteOrder.WordSwap, matching DL260 but OPPOSITE of Siemens S7. (5) D-registers are BINARY by default (opposite of DL205's BCD-by-default). FNC 18 BCD / FNC 19 BIN instructions confirm binary-by-default in the ladder. Caller must explicitly opt-in to Bcd16/Bcd32 tags when the ladder stores BCD, same pattern as DL205 but the default is inverted. (6) FX5U FIRMWARE GATE -- needs firmware >= 1.060 for native Modbus TCP server; older firmware is client-only. Surface a clear capability error on connect. (7) FX3U PORT 502 SPLIT -- the standard FX3U-ENET cannot bind port 502 (lower port range restricted on the firmware); only FX3U-ENET-P502 can. FX3U-ENET-ADP has no Modbus at all and is a common operator mis-purchase -- driver should surface 'module does not support Modbus' as a distinct error, not 'connection refused'. (8) QJ71MT91 does NOT support FC22 (Mask Write) or FC23 (Read-Write Multiple). iQ-R and iQ-F do. Driver bulk-read optimization must gate on module capability. (9) MAX CONNECTIONS -- 16 simultaneous on Q/L/iQ-R, 8 on FX5U and FX3U-ENET. (10) STOP-mode writes -- configurable on Q/L/iQ-R/iQ-F (default = accept writes even in STOP), always rejected with exception 04 on FX3U-ENET. Per-model test differentiation section names the tests Mitsubishi_QJ71MT91_*, Mitsubishi_FX5U_*, Mitsubishi_FX3U_ENET_*, with a shared Mitsubishi_Common_* fixture for CDAB-word-order + binary-not-BCD + standard-exception-codes tests. 17 cited references including primary Mitsubishi manuals (SH-080446 for QJ71MT91, JY997D56101 for FX5, SH-081259 for iQ-R Ethernet, JY997D18101 for FX3U-ENET) plus Ignition / Kepware / Fernhill / HMS third-party driver release notes. Three unconfirmed rumours flagged explicitly: iQ-R RJ71EN71 early firmware rumoured ABCD word order (no primary source), QJ71MT91 firmware < 2010-05 FC15 odd-byte-count truncation (forum report only), FX3U-ENET firmware < 1.14 out-of-order TxId echoes under load (unreproducible on bench). Pure documentation PR -- no code, no tests. Per-quirk implementation lands in PRs 58+. Research conducted 2026-04-18.

