The user explicitly flagged that DL205/DL260 strings don't follow Modbus convention; research turned up that and a lot more. Headline findings:
String packing — TWO chars per V-memory register but the FIRST char is in the LOW byte (opposite of the big-endian Modbus convention generic drivers default to). 'Hello' in V2000 reads back as 'eHll o\0' on a textbook decoder. Kepware's DirectLogic driver exposes a per-tag 'String Byte Order = Low/High' toggle specifically for this; we'll need the same. Null-terminated, no length prefix, no dedicated KSTR address space — strings live wherever ladder allocates them in V-memory.
V-memory addressing — DirectLOGIC's native V-memory is OCTAL (V2000, V40400) but Modbus is decimal. The CPU translates: V2000 octal = decimal 1024 = Modbus PDU 0x0400. The widespread 'V40400 = register 0' shorthand is wrong on modern firmware (that was DL05/DL06 relative mode); on H2-ECOM100 absolute mode (factory default) V40400 = PDU 0x2100. We'd surface this with an address-format helper in the device profile so operators write V2000 instead of computing 1024 by hand.
Word order CDAB for all 32-bit values — DL205 and DL260 agree, ECOM modules don't re-swap. Already supported via ModbusByteOrder.WordSwap; just needs to be the default in the DL205 profile.
BCD-as-default numeric storage — bit one I didn't expect. DirectLOGIC stores 'V2000 = 1234' as 0x1234 on the wire (BCD nibbles), not as 0x04D2 (decimal 1234). IEEE 754 Float32 only works when ladder used the explicit R type (LDR/OUTR instructions). We need a new decoder mode for BCD-encoded registers — current code assumes binary integers.
FC quantity caps — FC03/04 cap at 128 (above spec's 125 — Bonus territory, current code already respects 125), FC16 caps at 100 (BELOW spec's 123 — important bulk-write batching gotcha). Quantity overrun returns exception 03 IllegalDataValue.
Coil/discrete mappings — DL260: X0->discrete input 0, Y0->coil 2048, C0->coil 3072. SP specials at discrete input 1024-1535 RO. These are CPU-wired constants and cannot be remapped; need to be hardcoded in the DL205/DL260 device profile.
Register 0 — accepted on DL205/DL260 with ECOM in absolute mode, contrary to the widespread internet claim that 'DirectLOGIC rejects register 0'. That rumour was an older DL05/DL06 relative-mode artefact. Our ModbusProbeOptions.ProbeAddress default of 0 is therefore safe for DL205/DL260.
Exception codes — only the standard 01-04. Write-to-protected-bit returns 02 on newer firmware, 04 on older (firmware-transition revision unconfirmed); driver should map both to BadNotWritable. No proprietary exception codes.
Behavioral oddities — H2-ECOM100 accepts MAX 4 simultaneous TCP connections (5th refused at TCP accept). No TCP keepalive (intermediate NAT/firewall drops idle sockets after 2-5 min — periodic probe required). No mid-stream resync on malformed MBAP — driver must reconnect + replay. TxId-drop-under-load forum rumour is unconfirmed; our single-flight + TxId-match guard handles it either way.
Each H2 section ends with the integration-test names we'd ship per the modbus-test-plan.md DL205_<behavior> convention — twelve named test slots ready for PR 42+ to fill in one at a time. References (8) cited inline, primarily D2-USER-M, HA-ECOM-M, and the Kepware DirectLogic Ethernet driver manual which documents these vendor quirks explicitly because they have to cope with them.
modbus-test-plan.md DL205 section rewritten as a priority-ordered table with three columns (quirk / driver impact / test name), pointing the reader at dl205.md for the full reference. Operator-reported items separated into a tail subsection so future-me knows which behaviors are documented vs reproduced-on-hardware.
Pure documentation PR — no code changes. The actual driver work (string-byte-order option, BCD decoder mode, V-memory address helper, FC16 cap-per-device-family, multi-client TCP handling) lands one PR per quirk in PR 42+ as ModbusPal validation completes.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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