Two JSON-config profiles for pymodbus's ModbusSimulatorServer. Replaces the ModbusPal .xmpp profiles that lived here in PR 42 — pymodbus is headless, maintained, semantic about register layout, and pip-installable on Windows.

File	What it simulates	Test category
`standard.json`	Generic Modbus TCP server — HR[0..31] = address-as-value, HR[100] declarative auto-increment via `"action": "increment"`, alternating coils, scratch ranges for write tests.	`Trait=Standard`
`dl205.json`	AutomationDirect DirectLOGIC DL205 / DL260 quirks per `docs/v2/dl205.md`: low-byte-first string packing, CDAB Float32, BCD numerics, V-memory address markers, Y/C coil mappings. Inline `_quirk` comments per register name the behavior.	`Trait=DL205`

Both bind TCP 5020 (pymodbus convention; sidesteps the Windows admin requirement for privileged port 502). The integration-test fixture (ModbusSimulatorFixture) defaults to localhost:5020 to match — override via MODBUS_SIM_ENDPOINT to point at a real PLC on its native port 502.

Run only one profile at a time (they share TCP 5020).

Install

pip install "pymodbus[simulator]==3.13.0"

The [simulator] extra pulls in aiohttp for the optional web UI / REST API. Pinned to 3.13.0 for reproducibility — avoid 4.x dev releases until stabilized. Requires Python ≥ 3.10. Windows Firewall will prompt on first bind; allow Private network.

Run

Foreground (Ctrl+C to stop). Use the serve.ps1 wrapper:

.\serve.ps1 -Profile standard
.\serve.ps1 -Profile dl205

Or invoke pymodbus directly:

pymodbus.simulator `
    --modbus_server srv `
    --modbus_device dev `
    --json_file .\standard.json `
    --http_port 8080

Web UI at http://localhost:8080 lets you inspect + poke registers manually. Pass --no_http (or -HttpPort 0 to serve.ps1) to disable.

Run the integration tests

In a separate shell, with the simulator running:

cd C:\Users\dohertj2\Desktop\lmxopcua
dotnet test tests\ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests

Tests auto-skip with a clear SkipReason if localhost:5020 isn't reachable within 2 seconds. Filter by trait when both profiles' tests coexist:

dotnet test ... --filter "Trait=Standard"
dotnet test ... --filter "Trait=DL205"

What's encoded in each profile

standard.json

HR[0..31]: each register's value equals its address. Easy mental map.
HR[100]: "action": "increment" ticks 0..65535 on every register access — drives subscribe-and-receive tests so they have a register that changes without a write.
HR[200..209]: scratch range for write-roundtrip tests.
Coils[0..31]: alternating on/off (even=on).
Coils[100..109]: scratch.
All addresses 0..1023 are writable ("write": [[0, 1023]]).

dl205.json (per `docs/v2/dl205.md`)

HR address	Quirk demonstrated	Raw value	Decoded
`0` (V0)	Register 0 is valid (rejects-register-0 rumour disproved)	`51966` (0xCAFE)	marker
`1024` (V2000 octal)	V-memory octal-to-decimal mapping	`8192` (0x2000)	marker
`8448` (V40400 octal)	V40400 → PDU 0x2100 (NOT register 0)	`16448` (0x4040)	marker
`1040..1042`	String "Hello" packed first-char-low-byte	`25928, 27756, 111`	`"Hello"`
`1056..1057`	Float32 1.5f in CDAB word order	`0, 16320`	`1.5f`
`1072`	Decimal 1234 in BCD encoding	`4660` (0x1234)	`1234`
`1280..1407`	128-register block (FC03 cap = 128 above spec's 125)	first/last/mid markers; rest defaults to 0	for FC03 cap test

Coil address	Quirk demonstrated
`2048`	Y0 maps to coil 2048 (DL260 layout)
`3072`	C0 maps to coil 3072 (DL260 layout)
`4000..4007`	Scratch C-relay range for write-roundtrip tests

The DL260 X-input markers (FC02 discrete inputs) are not encoded separately because the profile uses shared blocks: true (matches DL series memory model) — coils/DI/HR/IR overlay the same word address space. Tests that target FC02 against this profile end up reading the same bit positions as the coils they share with.

What's IN pymodbus that wasn't in ModbusPal

All four standard tables (HR, IR, coils, DI) configurable via co size / di size / hr size / ir size setup keys.
Per-register raw uint16 seeding — {"addr": 1040, "value": 25928} puts exactly that 16-bit value on the wire. No interpretation.
Built-in actions: increment, random, timestamp, reset, uptime for declarative dynamic registers. No Python script alongside the config required.
Custom actions — point --custom_actions_module at a .py file exposing callables to express anything more complex (per-second wall-clock ticks, BCD synthesis, etc.).
Headless — pure CLI process, no Java, no Swing. Pip-installable. Plays well with CI runners.
Web UI / REST API — --http_port 8080 adds an aiohttp server for live inspection. Optional.
Maintained — current stable 3.13.0 (April 2026), active development on 4.0 dev branch.

Trade-offs vs the hand-authored ModbusPal profiles

pymodbus's built-in float32 type stores in pymodbus's word order; for explicit DL205 CDAB control we seed two raw uint16 entries instead. Documented inline in dl205.json.
increment action ticks per-access, not wall-clock. A 250ms-poll integration test sees variation either way; for strict 1Hz cadence add --custom_actions_module my_actions.py with a time.time()-based callable.
dl205.json uses shared blocks: true because it matches DL series memory model; standard.json uses shared blocks: false so coils and HR address spaces are independent (more like a textbook PLC).

File format reference

{
  "server_list": {
    "<server-name>": {
      "comm": "tcp",
      "host": "0.0.0.0",
      "port": 5020,
      "framer": "socket",
      "device_id": 1
    }
  },
  "device_list": {
    "<device-name>": {
      "setup": {
        "co size": N, "di size": N, "hr size": N, "ir size": N,
        "shared blocks": false,
        "type exception": false,
        "defaults": { "value": {...}, "action": {...} }
      },
      "invalid": [],
      "write": [[<from>, <to>]],
      "bits":    [{"addr": N, "value": 0|1}],
      "uint16":  [{"addr": N, "value": <0..65535>, "action"?: "increment", "parameters"?: {...}}],
      "uint32":  [{"addr": N, "value": <int>}],
      "float32": [{"addr": N, "value": <float>}],
      "string":  [{"addr": N, "value": "<text>"}],
      "repeat":  []
    }
  }
}

The CLI args --modbus_server <server-name> --modbus_device <device-name> pick which entries the simulator binds.

References

pymodbus on PyPI — install, version pin
Simulator config docs — full schema reference
Simulator REST API — for the optional web UI
docs/v2/dl205.md — what each DL205 profile entry simulates
docs/v2/modbus-test-plan.md — the DL205_<behavior> test naming convention

README.md

pymodbus simulator profiles