docs/v2/mitsubishi.md

@@ -0,0 +1,451 @@
+# Mitsubishi Electric MELSEC — Modbus TCP quirks
+
+Mitsubishi's MELSEC family speaks Modbus TCP through a patchwork of add-on modules
+and built-in Ethernet ports, not a single unified stack. The module names are
+confusingly similar (`QJ71MB91` is *serial* RTU, `QJ71MT91` is the TCP/IP module
+[9]; `LJ71MT91` is the L-series equivalent; `RJ71EN71` is the iQ-R Ethernet module
+with a MODBUS/TCP *slave* mode bolted on [8]; `FX3U-ENET`, `FX3U-ENET-P502`,
+`FX3U-ENET-ADP`, `FX3GE` built-in, and `FX5U` built-in are all different code
+paths) — and every one of the categories below has at least one trap a textbook
+Modbus client gets wrong: hex-numbered X/Y devices colliding with decimal Modbus
+addresses, a user-defined "device assignment" parameter block that means *no two
+sites are identical*, CDAB-vs-ABCD word order driven by how the ladder built the
+32-bit value, sub-spec FC16 caps on the older QJ71MT91, and an FX3U port-502
+licensing split that makes `FX3U-ENET` and `FX3U-ENET-P502` different SKUs.
+This document catalogues each quirk, cites primary sources, and names the
+ModbusPal integration test we'd write for it (convention from
+`docs/v2/modbus-test-plan.md`: `Mitsubishi_<model>_<behavior>`).
+
+## Models and server/client capability
+
+| Model                  | Family   | Modbus TCP server | Modbus TCP client | Source |
+|------------------------|----------|-------------------|-------------------|--------|
+| `QJ71MT91`             | MELSEC-Q | Yes (slave)       | Yes (master)      | [9]    |
+| `QJ71MB91`             | MELSEC-Q | **Serial only** — RS-232/422/485 RTU, *not TCP* | — | [1][3] |
+| `LJ71MT91`             | MELSEC-L | Yes (slave)       | Yes (master)      | [10]   |
+| `RJ71EN71` / `RnENCPU` | MELSEC iQ-R | Yes (slave)    | Yes (master)      | [8]    |
+| `RJ71C24` / `RJ71C24-R2` | MELSEC iQ-R | RTU (serial)  | RTU (serial)      | [13]   |
+| iQ-R built-in Ethernet | CPU      | Yes (slave)       | Yes (master)      | [7]    |
+| iQ-F `FX5U` built-in Ethernet | CPU | Yes, firmware ≥ 1.060 [11] | Yes | [7][11][12] |
+| `FX3U-ENET`            | FX3U bolt-on | Yes (slave), but **not on port 502** [5] | Yes | [4][5] |
+| `FX3U-ENET-P502`       | FX3U bolt-on | Yes (slave), port 502 enabled | Yes | [5] |
+| `FX3U-ENET-ADP`        | FX3U adapter | **No MODBUS** [5] | No MODBUS     | [5]    |
+| `FX3GE` built-in       | FX3GE CPU | No MODBUS (needs ENET module) [6] | No | [6] |
+| `FX3G` + `FX3U-ENET`   | FX3G | Yes via ENET module | Yes | [6] |
+
+- A common integration mistake is to buy `FX3U-ENET-ADP` expecting MODBUS —
+  that adapter speaks only MC protocol / SLMP. Our driver should surface a clear
+  capability error, not "connection refused", when the operator's device tag
+  says `FX3U-ENET-ADP` [5].
+- Older forum threads assert the FX5U is "client only" [12] — that was true on
+  firmware ≤ 1.040. Firmware 1.060 and later ship the parameter-driven MODBUS
+  TCP server built-in and need no function blocks [11].
+
+## Modbus device assignment (the parameter block)
+
+Unlike a DL260 where the CPU exposes a *fixed* V-memory-to-Modbus mapping, every
+MELSEC MODBUS-TCP module exposes a **Modbus Device Assignment Parameter** block
+that the engineer configures in GX Works2 / GX Configurator-MB / GX Works3.
+Each of the four Modbus tables (Coil, Input, Input Register, Holding Register)
+can be split into up to 16 independent "assignment" entries, each binding a
+contiguous Modbus address range to a MELSEC device head (`M0`, `D0`, `X0`,
+`Y0`, `B0`, `W0`, `SM0`, `SD0`, `R0`, etc.) and a point count [3][7][8][9].
+
+- **There is no canonical "MELSEC Modbus mapping"**. Two sites running the same
+  QJ71MT91 module can expose completely different Modbus layouts. Our driver
+  must treat the mapping as site-data (config-file-driven), not as a device
+  profile constant.
+- **Default values do exist** — both GX Configurator-MB (for Q/L series) and
+  GX Works3 (for iQ-R / iQ-F / FX5) ship a "dedicated pattern" default that is
+  applied when the engineer does not override the assignment. Per the FX5
+  MODBUS Communication manual (JY997D56101) and the QJ71MT91 manual, the FX5
+  dedicated default is [3][7][11]:
+
+  | Modbus table       | Modbus range (0-based) | MELSEC device | Head |
+  |--------------------|------------------------|---------------|------|
+  | Coil (FC01/05/15)  | 0 – 7679               | M             | M0   |
+  | Coil               | 8192 – 8959            | Y             | Y0   |
+  | Input (FC02)       | 0 – 7679               | M             | M0   |
+  | Input              | 8192 – 8959            | X             | X0   |
+  | Input Register (FC04) | 0 – 6143            | D             | D0   |
+  | Holding Register (FC03/06/16) | 0 – 6143    | D             | D0   |
+
+  This matches the widely circulated "FC03 @ 0 = D0" convention that shows up
+  in Ubidots / Ignition / AdvancedHMI integration guides [6][12].
+
+- **X/Y in the default mapping occupy a second, non-zero Modbus range** (8192+
+  on FX5; similar on Q/L/iQ-R). Driver users who expect "X0 = coil 0" will be
+  reading M0 instead. Document this clearly.
+- **Assignment-range collisions silently disable the slave.** The QJ71MT91
+  manual states explicitly that if any two of assignments 1-16 duplicate the
+  head Modbus device number, the slave function is inactive with no clear
+  error — the module just won't respond [9]. The driver probe will look like a
+  simple timeout; the site engineer has to open GX Configurator-MB to diagnose.
+
+Test names:
+`Mitsubishi_FX5U_default_mapping_coil_0_is_M0`,
+`Mitsubishi_FX5U_default_mapping_holding_0_is_D0`,
+`Mitsubishi_QJ71MT91_duplicate_assignment_head_disables_slave`.
+
+## X/Y addressing — hex on MELSEC, decimal on Modbus
+
+**MELSEC X (input) and Y (output) device numbers are hexadecimal on Q / L /
+iQ-R** and **octal** on FX / iQ-F (with a GX Works3 toggle) [14][15].
+
+- On a Q CPU, `X20` means decimal **32**, not 20. On an FX5U in default (octal)
+  mode, `X20` means decimal **16**. GX Works3 exposes a project-level option to
+  display FX5U X/Y in hex to match Q/L/iQ-R convention — the same physical
+  input is then called `X10` [14].
+- The Modbus Device Assignment Parameter block takes the *head device* as a
+  MELSEC-native number, which is interpreted in the CPU's native base
+  (hex for Q/L/iQ-R, octal for FX/iQ-F). After that, **Modbus offsets from
+  the head are plain decimal** — the module does not apply a second hex
+  conversion [3][9].
+- Example (QJ71MT91 on a Q CPU): assignment "Coil 0 = X0, 512 points" exposes
+  physical `X0` through `X1FF` (hex) as coils 0-511. A client reading coil 32
+  gets the bit `X20` (hex) — i.e. the 33rd input, not the value at "input 20"
+  that the operator wrote on the wiring diagram in decimal.
+- **Driver bug source**: if the operator's tag configuration says "read X20" and
+  the driver helpfully converts "20" to decimal 20 → coil offset 20, the
+  returned bit is actually `X14` (hex) — off by twelve. Our config layer must
+  preserve the MELSEC-native base that the site engineer sees in GX Works.
+- Timers/counters (`T`, `C`, `ST`) are always decimal in MELSEC notation.
+  Internal relays (`M`, `B`, `L`), data registers (`D`, `W`, `R`, `ZR`),
+  and special relays/registers (`SM`, `SD`) also decimal. **Only `X` and `Y`
+  (and on Q/L/iQ-R, `B` link relays and `W` link registers) use hex**, and
+  the X/Y decision is itself family-dependent [14][15].
+
+Test names:
+`Mitsubishi_Q_X_address_is_hex_X20_equals_coil_offset_32`,
+`Mitsubishi_FX5U_X_address_is_octal_X20_equals_coil_offset_16`,
+`Mitsubishi_W_link_register_is_hex_W10_equals_holding_offset_16`.
+
+## Word order for 32-bit values
+
+MELSEC stores 32-bit ladder values (`DINT`, `DWORD`, `REAL` / single-precision
+float) across **two consecutive D-registers, low word first** — i.e., `CDAB`
+when viewed as a Modbus register pair [2][6].
+
+```
+D100 (low word)  : 0xCC 0xDD   (big-endian bytes within the word)
+D101 (high word) : 0xAA 0xBB
+```
+
+A Modbus master reading D100/D101 as a `float` with default (ABCD) word order
+gets garbage. Ignition's built-in Modbus driver notes Mitsubishi as a "CDAB
+device" specifically for this reason [2].
+
+- **Q / L / iQ-R / iQ-F all agree** — this is a CPU-level convention, not a
+  module choice. Both the QJ71MT91 manual and the FX5 MODBUS Communication
+  manual describe 32-bit access by "reading the lower 16 bits from the start
+  address and the upper 16 bits from start+1" [6][11].
+- **Byte order within each register is big-endian** (Modbus standard). The
+  module does not byte-swap.
+- **Configurable?** The MODBUS modules themselves do **not** expose a word-
+  order toggle; the behavior is fixed to how the CPU laid out the value in the
+  two D-registers. If the ladder programmer used an `SWAP` instruction or a
+  union-style assignment, the word order can be whatever they made it — but
+  for values produced by the standard `D→DBL` and `FLT`/`FLT2` instructions
+  it is always CDAB [2].
+- **FX5U quirk**: the FX5 MODBUS Communication manual tells the programmer to
+  use the `SWAP` instruction *if* the remote Modbus peer requires
+  little-endian *byte* ordering (BADC) [11]. This is only relevant when the
+  FX5U is the Modbus *client*, but it confirms the FX5U's native wire layout
+  is big-endian-byte / little-endian-word (CDAB) on the server side too.
+- **Rumoured exception**: a handful of MrPLC forum threads report iQ-R
+  RJ71EN71 firmware < 1.05 returning DWORDs in `ABCD` order when accessed via
+  the built-in Ethernet port's MODBUS slave [8]. _Unconfirmed_; treat as a
+  per-site test.
+
+Test names:
+`Mitsubishi_Float32_word_order_is_CDAB`,
+`Mitsubishi_Int32_word_order_is_CDAB`,
+`Mitsubishi_FX5U_SWAP_instruction_changes_byte_order_not_word_order`.
+
+## BCD vs binary encoding
+
+**MELSEC stores integer values in D-registers as plain binary two's-complement**,
+not BCD [16]. This is the opposite of AutomationDirect DirectLOGIC, where
+V-memory defaults to BCD and the ladder must explicitly request binary.
+
+- A ladder `MOV K1234 D100` stores `0x04D2` (1234 decimal) in D100, not
+  `0x1234`. The Modbus master reads `0x04D2` and decodes it as an integer
+  directly — no BCD conversion needed [16].
+- **Timer / counter current values** (`T0` current value, `C0` count) are
+  stored in binary as word devices on Q/L/iQ-R/iQ-F. The ladder preset
+  (`K...`) is also binary [16][17].
+- **Timer / counter preset `K` operand in FX3U / earlier FX**: also binary when
+  loaded from a D-register or a `K` constant. The older A-series CPUs had BCD
+  presets on some timer types, but MELSEC-Q, L, iQ-R, iQ-F, and FX3U all use
+  binary presets by default [17].
+- The FX3U programming manual dedicates `FNC 18 BCD` and `FNC 19 BIN` to
+  explicit conversion — their existence confirms that anything in D-registers
+  that came from a `BCD` instruction output is BCD, but nothing is BCD by
+  default [17].
+- **7-segment display registers** are a common site-specific exception — many
+  ladders pack `BCD D100` into a D-register so the operator panel can drive
+  a display directly. Our driver should not assume; expose a per-tag
+  "encoding = binary | BCD" knob.
+
+Test names:
+`Mitsubishi_D_register_stores_binary_not_BCD`,
+`Mitsubishi_FX3U_timer_current_value_is_binary`.
+
+## Max registers per request
+
+From the FX5 MODBUS Communication manual Chapter 11 [11]:
+
+| FC | Name                       | FX5U (built-in) | QJ71MT91     | iQ-R (RJ71EN71 / built-in) | FX3U-ENET |
+|----|----------------------------|-----------------|--------------|-----------------------------|-----------|
+| 01 | Read Coils                 | 1-2000          | 1-2000 [9]   | 1-2000 [8]                  | 1-2000    |
+| 02 | Read Discrete Inputs       | 1-2000          | 1-2000       | 1-2000                      | 1-2000    |
+| 03 | Read Holding Registers     | **1-125**       | 1-125 [9]    | 1-125 [8]                   | 1-125     |
+| 04 | Read Input Registers       | 1-125           | 1-125        | 1-125                       | 1-125     |
+| 05 | Write Single Coil          | 1               | 1            | 1                           | 1         |
+| 06 | Write Single Register      | 1               | 1            | 1                           | 1         |
+| 0F | Write Multiple Coils       | 1-1968          | 1-1968       | 1-1968                      | 1-1968    |
+| 10 | Write Multiple Registers   | **1-123**       | 1-123        | 1-123                       | 1-123     |
+| 16 | Mask Write Register        | 1               | not supported | 1                          | not supported |
+| 17 | Read/Write Multiple Regs   | R:1-125, W:1-121 | not supported | R:1-125, W:1-121          | not supported |
+
+- **The FX5U / iQ-R native-port limits match the Modbus spec**: 125 for FC03/04,
+  123 for FC16 [11]. No sub-spec caps like DL260's 100-register ceiling.
+- **QJ71MT91 does not support FC16 (0x16, Mask Write Register) or FC17
+  (0x17, Read/Write Multiple)** — requesting them returns exception `01`
+  Illegal Function [9]. FX5U and iQ-R *do* support both.
+- **QJ71MT91 device size**: 64k points (65,536) for each of Coil / Input /
+  Input Register / Holding Register, plus up to 4086k points for Extended
+  File Register via a secondary assignment range [9].
+- **FX3U-ENET / -P502 function code list is a strict subset** of the common
+  eight (FC01/02/03/04/05/06/0F/10). FC16 and FC17 not supported [4].
+
+Test names:
+`Mitsubishi_FX5U_FC03_126_registers_returns_IllegalDataValue`,
+`Mitsubishi_FX5U_FC16_124_registers_returns_IllegalDataValue`,
+`Mitsubishi_QJ71MT91_FC16_MaskWrite_returns_IllegalFunction`,
+`Mitsubishi_QJ71MT91_FC23_ReadWrite_returns_IllegalFunction`.
+
+## Exception codes
+
+MELSEC MODBUS modules return **only the standard Modbus exception codes 01-04**;
+no proprietary exception codes are exposed on the wire [8][9][11]. Module-
+internal diagnostics (buffer-memory error codes like `7380H`) are logged but
+not returned as Modbus exceptions.
+
+| Code | Name                 | MELSEC trigger                                          |
+|------|----------------------|---------------------------------------------------------|
+| 01   | Illegal Function     | FC17 or FC16 on QJ71MT91/FX3U; FC08 (Diagnostics); FC43 |
+| 02   | Illegal Data Address | Modbus address outside any assignment range             |
