v1.1.0

Date: 2026-07-06
Previous release: v1.0.0

Summary

Codec-aware fixtures: write typed values in JSON instead of hand-encoding Modbus registers. Still modbus-simulator.fixture.v1 — raw uint16/bool points remain fully supported.

• Codec register fields — optional codec (go-modbus stable ID) and typed value on holding/input register points. Register span is derived from the codec; values are encoded to wire registers at load time.
• Two-tier validation — JSON Schema (oneOf raw vs codec point) plus semantic checks: codec registry lookup, value coercion, encode/decode round-trip, non-overlapping address spans.
• codecs list — discover available codec IDs and metadata (--family, --format json). Wraps github.com/otfabric/go-modbus/codec.
• run --trace-decoded — optional decoded[] overlay on trace records when codec fields are fully contained in the request window.
• Examples — nineteen fixtures in examples/: ten function-code fixtures plus nine codec bundles (one per go-modbus codec family: integer, float, text, bcd, bytes, decimal_limb, time, network, hardware_address). Together they cover every stable ID from codecs list (195 codecs) with non-overlapping addresses and round-trip-validated sample values. Regenerate bundles with make generate-codec-examples (tools/generate-codec-examples). TCP integration tests exercise each fixture; TestExamples_AllCodecsHaveExample guards full codec coverage.

Fixture codec example

{
  "schemaVersion": "modbus-simulator.fixture.v1",
  "devices": [{
    "unitId": 1,
    "state": {
      "holdingRegisters": [
        { "address": 10, "codec": "float32/layout:4321", "value": 230.5, "name": "voltage" }
      ]
    }
  }]
}

Run modbus-simulator codecs list for all stable IDs. See go-modbus CODECS.md for layout and value-kind details.

Example fixtures (v1.1.0)

Fixture	Family	Codecs
codec-integers.json	integer	29
codec-floats.json	float	8
codec-text.json	text	60
codec-bcd.json	bcd	48
codec-bytes.json	bytes	26
codec-decimal-limb.json	decimal_limb	12
codec-datetime.json	time	8
codec-network.json	network	2
codec-hardware.json	hardware_address	2

Function-code fixtures (fc01–fc43, all-function-codes.json) are unchanged from v1.0.0.

Install

go install github.com/otfabric/modbus-simulator@v1.1.0

Or download a pre-built binary from GitHub Releases.

Requires Go 1.25+ when building from source. Depends on github.com/otfabric/go-modbus v1.1.0+ for the codec registry.

Breaking changes

None. Existing v1 fixtures with raw integer register values validate and run unchanged.

Known limitations

Unchanged from v1.0.0 unless noted:

• TCP only; RTU/serial server and TCP+TLS listen are deferred.
• timeout and connectionClose fault rules are recorded in traces but approximated on the wire.
• resetViaControl and reloadFixture state modes are deferred.
• Codec fields are register areas only (no coil/bit codecs). FC16 writes need not align to codec field boundaries; traces still emit per-register stateChanges.
• Trace decoded[] entries are omitted when a request only partially overlaps a multi-register codec field.

v1.0.0

Date: 2026-07-05
Previous release: none

Summary

First release of the Modbus TCP device simulator for gateway integration and CI testing.

• Fixture-driven simulation — modbus-simulator.fixture.v1 JSON defines one or more devices (coils, discrete inputs, holding/input registers, FC43 identity, exception/fault rules).
• Stateful runtime — writes persist in memory; optional resetOnConnection per device.
• JSONL traces — run --trace records modbus-simulator.trace.v1 logs with deterministic seq, responses, and stateChanges.
• CI assertions — assert-trace evaluates modbus-simulator.expect.v1 files (contains, notContains, count, ordered, exact).
• CLI — validate, run, assert-trace, version, and shell completion.
• Schemas — canonical JSON Schemas in schemas/ (fixture-v1.json, trace-v1.json, expect-v1.json), embedded in the binary for offline validation.
• Examples — ten ready-to-run fixtures in examples/ (one per supported FC plus an all-in-one fixture), each covered by TCP integration tests.
• License — MIT (LICENSE).

Install

Download the binary for your platform from GitHub Releases, or:

go install github.com/otfabric/modbus-simulator@v1.0.0

Requires Go 1.25+ when building from source.

Pre-built binaries are published for linux/amd64, linux/arm64, linux/arm/v7, darwin/amd64, darwin/arm64, windows/amd64, and windows/arm64.

Breaking changes

None (initial release).

Known limitations (v1)

• TCP only; RTU/serial server and TCP+TLS listen are deferred.
• timeout and connectionClose fault rules are recorded in traces but approximated on the wire (go-modbus handler constraints).
• resetViaControl and reloadFixture state modes are deferred.