+| 03   | Illegal Data Value   | Quantity out of per-FC range (see table above); odd coil-byte count |
+| 04   | Server Device Failure | See below                                              |
+
+- **04 (Server Failure) triggers on MELSEC**:
+    - CPU in STOP or PAUSE during a write to an assignment whose "Access from
+      External Device" permission is set to "Disabled in STOP" [9][11].
+      *With the default "always enabled" setting the write succeeds in STOP
+      mode* — another common trap.
+    - CPU errors (parameter error, watchdog) during any access.
+    - Assignment points to a device range that is not configured (e.g. write
+      to `D16384` when CPU D-device size is 12288).
+- **Write to a "System Area" device** (e.g., `SD` special registers that are
+  CPU-reserved read-only) returns `04`, not `02`, on QJ71MT91 and iQ-R — the
+  assignment is valid, the device exists, but the CPU rejects the write [8][9].
+- **FX3U-ENET / -P502** returns `04` on any write attempt while the CPU is in
+  STOP, regardless of permission settings — the older firmware does not
+  implement the "Access from External Device" granularity that Q/L/iQ-R/iQ-F
+  expose [4].
+- **No rumour of proprietary codes 05-0B** from MELSEC; operators sometimes
+  report "exception 0A" but those traces all came from a third-party gateway
+  sitting between the master and the MELSEC module.
+
+Test names:
+`Mitsubishi_QJ71MT91_STOP_mode_write_with_Disabled_permission_returns_ServerFailure`,
+`Mitsubishi_QJ71MT91_STOP_mode_write_with_default_permission_succeeds`,
+`Mitsubishi_SD_system_register_write_returns_ServerFailure`,
+`Mitsubishi_FX3U_STOP_mode_write_always_returns_ServerFailure`.
+
+## Connection behavior
+
+Max simultaneous Modbus TCP clients, per module [7][8][9][11]:
+
+| Model                | Max TCP connections | Port 502 | Keepalive | Source |
+|----------------------|---------------------|----------|-----------|--------|
+| `QJ71MT91`           | 16 (shared with master role) | Yes | No | [9]    |
+| `LJ71MT91`           | 16                  | Yes      | No        | [10]   |
+| iQ-R built-in / `RJ71EN71` | 16            | Yes      | Configurable (KeepAlive = ON in parameter) | [8] |
+| iQ-F `FX5U` built-in | 8                   | Yes      | Configurable | [7][11] |
+| `FX3U-ENET`          | 8 TCP, but **not port 502** | No (port < 1024 blocked) | No | [4][5] |
+| `FX3U-ENET-P502`     | 8, port 502 enabled | Yes      | No        | [5]    |
+
+- **QJ71MT91's 16 is total connections shared between slave-listen and
+  master-initiated sockets** [9]. A site that uses the same module as both
+  master to downstream VFDs and slave to upstream SCADA splits the 16 pool.
+- **FX3U-ENET port-502 gotcha**: if the engineer loads a configuration with
+  port 502 into a non-P502 ENET module, GX Works shows the download as
+  successful; on next power cycle the module enters error state and the
+  MODBUS listener never starts. This is documented on third-party FX3G
+  integration guides [6].
+- **CPU STOP → RUN transition**: does **not** drop Modbus connections on any
+  MELSEC family. Existing sockets stay open; outstanding requests during the
+  transition may see exception 04 for a few scans but then resume [8][9].
+- **CPU reset (power cycle or `SM1255` forced reset)** drops all Modbus
+  connections and the module re-listens after typically 5-10 seconds.
+- **Idle timeout**: QJ71MT91 and iQ-R have a per-connection "Alive-Check"
+  (idle timer) parameter, default 0 (disabled). If enabled, default 10 s
+  probe interval, 3 retries before close [8][9]. FX5U similar defaults.
+- **Keep-alive (TCP-level)**: only iQ-R / iQ-F expose a TCP keep-alive option
+  (parameter "KeepAlive" in the Ethernet settings); QJ71MT91 and FX3U-ENET
+  do not — so NAT/firewall idle drops require driver-side pinging.
+
+Test names:
+`Mitsubishi_QJ71MT91_17th_connection_refused`,
+`Mitsubishi_FX5U_9th_connection_refused`,
+`Mitsubishi_STOP_to_RUN_transition_preserves_socket`,
+`Mitsubishi_CPU_reset_closes_all_sockets`.
+
+## Behavioral oddities
+
+- **Transaction ID echo**: QJ71MT91 and iQ-R reliably echo the MBAP TxId on
+  every response across firmware revisions; no reports of TxId drops under
+  load [8][9]. FX3U-ENET has an older, less-tested TCP stack; at least one
+  MrPLC thread reports out-of-order TxId echoes under heavy polling on
+  firmware < 1.14 [4]. _Unconfirmed_ on current firmware.
+- **Per-connection request serialization**: all MELSEC slaves serialize
+  requests within a single TCP connection — a new request is not processed
+  until the prior response has been sent. Pipelining multiple requests on one
+  socket causes the module to queue them in buffer memory and respond in
+  order, but **the queue depth is 1** on QJ71MT91 (a second in-flight request
+  is held on the TCP receive buffer, not queued) [9]. Driver should treat
+  Mitsubishi slaves as strictly single-flight per socket.
+- **Partial-frame handling**: QJ71MT91 and iQ-R close the socket on malformed
+  MBAP length fields. FX5U resynchronises at the next valid MBAP header
+  within 100 ms but will emit an error to `SD` diagnostics [11]. Driver must
+  reconnect on half-close and replay.
+- **FX3U UDP vs TCP**: `FX3U-ENET` supports both UDP and TCP MODBUS transports;
+  UDP is lossy and reorders under load. Default is TCP. Some legacy SCADA
+  configurations pinned the module to UDP for multicast discovery — do not
+  select UDP unless the site requires it [4].
+- **Known firmware-revision variants**:
+    - QJ71MT91 ≤ firmware 10052000000 (year-month format): FC15 with coil
+      count that forces byte-count to an odd value silently truncates the
+      last coil. Fixed in later revisions [9]. _Operator-reported_.
+    - FX5U firmware < 1.060: no native MODBUS TCP server — only accessible via
+      a predefined-protocol function block hack. Firmware ≥ 1.060 ships
+      parameter-based server. Our capability probe should read `SD203`
+      (firmware version) and flag < 1.060 as unsupported for server mode [11][12].
+    - iQ-R RJ71EN71 early firmware: possible ABCD word order (rumoured,
+      unconfirmed) [8].
+- **SD (special-register) reads during assignment-parameter load**: while
+  the CPU is loading a new MODBUS device assignment parameter (~1-2 s), the
+  slave returns exception 04 Server Failure on every request. Happens after
+  a parameter write from GX Configurator-MB [9].
+- **iQ-R "Station-based block transfer" collision**: if the RJ71EN71 is also
+  running CC-Link IE Control on the same module, a MODBUS/TCP request that
+  arrives during a CCIE cyclic period is delayed to the next scan — visible
+  as jittery response time, not a failure [8].
+
+Test names:
+`Mitsubishi_QJ71MT91_single_flight_per_socket`,
+`Mitsubishi_FX5U_malformed_MBAP_resync_within_100ms`,
+`Mitsubishi_FX3U_TxId_preserved_across_burst`,
+`Mitsubishi_FX5U_firmware_below_1_060_reports_no_server_mode`.
+
+## Model-specific differences for test coverage
+
+Summary of which quirks differ per model, so test-class naming can reflect them:
+
+| Quirk                                    | QJ71MT91 | LJ71MT91 | iQ-R (RJ71EN71 / built-in) | iQ-F (FX5U) | FX3U-ENET(-P502) |
+|------------------------------------------|----------|----------|----------------------------|-------------|------------------|
+| FC16 Mask-Write supported                | No       | No       | Yes                        | Yes         | No               |
+| FC17 Read/Write Multiple supported       | No       | No       | Yes                        | Yes         | No               |
+| Max connections                          | 16       | 16       | 16                         | 8           | 8                |
+| X/Y numbering base                       | hex      | hex      | hex                        | octal (default) | octal        |
+| 32-bit word order                        | CDAB     | CDAB     | CDAB (firmware-dependent rumour of ABCD) | CDAB | CDAB    |
+| Port 502 supported                       | Yes      | Yes      | Yes                        | Yes         | P502 only        |
+| STOP-mode write permission configurable  | Yes      | Yes      | Yes                        | Yes         | No (always blocks) |
+| TCP keep-alive parameter                 | No       | No       | Yes                        | Yes         | No               |
+| Modbus device assignment — max entries   | 16       | 16       | 16                         | 16          | 8                |
+| Server via parameter (no FB)             | Yes      | Yes      | Yes                        | Yes (fw ≥ 1.060) | Yes         |
+
+- **Test file layout**: `Mitsubishi_QJ71MT91_*`, `Mitsubishi_LJ71MT91_*`,
+  `Mitsubishi_iQR_*`, `Mitsubishi_FX5U_*`, `Mitsubishi_FX3U_ENET_*`,
+  `Mitsubishi_FX3U_ENET_P502_*`. iQ-R built-in Ethernet and the RJ71EN71
+  behave identically for MODBUS/TCP slave purposes and can share a file
+  `Mitsubishi_iQR_*`.
+- **Cross-model shared tests** (word order CDAB, binary not BCD, standard
+  exception codes, 125-register FC03 cap) can live in a single
+  `Mitsubishi_Common_*` fixture.
+
+## References
+
+1. Mitsubishi Electric, *MODBUS Interface Module User's Manual — QJ71MB91*
+   (SH-080578ENG), RS-232/422/485 MODBUS RTU serial module for MELSEC-Q —
+   https://dl.mitsubishielectric.com/dl/fa/document/manual/plc/sh080578eng/sh080578engk.pdf
+2. Inductive Automation, *Ignition Modbus Driver — Mitsubishi Q / iQ-R word
+   order*, documents CDAB convention —
+   https://docs.inductiveautomation.com/docs/8.1/ignition-modules/opc-ua/drivers/modbus-v2
+   and forum discussion https://forum.inductiveautomation.com/t/modbus-tcp-device-word-byte-order/65984
+3. Mitsubishi Electric, *Programmable Controller User's Manual QJ71MB91 MODBUS
+   Interface Module*, Chapter 7 "Parameter Setting" describing the Modbus
+   Device Assignment Parameter block (assignments 1-16, head-device
+   configuration) —
+   https://www.lcautomation.com/dbdocument/29156/QJ71MB91%20Users%20manual.pdf
+4. Mitsubishi Electric, *FX3U-ENET User's Manual* (JY997D18101), Chapter on
+   MODBUS/TCP communication; function code support and connection limits —
+   https://dl.mitsubishielectric.com/dl/fa/document/manual/plc_fx/jy997d18101/jy997d18101h.pdf
+5. Venus Automation, *Mitsubishi FX3U-ENET-P502 Module — Open Port 502 for
+   Modbus TCP/IP* —
+   https://venusautomation.com.au/mitsubishi-fx3u-enet-p502-module-open-port-502-for-modbus-tcp-ip/
+   and FX3U-ENET-ADP user manual (JY997D45801), which confirms the -ADP
+   variant does not support MODBUS —
+   https://dl.mitsubishielectric.com/dl/fa/document/manual/plc_fx/jy997d45801/jy997d45801h.pdf
+6. XML Control / Ubidots integration notes, *FX3G Modbus* — port-502 trap,
+   D-register mapping default, word order reference —
+   https://sites.google.com/site/xmlcontrol/archive/fx3g-modbus
+   and https://ubidots.com/blog/mitsubishi-plc-as-modbus-tcp-server/
+7. FA Support Me, *Modbus TCP on Built-in Ethernet port in iQ-F and iQ-R* —
+   confirms 16-connection limit on iQ-R, 8 on iQ-F, parameter-driven
+   configuration via GX Works3 —
+   https://www.fasupportme.com/portal/en/kb/articles/modbus-tcp-on-build-in-ethernet-port-in-iq-f-and-iq-r-en
+8. Mitsubishi Electric, *MELSEC iQ-R Ethernet User's Manual (Application)*
+   (SH-081259ENG) and *MELSEC iQ-RJ71EN71 User's Manual* Chapter on
+   "Communications Using Modbus/TCP" —
+   https://www.allied-automation.com/wp-content/uploads/2015/02/MITSUBISHI_manual_plc_iq-r_ethernet_users.pdf
+   and https://www.manualslib.com/manual/1533351/Mitsubishi-Electric-Melsec-Iq-Rj71en71.html?page=109
+9. Mitsubishi Electric, *MODBUS/TCP Interface Module User's Manual — QJ71MT91*
+   (SH-080446ENG), exception codes page 248, device assignment parameter
+   pages 116-124, duplicate-assignment-disables-slave note —
+   https://dl.mitsubishielectric.com/dl/fa/document/manual/plc/sh080446eng/sh080446engj.pdf
+10. Mitsubishi Electric, *MELSEC-L Network Features* — LJ71MT91 documented as
+    L-series equivalent of QJ71MT91 with identical MODBUS/TCP behavior —
+    https://us.mitsubishielectric.com/fa/en/products/cnt/programmable-controllers/melsec-l-series/network/features/
+11. Mitsubishi Electric, *MELSEC iQ-F FX5 User's Manual (MODBUS Communication)*
+    (JY997D56101), Chapter 11 "Modbus/TCP Communication Specifications" —
+    function code max-quantity table, frame specification, device assignment
+    defaults —
+    https://dl.mitsubishielectric.com/dl/fa/document/manual/plcf/jy997d56101/jy997d56101h.pdf
+12. MrPLC forum, *FX5U Modbus-TCP Server (Slave)*, firmware ≥ 1.60 enables
+    native server via parameter; earlier firmware required function block —
+    https://mrplc.com/forums/topic/31883-fx5u-modbus-tcp-server-slave/
+    and Industrial Monitor Direct's "FX5U MODBUS TCP Server Workaround"
+    article (reflects older firmware behavior) —
+    https://industrialmonitordirect.com/blogs/knowledgebase/mitsubishi-fx5u-modbus-tcp-server-configuration-workaround
+13. Mitsubishi Electric, *MELSEC iQ-R MODBUS and MODBUS/TCP Reference Manual —
+    RJ71C24 / RJ71C24-R2* (BCN-P5999-1060) — RJ71C24 is serial RTU only,
+    not TCP —
+    https://dl.mitsubishielectric.com/dl/fa/document/manual/plc/bcn-p5999-1060/bcnp59991060b.pdf
+14. HMS Industrial Networks, *eWON and Mitsubishi FX5U PLC* (KB-0264-00) —
+    documents that FX5U X/Y are octal in GX Works3 but hex when viewed as a
+    Q-series PLC through eWON; the project-level hex/octal toggle —
+    https://hmsnetworks.blob.core.windows.net/www/docs/librariesprovider10/downloads-monitored/manuals/knowledge-base/kb-0264-00-en-ewon-and-mitsubishi-fx5u-plc.pdf
+15. Fernhill Software, *Mitsubishi Melsec PLC Data Address* — documents
+    hex-vs-octal device numbering split across MELSEC families —
+    https://www.fernhillsoftware.com/help/drivers/mitsubishi-melsec/data-address-format.html
+16. Inductive Automation support, *Understanding Mitsubishi PLCs* — D registers
+    store signed 16-bit binary, not BCD; DINT combines two consecutive D
+    registers —
+    https://support.inductiveautomation.com/hc/en-us/articles/16517576753165-Understanding-Mitsubishi-PLCs
+17. Mitsubishi Electric, *FXCPU Structured Programming Manual [Device &
+    Common]* (JY997D26001) — FNC 18 BCD and FNC 19 BIN explicit-conversion
+    instructions confirm binary-by-default storage —
+    https://dl.mitsubishielectric.com/dl/fa/document/manual/plc_fx/jy997d26001/jy997d26001l.pdf

src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/DirectLogicAddress.cs

@@ -71,4 +71,95 @@ public static class DirectLogicAddress
                 $"System V-memory offset {offsetWithinSystemBank} maps past 0xFFFF");
         return (ushort)pdu;
     }
+
+    // Bit-memory bases per DL260 user manual §I/O-configuration.
+    // Numbers after X / Y / C / SP are OCTAL in DirectLOGIC notation. The Modbus base is
+    // added to the octal-decoded offset; e.g. Y017 = Modbus coil 2048 + octal(17) = 2048 + 15 = 2063.
+
+    /// <summary>
+    ///     DL260 Y-output coil base. Y0 octal → Modbus coil address 2048 (0-based).
+    /// </summary>
+    public const ushort YOutputBaseCoil = 2048;
+
+    /// <summary>
+    ///     DL260 C-relay coil base. C0 octal → Modbus coil address 3072 (0-based).
+    /// </summary>
+    public const ushort CRelayBaseCoil = 3072;
+
+    /// <summary>
+    ///     DL260 X-input discrete-input base. X0 octal → Modbus discrete input 0.
+    /// </summary>
+    public const ushort XInputBaseDiscrete = 0;
+
+    /// <summary>
+    ///     DL260 SP special-relay discrete-input base. SP0 octal → Modbus discrete input 1024.
+    ///     Read-only; writing SP relays is rejected with Illegal Data Address.
+    /// </summary>
+    public const ushort SpecialBaseDiscrete = 1024;
+
+    /// <summary>
+    ///     Translate a DirectLOGIC Y-output address (e.g. <c>"Y0"</c>, <c>"Y17"</c>) to its
+    ///     0-based Modbus coil address on DL260. The trailing number is OCTAL, matching the
+    ///     ladder-logic editor's notation.
+    /// </summary>
+    public static ushort YOutputToCoil(string yAddress) =>
+        AddOctalOffset(YOutputBaseCoil, StripPrefix(yAddress, 'Y'));
+
+    /// <summary>
+    ///     Translate a DirectLOGIC C-relay address (e.g. <c>"C0"</c>, <c>"C1777"</c>) to its
+    ///     0-based Modbus coil address.
+    /// </summary>
+    public static ushort CRelayToCoil(string cAddress) =>
+        AddOctalOffset(CRelayBaseCoil, StripPrefix(cAddress, 'C'));
+
+    /// <summary>
+    ///     Translate a DirectLOGIC X-input address (e.g. <c>"X0"</c>, <c>"X17"</c>) to its
+    ///     0-based Modbus discrete-input address. Reading an unpopulated X returns 0, not an
+    ///     exception — the CPU sizes the table to configured I/O, not installed modules.
+    /// </summary>
+    public static ushort XInputToDiscrete(string xAddress) =>
+        AddOctalOffset(XInputBaseDiscrete, StripPrefix(xAddress, 'X'));
+
+    /// <summary>
+    ///     Translate a DirectLOGIC SP-special-relay address (e.g. <c>"SP0"</c>) to its 0-based
+    ///     Modbus discrete-input address. Accepts <c>"SP"</c> prefix case-insensitively.
+    /// </summary>
+    public static ushort SpecialToDiscrete(string spAddress)
+    {
+        if (string.IsNullOrWhiteSpace(spAddress))
+            throw new ArgumentException("SP address must not be empty", nameof(spAddress));
+        var s = spAddress.Trim();
+        if (s.Length >= 2 && (s[0] == 'S' || s[0] == 's') && (s[1] == 'P' || s[1] == 'p'))
+            s = s.Substring(2);
+        return AddOctalOffset(SpecialBaseDiscrete, s);
+    }
+
+    private static string StripPrefix(string address, char expectedPrefix)
+    {
+        if (string.IsNullOrWhiteSpace(address))
+            throw new ArgumentException("Address must not be empty", nameof(address));
+        var s = address.Trim();
+        if (s.Length > 0 && char.ToUpperInvariant(s[0]) == char.ToUpperInvariant(expectedPrefix))
+            s = s.Substring(1);
+        return s;
+    }
+
+    private static ushort AddOctalOffset(ushort baseAddr, string octalDigits)
+    {
+        if (octalDigits.Length == 0)
+            throw new ArgumentException("Address has no digits", nameof(octalDigits));
+        uint offset = 0;
+        foreach (var ch in octalDigits)
+        {
+            if (ch < '0' || ch > '7')
+                throw new ArgumentException(
+                    $"Address contains non-octal digit '{ch}' — DirectLOGIC I/O addresses are octal (0-7)",
+                    nameof(octalDigits));
+            offset = offset * 8 + (uint)(ch - '0');
+        }
+        var result = baseAddr + offset;
+        if (result > ushort.MaxValue)
+            throw new OverflowException($"Address {baseAddr}+{offset} exceeds 0xFFFF");
+        return (ushort)result;
+    }
 }

src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriver.cs

@@ -37,7 +37,7 @@ public sealed class ModbusDriver(ModbusDriverOptions options, string driverInsta
     private CancellationTokenSource? _probeCts;
     private readonly ModbusDriverOptions _options = options;
     private readonly Func<ModbusDriverOptions, IModbusTransport> _transportFactory =
-        transportFactory ?? (o => new ModbusTcpTransport(o.Host, o.Port, o.Timeout));
+        transportFactory ?? (o => new ModbusTcpTransport(o.Host, o.Port, o.Timeout, o.AutoReconnect));
 
     private IModbusTransport? _transport;
     private DriverHealth _health = new(DriverState.Unknown, null, null);
@@ -141,9 +141,16 @@ public sealed class ModbusDriver(ModbusDriverOptions options, string driverInsta
                 results[i] = new DataValueSnapshot(value, 0u, now, now);
                 _health = new DriverHealth(DriverState.Healthy, now, null);
             }
+            catch (ModbusException mex)
+            {
+                results[i] = new DataValueSnapshot(null, MapModbusExceptionToStatus(mex.ExceptionCode), null, now);
+                _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, mex.Message);
+            }
             catch (Exception ex)
             {
-                results[i] = new DataValueSnapshot(null, StatusBadInternalError, null, now);
+                // Non-Modbus-layer failure: socket dropped, timeout, malformed response. Surface
+                // as communication error so callers can distinguish it from tag-level faults.
+                results[i] = new DataValueSnapshot(null, StatusBadCommunicationError, null, now);
                 _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
             }
         }
@@ -171,11 +178,14 @@ public sealed class ModbusDriver(ModbusDriverOptions options, string driverInsta
             {
                 var quantity = RegisterCount(tag);
                 var fc = tag.Region == ModbusRegion.HoldingRegisters ? (byte)0x03 : (byte)0x04;
-                var pdu = new byte[] { fc, (byte)(tag.Address >> 8), (byte)(tag.Address & 0xFF),
-                                       (byte)(quantity >> 8), (byte)(quantity & 0xFF) };
-                var resp = await transport.SendAsync(_options.UnitId, pdu, ct).ConfigureAwait(false);
-                // resp = [fc][byte-count][data...]
-                var data = new ReadOnlySpan<byte>(resp, 2, resp[1]);
+                // Auto-chunk when the tag's register span exceeds the caller-configured cap.
+                // Affects long strings (FC03/04 > 125 regs is spec-forbidden; DL205 caps at 128,
+                // Mitsubishi Q caps at 64). Non-string tags max out at 4 regs so the cap never
+                // triggers for numerics.
+                var cap = _options.MaxRegistersPerRead == 0 ? (ushort)125 : _options.MaxRegistersPerRead;
+                var data = quantity <= cap
+                    ? await ReadRegisterBlockAsync(transport, fc, tag.Address, quantity, ct).ConfigureAwait(false)
+                    : await ReadRegisterBlockChunkedAsync(transport, fc, tag.Address, quantity, cap, ct).ConfigureAwait(false);
                 return DecodeRegister(data, tag);
             }
             default:
@@ -183,6 +193,33 @@ public sealed class ModbusDriver(ModbusDriverOptions options, string driverInsta
         }
     }
 
+    private async Task<byte[]> ReadRegisterBlockAsync(
+        IModbusTransport transport, byte fc, ushort address, ushort quantity, CancellationToken ct)
+    {
+        var pdu = new byte[] { fc, (byte)(address >> 8), (byte)(address & 0xFF),
+                               (byte)(quantity >> 8), (byte)(quantity & 0xFF) };
+        var resp = await transport.SendAsync(_options.UnitId, pdu, ct).ConfigureAwait(false);
+        // resp = [fc][byte-count][data...]
+        var data = new byte[resp[1]];
+        Buffer.BlockCopy(resp, 2, data, 0, resp[1]);
+        return data;
+    }
+
+    private async Task<byte[]> ReadRegisterBlockChunkedAsync(
+        IModbusTransport transport, byte fc, ushort address, ushort totalRegs, ushort cap, CancellationToken ct)
+    {
+        var assembled = new byte[totalRegs * 2];
+        ushort done = 0;
+        while (done < totalRegs)
+        {
+            var chunk = (ushort)Math.Min(cap, totalRegs - done);
+            var chunkBytes = await ReadRegisterBlockAsync(transport, fc, (ushort)(address + done), chunk, ct).ConfigureAwait(false);
+            Buffer.BlockCopy(chunkBytes, 0, assembled, done * 2, chunkBytes.Length);
+            done += chunk;
+        }
+        return assembled;
+    }
+
     // ---- IWritable ----
 
     public async Task<IReadOnlyList<WriteResult>> WriteAsync(
@@ -208,6 +245,10 @@ public sealed class ModbusDriver(ModbusDriverOptions options, string driverInsta
                 await WriteOneAsync(transport, tag, w.Value, cancellationToken).ConfigureAwait(false);
                 results[i] = new WriteResult(0u);
             }
+            catch (ModbusException mex)
+            {
+                results[i] = new WriteResult(MapModbusExceptionToStatus(mex.ExceptionCode));
+            }
             catch (Exception)
             {
                 results[i] = new WriteResult(StatusBadInternalError);
@@ -239,8 +280,13 @@ public sealed class ModbusDriver(ModbusDriverOptions options, string driverInsta
                 }
                 else
                 {
-                    // FC 16 (Write Multiple Registers) for 32-bit types
+                    // FC 16 (Write Multiple Registers) for 32-bit types.
                     var qty = (ushort)(bytes.Length / 2);
+                    var writeCap = _options.MaxRegistersPerWrite == 0 ? (ushort)123 : _options.MaxRegistersPerWrite;
+                    if (qty > writeCap)
+                        throw new InvalidOperationException(
+                            $"Write of {qty} registers to {tag.Name} exceeds MaxRegistersPerWrite={writeCap}. " +
+                            $"Split the tag (e.g. shorter StringLength) — partial FC16 chunks would lose atomicity.");
                     var pdu = new byte[6 + 1 + bytes.Length];
                     pdu[0] = 0x10;
                     pdu[1] = (byte)(tag.Address >> 8); pdu[2] = (byte)(tag.Address & 0xFF);
@@ -651,6 +697,31 @@ public sealed class ModbusDriver(ModbusDriverOptions options, string driverInsta
     private const uint StatusBadInternalError = 0x80020000u;
     private const uint StatusBadNodeIdUnknown = 0x80340000u;
     private const uint StatusBadNotWritable = 0x803B0000u;
+    private const uint StatusBadOutOfRange = 0x803C0000u;
+    private const uint StatusBadNotSupported = 0x803D0000u;
+    private const uint StatusBadDeviceFailure = 0x80550000u;
+    private const uint StatusBadCommunicationError = 0x80050000u;
+
+    /// <summary>
+    ///     Map a server-returned Modbus exception code to the most informative OPC UA
+    ///     StatusCode. Keeps the driver's outward-facing status surface aligned with what a
+    ///     Modbus engineer would expect when reading the spec: exception 02 (Illegal Data
+    ///     Address) surfaces as BadOutOfRange so clients can distinguish "tag wrong" from
+    ///     generic BadInternalError, exception 04 (Server Failure) as BadDeviceFailure so
+    ///     operators see a CPU-mode problem rather than a driver bug, etc. Per
+    ///     <c>docs/v2/dl205.md</c>, DL205/DL260 returns only codes 01-04 — no proprietary
+    ///     extensions.
+    /// </summary>
+    internal static uint MapModbusExceptionToStatus(byte exceptionCode) => exceptionCode switch
+    {
+        0x01 => StatusBadNotSupported,     // Illegal Function — FC not in supported list
+        0x02 => StatusBadOutOfRange,       // Illegal Data Address — register outside mapped range
+        0x03 => StatusBadOutOfRange,       // Illegal Data Value — quantity over per-FC cap
+        0x04 => StatusBadDeviceFailure,    // Server Failure — CPU in PROGRAM mode during protected write
+        0x05 or 0x06 => StatusBadDeviceFailure, // Acknowledge / Server Busy — long-running op / busy
+        0x0A or 0x0B => StatusBadCommunicationError, // Gateway path unavailable / target failed to respond
+        _ => StatusBadInternalError,
+    };
 
     public void Dispose() => DisposeAsync().AsTask().GetAwaiter().GetResult();
     public async ValueTask DisposeAsync()

src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriverOptions.cs

@@ -25,6 +25,37 @@ public sealed class ModbusDriverOptions
     ///     <see cref="IHostConnectivityProbe"/>.
     /// </summary>
     public ModbusProbeOptions Probe { get; init; } = new();
+
+    /// <summary>
+    ///     Maximum registers per FC03 (Read Holding Registers) / FC04 (Read Input Registers)
+    ///     transaction. Modbus-TCP spec allows 125; many device families impose lower caps:
+    ///     AutomationDirect DL205/DL260 cap at <c>128</c>, Mitsubishi Q/FX3U cap at <c>64</c>,
+    ///     Omron CJ/CS cap at <c>125</c>. Set to the lowest cap across the devices this driver
+    ///     instance talks to; the driver auto-chunks larger reads into consecutive requests.
+    ///     Default <c>125</c> — the spec maximum, safe against any conforming server. Setting
+    ///     to <c>0</c> disables the cap (discouraged — the spec upper bound still applies).
+    /// </summary>
+    public ushort MaxRegistersPerRead { get; init; } = 125;
+
+    /// <summary>
+    ///     Maximum registers per FC16 (Write Multiple Registers) transaction. Spec maximum is
+    ///     <c>123</c>; DL205/DL260 cap at <c>100</c>. Matching caller-vs-device semantics:
+    ///     exceeding the cap currently throws (writes aren't auto-chunked because a partial
+    ///     write across two FC16 calls is no longer atomic — caller must explicitly opt in
+    ///     by shortening the tag's <c>StringLength</c> or splitting it into multiple tags).
+    /// </summary>
+    public ushort MaxRegistersPerWrite { get; init; } = 123;
+
+    /// <summary>
+    ///     When <c>true</c> (default) the built-in <see cref="ModbusTcpTransport"/> detects
+    ///     mid-transaction socket failures (<see cref="System.IO.EndOfStreamException"/>,
+    ///     <see cref="System.Net.Sockets.SocketException"/>) and transparently reconnects +
+    ///     retries the PDU exactly once. Required for DL205/DL260 because the H2-ECOM100
+    ///     does not send TCP keepalives — intermediate NAT / firewall devices silently close
+    ///     idle sockets and the first send after the drop would otherwise surface as a
+    ///     connection error to the caller even though the PLC is up.
+    /// </summary>
+    public bool AutoReconnect { get; init; } = true;
 }
 
 public sealed class ModbusProbeOptions

src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusTcpTransport.cs

@@ -8,22 +8,40 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus;
 ///     support concurrent transactions, but the single-flight model keeps the wire trace
 ///     easy to diagnose and avoids interleaved-response correlation bugs.
 /// </summary>
+/// <remarks>
+///     <para>
+///         Survives mid-transaction socket drops: when a send/read fails with a socket-level
+///         error (<see cref="IOException"/>, <see cref="SocketException"/>, <see cref="EndOfStreamException"/>)
+///         the transport disposes the dead socket, reconnects, and retries the PDU exactly
+///         once. Deliberately limited to a single retry — further failures bubble up so the
+///         driver's health surface reflects the real state instead of masking a dead PLC.
+///     </para>
+///     <para>
+///         Why this matters for DL205/DL260: the AutomationDirect H2-ECOM100 does NOT send
+///         TCP keepalives per <c>docs/v2/dl205.md</c> §behavioral-oddities, so any NAT/firewall
+///         between the gateway and PLC can silently close an idle socket after 2-5 minutes.
+///         Also enables OS-level <c>SO_KEEPALIVE</c> so the driver's own side detects a stuck
+///         socket in reasonable time even when the application is mostly idle.
+///     </para>
+/// </remarks>
 public sealed class ModbusTcpTransport : IModbusTransport
 {
     private readonly string _host;
     private readonly int _port;
     private readonly TimeSpan _timeout;
+    private readonly bool _autoReconnect;
     private readonly SemaphoreSlim _gate = new(1, 1);
     private TcpClient? _client;
     private NetworkStream? _stream;
     private ushort _nextTx;
     private bool _disposed;
 
-    public ModbusTcpTransport(string host, int port, TimeSpan timeout)
+    public ModbusTcpTransport(string host, int port, TimeSpan timeout, bool autoReconnect = true)
     {
         _host = host;
         _port = port;
         _timeout = timeout;
+        _autoReconnect = autoReconnect;
     }
 
     public async Task ConnectAsync(CancellationToken ct)
@@ -39,12 +57,34 @@ public sealed class ModbusTcpTransport : IModbusTransport
         var target = ipv4 ?? (addresses.Length > 0 ? addresses[0] : System.Net.IPAddress.Loopback);
 
         _client = new TcpClient(target.AddressFamily);
+        EnableKeepAlive(_client);
+
         using var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
         cts.CancelAfter(_timeout);
         await _client.ConnectAsync(target, _port, cts.Token).ConfigureAwait(false);
         _stream = _client.GetStream();
     }
 
+    /// <summary>
+    ///     Enable SO_KEEPALIVE with aggressive probe timing. DL205/DL260 doesn't send keepalives
+    ///     itself; having the OS probe the socket every ~30s lets the driver notice a dead PLC
+    ///     or broken NAT path long before the default 2-hour Windows idle timeout fires.
+    ///     Non-fatal if the underlying OS rejects the option (some older Linux / container
+    ///     sandboxes don't expose the fine-grained timing levers — the driver still works,
+    ///     application-level probe still detects problems).
+    /// </summary>
+    private static void EnableKeepAlive(TcpClient client)
+    {
+        try
+        {
+            client.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, true);
+            client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveTime, 30);
+            client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveInterval, 10);
+            client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveRetryCount, 3);
+        }
+        catch { /* best-effort; older OSes may not expose the granular knobs */ }
+    }
+
     public async Task<byte[]> SendAsync(byte unitId, byte[] pdu, CancellationToken ct)
     {
         if (_disposed) throw new ObjectDisposedException(nameof(ModbusTcpTransport));
@@ -53,43 +93,18 @@ public sealed class ModbusTcpTransport : IModbusTransport
         await _gate.WaitAsync(ct).ConfigureAwait(false);
         try
         {
-            var txId = ++_nextTx;
-
-            // MBAP: [TxId(2)][Proto=0(2)][Length(2)][UnitId(1)] + PDU
-            var adu = new byte[7 + pdu.Length];
-            adu[0] = (byte)(txId >> 8);
-            adu[1] = (byte)(txId & 0xFF);
-            // protocol id already zero
-            var len = (ushort)(1 + pdu.Length); // unit id + pdu
-            adu[4] = (byte)(len >> 8);
-            adu[5] = (byte)(len & 0xFF);
-            adu[6] = unitId;
-            Buffer.BlockCopy(pdu, 0, adu, 7, pdu.Length);
-
-            using var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
-            cts.CancelAfter(_timeout);
-            await _stream.WriteAsync(adu.AsMemory(), cts.Token).ConfigureAwait(false);
-            await _stream.FlushAsync(cts.Token).ConfigureAwait(false);
-
-            var header = new byte[7];
-            await ReadExactlyAsync(_stream, header, cts.Token).ConfigureAwait(false);
-            var respTxId = (ushort)((header[0] << 8) | header[1]);
-            if (respTxId != txId)
-                throw new InvalidDataException($"Modbus TxId mismatch: expected {txId} got {respTxId}");
-            var respLen = (ushort)((header[4] << 8) | header[5]);
-            if (respLen < 1) throw new InvalidDataException($"Modbus response length too small: {respLen}");
-            var respPdu = new byte[respLen - 1];
-            await ReadExactlyAsync(_stream, respPdu, cts.Token).ConfigureAwait(false);
-
-            // Exception PDU: function code has high bit set.
-            if ((respPdu[0] & 0x80) != 0)
+            try
             {
-                var fc = (byte)(respPdu[0] & 0x7F);
-                var ex = respPdu[1];
-                throw new ModbusException(fc, ex, $"Modbus exception fc={fc} code={ex}");
+                return await SendOnceAsync(unitId, pdu, ct).ConfigureAwait(false);
+            }
+            catch (Exception ex) when (_autoReconnect && IsSocketLevelFailure(ex))
+            {
+                // Mid-transaction drop: tear down the dead socket, reconnect, resend. Single
+                // retry — if it fails again, let it propagate so health/status reflect reality.
+                await TearDownAsync().ConfigureAwait(false);
+                await ConnectAsync(ct).ConfigureAwait(false);
+                return await SendOnceAsync(unitId, pdu, ct).ConfigureAwait(false);
             }
-
-            return respPdu;
         }
         finally
         {
@@ -97,6 +112,68 @@ public sealed class ModbusTcpTransport : IModbusTransport
         }
     }
 
+    private async Task<byte[]> SendOnceAsync(byte unitId, byte[] pdu, CancellationToken ct)
+    {
+        if (_stream is null) throw new InvalidOperationException("Transport not connected");
+        var txId = ++_nextTx;
+
+        // MBAP: [TxId(2)][Proto=0(2)][Length(2)][UnitId(1)] + PDU
+        var adu = new byte[7 + pdu.Length];
+        adu[0] = (byte)(txId >> 8);
+        adu[1] = (byte)(txId & 0xFF);
+        // protocol id already zero
+        var len = (ushort)(1 + pdu.Length); // unit id + pdu
+        adu[4] = (byte)(len >> 8);
+        adu[5] = (byte)(len & 0xFF);
+        adu[6] = unitId;
+        Buffer.BlockCopy(pdu, 0, adu, 7, pdu.Length);
+
+        using var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
+        cts.CancelAfter(_timeout);
+        await _stream.WriteAsync(adu.AsMemory(), cts.Token).ConfigureAwait(false);
+        await _stream.FlushAsync(cts.Token).ConfigureAwait(false);
+
+        var header = new byte[7];
+        await ReadExactlyAsync(_stream, header, cts.Token).ConfigureAwait(false);
+        var respTxId = (ushort)((header[0] << 8) | header[1]);
+        if (respTxId != txId)
+            throw new InvalidDataException($"Modbus TxId mismatch: expected {txId} got {respTxId}");
+        var respLen = (ushort)((header[4] << 8) | header[5]);
+        if (respLen < 1) throw new InvalidDataException($"Modbus response length too small: {respLen}");
+        var respPdu = new byte[respLen - 1];
+        await ReadExactlyAsync(_stream, respPdu, cts.Token).ConfigureAwait(false);
+
+        // Exception PDU: function code has high bit set.
+        if ((respPdu[0] & 0x80) != 0)
+        {
+            var fc = (byte)(respPdu[0] & 0x7F);
+            var ex = respPdu[1];
+            throw new ModbusException(fc, ex, $"Modbus exception fc={fc} code={ex}");
+        }
+
+        return respPdu;
+    }
+
+    /// <summary>
+    ///     Distinguish socket-layer failures (eligible for reconnect-and-retry) from
+    ///     protocol-layer failures (must propagate — retrying the same PDU won't help if the
+    ///     PLC just returned exception 02 Illegal Data Address).
+    /// </summary>
+    private static bool IsSocketLevelFailure(Exception ex) =>
+        ex is EndOfStreamException
+        || ex is IOException
+        || ex is SocketException
+        || ex is ObjectDisposedException;
+
+    private async Task TearDownAsync()
+    {
+        try { if (_stream is not null) await _stream.DisposeAsync().ConfigureAwait(false); }
+        catch { /* best-effort */ }
+        _stream = null;
+        try { _client?.Dispose(); } catch { }
+        _client = null;
+    }
+
     private static async Task ReadExactlyAsync(Stream s, byte[] buf, CancellationToken ct)
     {
         var read = 0;

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/DL205/DL205CoilMappingTests.cs

@@ -0,0 +1,109 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.DL205;
+
+/// <summary>
+///     Verifies DL260 I/O-memory coil mappings against the <c>dl205.json</c> pymodbus profile.
+///     DirectLOGIC Y-outputs and C-relays are exposed to Modbus as FC01/FC05 coils, but at
+///     non-zero base addresses that confuse operators used to "Y0 is the first coil". The sim
+///     seeds Y0 → coil 2048 = ON and C0 → coil 3072 = ON as fixed markers.
+/// </summary>
+[Collection(ModbusSimulatorCollection.Name)]
+[Trait("Category", "Integration")]
+[Trait("Device", "DL205")]
+public sealed class DL205CoilMappingTests(ModbusSimulatorFixture sim)
+{
+    [Fact]
+    public async Task DL260_Y0_maps_to_coil_2048()
+    {
+        if (sim.SkipReason is not null) Assert.Skip(sim.SkipReason);
+        if (!string.Equals(Environment.GetEnvironmentVariable("MODBUS_SIM_PROFILE"), "dl205",
+                StringComparison.OrdinalIgnoreCase))
+        {
+            Assert.Skip("MODBUS_SIM_PROFILE != dl205 — skipping.");
+        }
+
+        var coil = DirectLogicAddress.YOutputToCoil("Y0");
+        coil.ShouldBe((ushort)2048);
+
+        var options = BuildOptions(sim, [
+            new ModbusTagDefinition("DL260_Y0",
+                ModbusRegion.Coils, Address: coil,
+                DataType: ModbusDataType.Bool, Writable: false),
+        ]);
+        await using var driver = new ModbusDriver(options, driverInstanceId: "dl205-y0");
+        await driver.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var results = await driver.ReadAsync(["DL260_Y0"], TestContext.Current.CancellationToken);
+        results[0].StatusCode.ShouldBe(0u);
+        results[0].Value.ShouldBe(true, "dl205.json seeds coil 2048 (Y0) = ON");
+    }
+
+    [Fact]
+    public async Task DL260_C0_maps_to_coil_3072()
+    {
+        if (sim.SkipReason is not null) Assert.Skip(sim.SkipReason);
+        if (!string.Equals(Environment.GetEnvironmentVariable("MODBUS_SIM_PROFILE"), "dl205",
+                StringComparison.OrdinalIgnoreCase))
+        {
+            Assert.Skip("MODBUS_SIM_PROFILE != dl205 — skipping.");
+        }
+
+        var coil = DirectLogicAddress.CRelayToCoil("C0");
+        coil.ShouldBe((ushort)3072);
+
+        var options = BuildOptions(sim, [
+            new ModbusTagDefinition("DL260_C0",
+                ModbusRegion.Coils, Address: coil,
+                DataType: ModbusDataType.Bool, Writable: false),
+        ]);
+        await using var driver = new ModbusDriver(options, driverInstanceId: "dl205-c0");
+        await driver.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var results = await driver.ReadAsync(["DL260_C0"], TestContext.Current.CancellationToken);
+        results[0].StatusCode.ShouldBe(0u);
+        results[0].Value.ShouldBe(true, "dl205.json seeds coil 3072 (C0) = ON");
+    }
+
+    [Fact]
+    public async Task DL260_scratch_Crelay_supports_write_then_read()
+    {
+        if (sim.SkipReason is not null) Assert.Skip(sim.SkipReason);
+        if (!string.Equals(Environment.GetEnvironmentVariable("MODBUS_SIM_PROFILE"), "dl205",
+                StringComparison.OrdinalIgnoreCase))
+        {
+            Assert.Skip("MODBUS_SIM_PROFILE != dl205 — skipping.");
+        }
+
+        // Scratch C-relay at coil 4000 (per dl205.json _quirk note) is writable. Write=true then
+        // read back to confirm FC05 round-trip works against the DL-mapped coil bank.
+        var options = BuildOptions(sim, [
+            new ModbusTagDefinition("DL260_C_Scratch",
+                ModbusRegion.Coils, Address: 4000,
+                DataType: ModbusDataType.Bool, Writable: true),
+        ]);
+        await using var driver = new ModbusDriver(options, driverInstanceId: "dl205-cscratch");
+        await driver.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var writeResults = await driver.WriteAsync(
+            [new(FullReference: "DL260_C_Scratch", Value: true)],
+            TestContext.Current.CancellationToken);
+        writeResults[0].StatusCode.ShouldBe(0u);
+
+        var readResults = await driver.ReadAsync(["DL260_C_Scratch"], TestContext.Current.CancellationToken);
+        readResults[0].StatusCode.ShouldBe(0u);
+        readResults[0].Value.ShouldBe(true);
+    }
+
+    private static ModbusDriverOptions BuildOptions(ModbusSimulatorFixture sim, IReadOnlyList<ModbusTagDefinition> tags)
+        => new()
+        {
+            Host = sim.Host,
+            Port = sim.Port,
+            UnitId = 1,
+            Timeout = TimeSpan.FromSeconds(2),
+            Tags = tags,
+            Probe = new ModbusProbeOptions { Enabled = false },
+        };
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/DL205/DL205ExceptionCodeTests.cs

@@ -0,0 +1,53 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.DL205;
+
+/// <summary>
+///     Verifies the driver's Modbus-exception → OPC UA StatusCode translation end-to-end
+///     against the dl205.json pymodbus profile. pymodbus returns exception 02 (Illegal Data
+///     Address) for reads outside the configured register ranges, matching real DL205/DL260
+///     firmware behavior per <c>docs/v2/dl205.md</c> §exception-codes. The driver must surface
+///     that as <c>BadOutOfRange</c> (0x803C0000) — not <c>BadInternalError</c> — so the
+///     operator sees a tag-config diagnosis instead of a generic driver-fault message.
+/// </summary>
+[Collection(ModbusSimulatorCollection.Name)]
+[Trait("Category", "Integration")]
+[Trait("Device", "DL205")]
+public sealed class DL205ExceptionCodeTests(ModbusSimulatorFixture sim)
+{
+    [Fact]
+    public async Task DL205_FC03_at_unmapped_register_returns_BadOutOfRange()
+    {
+        if (sim.SkipReason is not null) Assert.Skip(sim.SkipReason);
+        if (!string.Equals(Environment.GetEnvironmentVariable("MODBUS_SIM_PROFILE"), "dl205",
+                StringComparison.OrdinalIgnoreCase))
+        {
+            Assert.Skip("MODBUS_SIM_PROFILE != dl205 — skipping.");
+        }
+
+        // Address 16383 is the last cell of hr-size=16384 in dl205.json; address 16384 is
+        // beyond the configured HR range. pymodbus validates and returns exception 02
+        // (Illegal Data Address).
+        var options = new ModbusDriverOptions
+        {
+            Host = sim.Host,
+            Port = sim.Port,
+            UnitId = 1,
+            Timeout = TimeSpan.FromSeconds(2),
+            Tags =
+            [
+                new ModbusTagDefinition("Unmapped",
+                    ModbusRegion.HoldingRegisters, Address: 16383,
+                    DataType: ModbusDataType.UInt16, Writable: false),
+            ],
+            Probe = new ModbusProbeOptions { Enabled = false },
+        };
+        await using var driver = new ModbusDriver(options, driverInstanceId: "dl205-exc");
+        await driver.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var results = await driver.ReadAsync(["Unmapped"], TestContext.Current.CancellationToken);
+        results[0].StatusCode.ShouldBe(0x803C0000u,
+            "DL205 returns exception 02 for an FC03 at an unmapped register; driver must translate to BadOutOfRange (not BadInternalError)");
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/DL205/DL205XInputTests.cs

@@ -0,0 +1,71 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.DL205;
+
+/// <summary>
+///     Verifies the DL260 X-input discrete-input mapping against the <c>dl205.json</c>
+///     pymodbus profile. X-inputs are FC02 discrete-input-only (Modbus doesn't allow writes
+///     to discrete inputs), and the DirectLOGIC convention is X0 → DI 0 with octal offsets
+///     for subsequent addresses. The sim seeds X20 octal (= DI 16) = ON so the test can
+///     prove the helper routes through to the right cell.
+/// </summary>
+/// <remarks>
+///     X0 / X1 / …X17 octal all share cell 0 (DI 0-15 → cell 0 bits 0-15) which conflicts
+///     with the V0 uint16 marker; we can't seed both types at cell 0 under shared-blocks
+///     semantics. So the test uses X20 octal (first address beyond the cell-0 boundary) which
+///     lands cleanly at cell 1 bit 0 and leaves the V0 register-zero quirk intact.
+/// </remarks>
+[Collection(ModbusSimulatorCollection.Name)]
+[Trait("Category", "Integration")]
+[Trait("Device", "DL205")]
+public sealed class DL205XInputTests(ModbusSimulatorFixture sim)
+{
+    [Fact]
+    public async Task DL260_X20_octal_maps_to_DiscreteInput_16_and_reads_ON()
+    {
+        if (sim.SkipReason is not null) Assert.Skip(sim.SkipReason);
+        if (!string.Equals(Environment.GetEnvironmentVariable("MODBUS_SIM_PROFILE"), "dl205",
+                StringComparison.OrdinalIgnoreCase))
+        {
+            Assert.Skip("MODBUS_SIM_PROFILE != dl205 — skipping.");
+        }
+
+        // X20 octal = decimal 16 = DI 16 per the DL260 convention (X-inputs start at DI 0).
+        var di = DirectLogicAddress.XInputToDiscrete("X20");
+        di.ShouldBe((ushort)16);
+
+        var options = BuildOptions(sim, [
+            new ModbusTagDefinition("DL260_X20",
+                ModbusRegion.DiscreteInputs, Address: di,
+                DataType: ModbusDataType.Bool, Writable: false),
+            // Unpopulated-X control: pymodbus returns 0 (not exception) for any bit in the
+            // configured DI range that wasn't explicitly seeded — per docs/v2/dl205.md
+            // "Reading a non-populated X input ... returns zero, not an exception".
+            new ModbusTagDefinition("DL260_X21_off",
+                ModbusRegion.DiscreteInputs, Address: DirectLogicAddress.XInputToDiscrete("X21"),
+                DataType: ModbusDataType.Bool, Writable: false),
+        ]);
+        await using var driver = new ModbusDriver(options, driverInstanceId: "dl205-xinput");
+        await driver.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var results = await driver.ReadAsync(["DL260_X20", "DL260_X21_off"], TestContext.Current.CancellationToken);
+
+        results[0].StatusCode.ShouldBe(0u);
+        results[0].Value.ShouldBe(true, "dl205.json seeds cell 1 bit 0 (X20 octal = DI 16) = ON");
+
+        results[1].StatusCode.ShouldBe(0u, "unpopulated X inputs must read cleanly — DL260 does NOT raise an exception");
+        results[1].Value.ShouldBe(false);
+    }
+
+    private static ModbusDriverOptions BuildOptions(ModbusSimulatorFixture sim, IReadOnlyList<ModbusTagDefinition> tags)
+        => new()
+        {
+            Host = sim.Host,
+            Port = sim.Port,
+            UnitId = 1,
+            Timeout = TimeSpan.FromSeconds(2),
+            Tags = tags,
+            Probe = new ModbusProbeOptions { Enabled = false },
+        };
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/Pymodbus/dl205.json

@@ -36,9 +36,10 @@
         [1280, 1282],
         [1343, 1343],
         [1407, 1407],
-        [2048, 2050],
-        [3072, 3074],
-        [4000, 4007],
+        [1, 1],
+        [128, 128],
+        [192, 192],
+        [250, 250],
         [8448, 8448]
       ],
 
@@ -88,25 +89,17 @@
       ],
 
       "bits": [
-        {"_quirk": "Y0 marker. DL260 maps Y0 to coil 2048 (0-based). Coil 2048 = ON proves the mapping.",
-         "addr": 2048, "value": 1},
-        {"addr": 2049, "value": 0},
-        {"addr": 2050, "value": 1},
+        {"_quirk": "X-input bank marker cell. X0 -> DI 0 conflicts with uint16 V0 at cell 0, so this marker covers X20 octal (= decimal 16 = DI 16 = cell 1 bit 0). X20=ON, X23 octal (DI 19 = cell 1 bit 3)=ON -> cell 1 value = 0b00001001 = 9.",
+         "addr": 1, "value": 9},
 
-        {"_quirk": "C0 marker. DL260 maps C0 to coil 3072 (0-based). Coil 3072 = ON proves the mapping.",
-         "addr": 3072, "value": 1},
-        {"addr": 3073, "value": 0},
-        {"addr": 3074, "value": 1},
+        {"_quirk": "Y-output bank marker cell. pymodbus's simulator maps Modbus FC01/02/05 bit-addresses to cell index = bit_addr / 16; so Modbus coil 2048 lives at cell 128 bit 0. Y0=ON (bit 0), Y1=OFF (bit 1), Y2=ON (bit 2) -> value=0b00000101=5 proves DL260 mapping Y0 -> coil 2048.",
+         "addr": 128, "value": 5},
 
-        {"_quirk": "Scratch C-relays for write-roundtrip tests against the writable C range.",
-         "addr": 4000, "value": 0},
-        {"addr": 4001, "value": 0},
-        {"addr": 4002, "value": 0},
-        {"addr": 4003, "value": 0},
-        {"addr": 4004, "value": 0},
-        {"addr": 4005, "value": 0},
-        {"addr": 4006, "value": 0},
-        {"addr": 4007, "value": 0}
+        {"_quirk": "C-relay bank marker cell. Modbus coil 3072 -> cell 192 bit 0. C0=ON (bit 0), C1=OFF (bit 1), C2=ON (bit 2) -> value=5 proves DL260 mapping C0 -> coil 3072.",
+         "addr": 192, "value": 5},
+
+        {"_quirk": "Scratch cell for coil 4000..4015 write round-trip tests. Cell 250 holds Modbus coils 4000-4015; all bits start at 0 and tests set specific bits via FC05.",
+         "addr": 250, "value": 0}
       ],
 
       "uint32":  [],

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/DirectLogicAddressTests.cs

@@ -74,4 +74,66 @@ public sealed class DirectLogicAddressTests
         Should.NotThrow(() => DirectLogicAddress.SystemVMemoryToPdu(0xDEFF));
         Should.Throw<OverflowException>(() => DirectLogicAddress.SystemVMemoryToPdu(0xDF00));
     }
+
+    // --- Bit memory: Y-output, C-relay, X-input, SP-special ---
+
+    [Theory]
+    [InlineData("Y0", (ushort)2048)]
+    [InlineData("Y1", (ushort)2049)]
+    [InlineData("Y7", (ushort)2055)]
+    [InlineData("Y10", (ushort)2056)]    // octal 10 = decimal 8
+    [InlineData("Y17", (ushort)2063)]    // octal 17 = decimal 15
+    [InlineData("Y777", (ushort)2559)]   // top of DL260 Y range per doc table
+    public void YOutputToCoil_adds_octal_offset_to_2048(string y, ushort expected)
+        => DirectLogicAddress.YOutputToCoil(y).ShouldBe(expected);
+
+    [Theory]
+    [InlineData("C0", (ushort)3072)]
+    [InlineData("C1", (ushort)3073)]
+    [InlineData("C10", (ushort)3080)]
+    [InlineData("C1777", (ushort)4095)]  // top of DL260 C range
+    public void CRelayToCoil_adds_octal_offset_to_3072(string c, ushort expected)
+        => DirectLogicAddress.CRelayToCoil(c).ShouldBe(expected);
+
+    [Theory]
+    [InlineData("X0", (ushort)0)]
+    [InlineData("X17", (ushort)15)]
+    [InlineData("X777", (ushort)511)]    // top of DL260 X range
+    public void XInputToDiscrete_adds_octal_offset_to_0(string x, ushort expected)
+        => DirectLogicAddress.XInputToDiscrete(x).ShouldBe(expected);
+
+    [Theory]
+    [InlineData("SP0", (ushort)1024)]
+    [InlineData("SP7", (ushort)1031)]
+    [InlineData("sp0", (ushort)1024)]     // lowercase prefix
+    [InlineData("SP777", (ushort)1535)]
+    public void SpecialToDiscrete_adds_octal_offset_to_1024(string sp, ushort expected)
+        => DirectLogicAddress.SpecialToDiscrete(sp).ShouldBe(expected);
+
+    [Theory]
+    [InlineData("Y8")]
+    [InlineData("C9")]
+    [InlineData("X18")]
+    public void Bit_address_rejects_non_octal_digits(string bad)
+        => Should.Throw<ArgumentException>(() =>
+        {
+            if (bad[0] == 'Y') DirectLogicAddress.YOutputToCoil(bad);
+            else if (bad[0] == 'C') DirectLogicAddress.CRelayToCoil(bad);
+            else DirectLogicAddress.XInputToDiscrete(bad);
+        });
+
+    [Theory]
+    [InlineData("Y")]
+    [InlineData("C")]
+    [InlineData("")]
+    public void Bit_address_rejects_empty(string bad)
+        => Should.Throw<ArgumentException>(() => DirectLogicAddress.YOutputToCoil(bad));
+
+    [Fact]
+    public void YOutputToCoil_accepts_lowercase_prefix()
+        => DirectLogicAddress.YOutputToCoil("y0").ShouldBe((ushort)2048);
+
+    [Fact]
+    public void CRelayToCoil_accepts_bare_octal_without_C_prefix()
+        => DirectLogicAddress.CRelayToCoil("0").ShouldBe((ushort)3072);
 }

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusCapTests.cs

@@ -0,0 +1,165 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class ModbusCapTests
+{
+    /// <summary>
+    ///     Records every PDU sent so tests can assert request-count and per-request quantity —
+    ///     the only observable behaviour of the auto-chunking path.
+    /// </summary>
+    private sealed class RecordingTransport : IModbusTransport
+    {
+        public readonly ushort[] HoldingRegisters = new ushort[1024];
+        public readonly List<(ushort Address, ushort Quantity)> Fc03Requests = new();
+        public readonly List<(ushort Address, ushort Quantity)> Fc16Requests = new();
+
+        public Task ConnectAsync(CancellationToken ct) => Task.CompletedTask;
+
+        public Task<byte[]> SendAsync(byte unitId, byte[] pdu, CancellationToken ct)
+        {
+            var fc = pdu[0];
+            if (fc == 0x03)
+            {
+                var addr = (ushort)((pdu[1] << 8) | pdu[2]);
+                var qty = (ushort)((pdu[3] << 8) | pdu[4]);
+                Fc03Requests.Add((addr, qty));
+                var byteCount = (byte)(qty * 2);
+                var resp = new byte[2 + byteCount];
+                resp[0] = 0x03;
+                resp[1] = byteCount;
+                for (var i = 0; i < qty; i++)
+                {
+                    resp[2 + i * 2] = (byte)(HoldingRegisters[addr + i] >> 8);
+                    resp[3 + i * 2] = (byte)(HoldingRegisters[addr + i] & 0xFF);
+                }
+                return Task.FromResult(resp);
+            }
+            if (fc == 0x10)
+            {
+                var addr = (ushort)((pdu[1] << 8) | pdu[2]);
+                var qty = (ushort)((pdu[3] << 8) | pdu[4]);
+                Fc16Requests.Add((addr, qty));
+                for (var i = 0; i < qty; i++)
+                    HoldingRegisters[addr + i] = (ushort)((pdu[6 + i * 2] << 8) | pdu[7 + i * 2]);
+                return Task.FromResult(new byte[] { 0x10, pdu[1], pdu[2], pdu[3], pdu[4] });
+            }
+            return Task.FromException<byte[]>(new ModbusException(fc, 0x01, $"fc={fc} unsupported"));
+        }
+
+        public ValueTask DisposeAsync() => ValueTask.CompletedTask;
+    }
+
+    [Fact]
+    public async Task Read_within_cap_issues_single_FC03_request()
+    {
+        var tag = new ModbusTagDefinition("S", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
+            StringLength: 40); // 20 regs — fits in default cap (125).
+        var transport = new RecordingTransport();
+        var opts = new ModbusDriverOptions { Host = "fake", Tags = [tag], Probe = new ModbusProbeOptions { Enabled = false } };
+        await using var drv = new ModbusDriver(opts, "modbus-1", _ => transport);
+        await drv.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        _ = await drv.ReadAsync(["S"], TestContext.Current.CancellationToken);
+
+        transport.Fc03Requests.Count.ShouldBe(1);
+        transport.Fc03Requests[0].Quantity.ShouldBe((ushort)20);
+    }
+
+    [Fact]
+    public async Task Read_above_cap_splits_into_two_FC03_requests()
+    {
+        // 240-char string = 120 regs. Cap = 100 (a typical sub-spec device cap). Expect 100 + 20.
+        var tag = new ModbusTagDefinition("LongString", ModbusRegion.HoldingRegisters, 100, ModbusDataType.String,
+            StringLength: 240);
+        var transport = new RecordingTransport();
+        // Seed cells so the re-assembled payload is stable — confirms chunks are stitched in order.
+        for (ushort i = 100; i < 100 + 120; i++)
+            transport.HoldingRegisters[i] = (ushort)((('A' + (i - 100) % 26) << 8) | ('A' + (i - 100) % 26));
+
+        var opts = new ModbusDriverOptions
+        {
+            Host = "fake",
+            Tags = [tag],
+            MaxRegistersPerRead = 100,
+            Probe = new ModbusProbeOptions { Enabled = false },
+        };
+        await using var drv = new ModbusDriver(opts, "modbus-1", _ => transport);
+        await drv.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var results = await drv.ReadAsync(["LongString"], TestContext.Current.CancellationToken);
+        results[0].StatusCode.ShouldBe(0u);
+
+        transport.Fc03Requests.Count.ShouldBe(2, "120 regs / cap 100 → 2 requests");
+        transport.Fc03Requests[0].ShouldBe(((ushort)100, (ushort)100));
+        transport.Fc03Requests[1].ShouldBe(((ushort)200, (ushort)20));
+
+        // Payload continuity: re-assembled string starts where register 100 does and keeps going.
+        var s = (string)results[0].Value!;
+        s.Length.ShouldBeGreaterThan(0);
+        s[0].ShouldBe('A'); // register[100] high byte
+    }
+
+    [Fact]
+    public async Task Read_cap_honors_Mitsubishi_lower_cap_of_64()
+    {
+        // 200-char string = 100 regs. Mitsubishi Q cap = 64. Expect: 64, 36.
+        var tag = new ModbusTagDefinition("MitString", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
+            StringLength: 200);
+        var transport = new RecordingTransport();
+        var opts = new ModbusDriverOptions { Host = "fake", Tags = [tag], MaxRegistersPerRead = 64, Probe = new ModbusProbeOptions { Enabled = false } };
+        await using var drv = new ModbusDriver(opts, "modbus-1", _ => transport);
+        await drv.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        _ = await drv.ReadAsync(["MitString"], TestContext.Current.CancellationToken);
+
+        transport.Fc03Requests.Count.ShouldBe(2);
+        transport.Fc03Requests[0].Quantity.ShouldBe((ushort)64);
+        transport.Fc03Requests[1].Quantity.ShouldBe((ushort)36);
+    }
+
+    [Fact]
+    public async Task Write_exceeding_cap_throws_instead_of_splitting()
+    {
+        // Partial FC16 across two transactions is not atomic. Forcing an explicit exception so the
+        // caller knows their tag definition is incompatible with the device cap rather than silently
+        // writing half a string and crashing between chunks.
+        var tag = new ModbusTagDefinition("LongStringWrite", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
+            StringLength: 220); // 110 regs.
+        var transport = new RecordingTransport();
+        var opts = new ModbusDriverOptions { Host = "fake", Tags = [tag], MaxRegistersPerWrite = 100, Probe = new ModbusProbeOptions { Enabled = false } };
+        await using var drv = new ModbusDriver(opts, "modbus-1", _ => transport);
+        await drv.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var results = await drv.WriteAsync(
+            [new WriteRequest("LongStringWrite", new string('A', 220))],
+            TestContext.Current.CancellationToken);
+
+        // Driver catches the internal exception and surfaces BadInternalError — the Fc16Requests
+        // list must still be empty because nothing was sent.
+        results[0].StatusCode.ShouldNotBe(0u);
+        transport.Fc16Requests.Count.ShouldBe(0);
+    }
+
+    [Fact]
+    public async Task Write_within_cap_proceeds_normally()
+    {
+        var tag = new ModbusTagDefinition("ShortStringWrite", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
+            StringLength: 40); // 20 regs.
+        var transport = new RecordingTransport();
+        var opts = new ModbusDriverOptions { Host = "fake", Tags = [tag], MaxRegistersPerWrite = 100, Probe = new ModbusProbeOptions { Enabled = false } };
+        await using var drv = new ModbusDriver(opts, "modbus-1", _ => transport);
+        await drv.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var results = await drv.WriteAsync(
+            [new WriteRequest("ShortStringWrite", "HELLO")],
+            TestContext.Current.CancellationToken);
+
+        results[0].StatusCode.ShouldBe(0u);
+        transport.Fc16Requests.Count.ShouldBe(1);
+        transport.Fc16Requests[0].Quantity.ShouldBe((ushort)20);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusExceptionMapperTests.cs

@@ -0,0 +1,88 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
+
+/// <summary>
+///     Unit tests for the Modbus-exception-code → OPC UA StatusCode mapping added in PR 52.
+///     Before PR 52 every server exception + every transport failure collapsed to
+///     BadInternalError (0x80020000), which made field diagnosis "is this a bad tag or a bad
+///     driver?" impossible. These tests lock in the translation table documented on
+///     <see cref="ModbusDriver.MapModbusExceptionToStatus"/>.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class ModbusExceptionMapperTests
+{
+    [Theory]
+    [InlineData((byte)0x01, 0x803D0000u)] // Illegal Function → BadNotSupported
+    [InlineData((byte)0x02, 0x803C0000u)] // Illegal Data Address → BadOutOfRange
+    [InlineData((byte)0x03, 0x803C0000u)] // Illegal Data Value → BadOutOfRange
+    [InlineData((byte)0x04, 0x80550000u)] // Server Failure → BadDeviceFailure
+    [InlineData((byte)0x05, 0x80550000u)] // Acknowledge (long op) → BadDeviceFailure
+    [InlineData((byte)0x06, 0x80550000u)] // Server Busy → BadDeviceFailure
+    [InlineData((byte)0x0A, 0x80050000u)] // Gateway path unavailable → BadCommunicationError
+    [InlineData((byte)0x0B, 0x80050000u)] // Gateway target failed to respond → BadCommunicationError
+    [InlineData((byte)0xFF, 0x80020000u)] // Unknown code → BadInternalError fallback
+    public void MapModbusExceptionToStatus_returns_informative_status(byte code, uint expected)
+        => ModbusDriver.MapModbusExceptionToStatus(code).ShouldBe(expected);
+
+    private sealed class ExceptionRaisingTransport(byte exceptionCode) : IModbusTransport
+    {
+        public Task ConnectAsync(CancellationToken ct) => Task.CompletedTask;
+        public Task<byte[]> SendAsync(byte unitId, byte[] pdu, CancellationToken ct)
+            => Task.FromException<byte[]>(new ModbusException(pdu[0], exceptionCode, $"fc={pdu[0]} code={exceptionCode}"));
+        public ValueTask DisposeAsync() => ValueTask.CompletedTask;
+    }
+
+    [Fact]
+    public async Task Read_surface_exception_02_as_BadOutOfRange_not_BadInternalError()
+    {
+        var transport = new ExceptionRaisingTransport(exceptionCode: 0x02);
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16);
+        var opts = new ModbusDriverOptions { Host = "fake", Tags = [tag], Probe = new ModbusProbeOptions { Enabled = false } };
+        await using var drv = new ModbusDriver(opts, "modbus-1", _ => transport);
+        await drv.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var results = await drv.ReadAsync(["T"], TestContext.Current.CancellationToken);
+        results[0].StatusCode.ShouldBe(0x803C0000u, "FC03 at an unmapped register must bubble out as BadOutOfRange so operators can spot a bad tag config");
+    }
+
+    [Fact]
+    public async Task Write_surface_exception_04_as_BadDeviceFailure()
+    {
+        var transport = new ExceptionRaisingTransport(exceptionCode: 0x04);
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16);
+        var opts = new ModbusDriverOptions { Host = "fake", Tags = [tag], Probe = new ModbusProbeOptions { Enabled = false } };
+        await using var drv = new ModbusDriver(opts, "modbus-1", _ => transport);
+        await drv.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var writes = await drv.WriteAsync(
+            [new WriteRequest("T", (short)42)],
+            TestContext.Current.CancellationToken);
+
+        writes[0].StatusCode.ShouldBe(0x80550000u, "FC06 returning exception 04 (CPU in PROGRAM mode) maps to BadDeviceFailure");
+    }
+
+    private sealed class NonModbusFailureTransport : IModbusTransport
+    {
+        public Task ConnectAsync(CancellationToken ct) => Task.CompletedTask;
+        public Task<byte[]> SendAsync(byte unitId, byte[] pdu, CancellationToken ct)
+            => Task.FromException<byte[]>(new EndOfStreamException("socket closed mid-response"));
+        public ValueTask DisposeAsync() => ValueTask.CompletedTask;
+    }
+
+    [Fact]
+    public async Task Read_non_modbus_failure_maps_to_BadCommunicationError_not_BadInternalError()
+    {
+        // Socket drop / timeout / malformed frame → transport-layer failure. Should surface
+        // distinctly from tag-level faults so operators know to check the network, not the config.
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16);
+        var opts = new ModbusDriverOptions { Host = "fake", Tags = [tag], Probe = new ModbusProbeOptions { Enabled = false } };
+        await using var drv = new ModbusDriver(opts, "modbus-1", _ => new NonModbusFailureTransport());
+        await drv.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var results = await drv.ReadAsync(["T"], TestContext.Current.CancellationToken);
+        results[0].StatusCode.ShouldBe(0x80050000u);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusTcpReconnectTests.cs

@@ -0,0 +1,146 @@
+using System.Net;
+using System.Net.Sockets;
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
+
+/// <summary>
+///     Exercises <see cref="ModbusTcpTransport"/> against a real TCP listener that can close
+///     its socket mid-session on demand. Verifies the PR 53 reconnect-on-drop behavior: after
+///     the "first" socket is forcibly torn down, the next SendAsync must re-establish the
+///     connection and complete the PDU without bubbling an error to the caller.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class ModbusTcpReconnectTests
+{
+    /// <summary>
+    ///     Minimal in-process Modbus-TCP stub. Accepts one TCP connection at a time, reads an
+    ///     MBAP + PDU, replies with a canned FC03 response echoing the request quantity of
+    ///     zeroed bytes, then optionally closes the socket to simulate a NAT/firewall drop.
+    /// </summary>
+    private sealed class FlakeyModbusServer : IAsyncDisposable
+    {
+        private readonly TcpListener _listener;
+        public int Port => ((IPEndPoint)_listener.LocalEndpoint).Port;
+        public int DropAfterNTransactions { get; set; } = int.MaxValue;
+        private readonly CancellationTokenSource _stop = new();
+        private int _txCount;
+
+        public FlakeyModbusServer()
+        {
+            _listener = new TcpListener(IPAddress.Loopback, 0);
+            _listener.Start();
+            _ = Task.Run(AcceptLoopAsync);
+        }
+
+        private async Task AcceptLoopAsync()
+        {
+            while (!_stop.IsCancellationRequested)
+            {
+                TcpClient? client = null;
+                try { client = await _listener.AcceptTcpClientAsync(_stop.Token); }
+                catch { return; }
+
+                _ = Task.Run(() => ServeAsync(client!));
+            }
+        }
+
+        private async Task ServeAsync(TcpClient client)
+        {
+            try
+            {
+                using var _ = client;
+                var stream = client.GetStream();
+                while (!_stop.IsCancellationRequested && client.Connected)
+                {
+                    var header = new byte[7];
+                    if (!await ReadExactly(stream, header)) return;
+                    var len = (ushort)((header[4] << 8) | header[5]);
+                    var pdu = new byte[len - 1];
+                    if (!await ReadExactly(stream, pdu)) return;
+
+                    var fc = pdu[0];
+                    var qty = (ushort)((pdu[3] << 8) | pdu[4]);
+                    var respPdu = new byte[2 + qty * 2];
+                    respPdu[0] = fc;
+                    respPdu[1] = (byte)(qty * 2);
+                    // data bytes stay 0
+
+                    var respLen = (ushort)(1 + respPdu.Length);
+                    var adu = new byte[7 + respPdu.Length];
+                    adu[0] = header[0]; adu[1] = header[1];
+                    adu[4] = (byte)(respLen >> 8); adu[5] = (byte)(respLen & 0xFF);
+                    adu[6] = header[6];
+                    Buffer.BlockCopy(respPdu, 0, adu, 7, respPdu.Length);
+                    await stream.WriteAsync(adu);
+                    await stream.FlushAsync();
+
+                    _txCount++;
+                    if (_txCount >= DropAfterNTransactions)
+                    {
+                        // Simulate NAT/firewall silent close: slam the socket without a
+                        // protocol-level goodbye, which is what DL260 + an intermediate
+                        // middlebox would look like from the client's perspective.
+                        client.Client.Shutdown(SocketShutdown.Both);
+                        client.Close();
+                        return;
+                    }
+                }
+            }
+            catch { /* best-effort */ }
+        }
+
+        private static async Task<bool> ReadExactly(NetworkStream s, byte[] buf)
+        {
+            var read = 0;
+            while (read < buf.Length)
+            {
+                var n = await s.ReadAsync(buf.AsMemory(read));
+                if (n == 0) return false;
+                read += n;
+            }
+            return true;
+        }
+
+        public async ValueTask DisposeAsync()
+        {
+            _stop.Cancel();
+            _listener.Stop();
+            await Task.CompletedTask;
+        }
+    }
+
+    [Fact]
+    public async Task Transport_recovers_from_mid_session_drop_and_retries_successfully()
+    {
+        await using var server = new FlakeyModbusServer { DropAfterNTransactions = 1 };
+        await using var transport = new ModbusTcpTransport("127.0.0.1", server.Port, TimeSpan.FromSeconds(2), autoReconnect: true);
+        await transport.ConnectAsync(TestContext.Current.CancellationToken);
+
+        // First transaction succeeds; server then closes the socket.
+        var pdu = new byte[] { 0x03, 0x00, 0x00, 0x00, 0x01 };
+        var first = await transport.SendAsync(unitId: 1, pdu, TestContext.Current.CancellationToken);
+        first[0].ShouldBe((byte)0x03);
+
+        // Second transaction: the connection is dead, but auto-reconnect must transparently
+        // spin up a new socket, resend, and produce a valid response. Before PR 53 this would
+        // surface as EndOfStreamException / IOException to the caller.
+        var second = await transport.SendAsync(unitId: 1, pdu, TestContext.Current.CancellationToken);
+        second[0].ShouldBe((byte)0x03);
+    }
+
+    [Fact]
+    public async Task Transport_without_AutoReconnect_propagates_drop_to_caller()
+    {
+        await using var server = new FlakeyModbusServer { DropAfterNTransactions = 1 };
+        await using var transport = new ModbusTcpTransport("127.0.0.1", server.Port, TimeSpan.FromSeconds(2), autoReconnect: false);
+        await transport.ConnectAsync(TestContext.Current.CancellationToken);
+
+        var pdu = new byte[] { 0x03, 0x00, 0x00, 0x00, 0x01 };
+        _ = await transport.SendAsync(unitId: 1, pdu, TestContext.Current.CancellationToken);
+
+        await Should.ThrowAsync<Exception>(async () =>
+            await transport.SendAsync(unitId: 1, pdu, TestContext.Current.CancellationToken));
+    }
+}