modpoll2mqtt — Modbus to MQTT Gateway

Documentation: yoch.github.io/modpoll2mqtt

modpoll2mqtt is a command-line Modbus-to-MQTT gateway. It polls Modbus devices (RTU, TCP, UDP) using CSV configuration files, publishes values to an MQTT broker, and accepts write commands by CSV reference name.

Install the PyPI package as modpoll2mqtt; the executable command remains modpoll.

Fork of modpoll, using the latest pymodbus release, with semantic MQTT writes by CSV reference name and numerous bugfixes and improvements.

Features

• Modbus RTU, TCP, and UDP (CSV configuration files)
• Local display of polled data (debug mode)
• MQTT publishing of references (configurable topics)
• MQTT writes by CSV reference name (modpoll/<device>/set)
• Atomic JSON export of all configured devices
• Bit-level access on registers and coils

Installation

Python 3.10+ required.

pip install modpoll2mqtt

Optional serial support (pyserial):

pip install 'modpoll2mqtt[serial]'

Upgrade:

pip install -U modpoll2mqtt

On Windows, pipx is recommended:

pipx install modpoll2mqtt

Quickstart

Single poll of a Modbus TCP device (replace the IP address with yours):

modpoll --once \
  --tcp 192.168.1.10 \
  --config examples/modsim.csv

Publish to an MQTT broker:

modpoll \
  --tcp 192.168.1.10 \
  --mqtt-host broker.emqx.io \
  --config examples/modsim.csv

Data is published to modpoll/<device_name>/data by default.

Add --mqtt-retain so the broker keeps the last data and diagnostics message per topic for new subscribers. The status topic modpoll/status is always retained. If a device goes offline, retained data values may still appear live until the next successful publish.

MQTT write by reference

Once connected to the broker, modpoll subscribes to modpoll/+/set. Publish to modpoll/<device>/set:

{
  "holding_reg01": 100,
  "coil01-08": [true, false, false, false, false, false, false, false]
}

Unknown reference names in legacy payloads are skipped with a warning. Known writes are validated together before bus I/O; results are returned on the request topic followed by /response. Responses are never retained. Never retain a command.

bool8 / bool16 references require a JSON array of 8 or 16 booleans at write time. Single-coil bool references accept a scalar boolean.

Example: an int16 reference with scale 0.1 accepts 21.5 as input; the raw register value 215 is written.

Only coil and holding_register references can be written. input_register and discrete_input are read-only at the Modbus protocol level even when marked rw in the CSV.

Migration from modpoll 1.6.x: the low-level format (object_type, address, value) is no longer supported.

Migration from modpoll2mqtt 2.0.x: the {"ref": "...", "value": ...} object format was removed in 2.1.0; use a reference map ({"ref_name": value}) instead.

Configuration pitfalls

• On coil or discrete_input pollers, use bool with the absolute Modbus coil address for a single boolean.
• bool8 / bool16: legacy grouped reads (group index, not a direct coil address).
• On holding_register or input_register, use bool with address:bit (0–15), e.g. 40019:15,bool.
• <endian> must be BE_BE, LE_BE, LE_LE, or BE_LE only.
• --autoremove disables a poller after 3 consecutive attempted failures; add --poller-recover-after 60 to probe it again.
• Addresses are passed literally as zero-based protocol addresses. A vendor label such as 40001 is not automatically converted to 0.
• All files are validated before starting; run modpoll --check-config --config file.csv to check without a Modbus transport.

Reliable commands and operation

Use --mqtt-namespace site/gateway1 to isolate the default data, command, diagnostic and retained presence topics for each gateway. Configure MQTT ACLs and use --mqtt-use-tls (TLS 1.2 minimum) with --mqtt-pass-file /run/secrets/mqtt_password. --read-only blocks all writes. --mqtt-insecure explicitly disables certificate verification and should only be used for controlled diagnostics.

The versioned command topic is modpoll/<device>/commands (customize with --mqtt-command-topic-pattern). For example:

{"version": 1, "request_id": "control-001", "operation": "set", "values": {"holding_reg01": 100}}

Optional expires_at is a UTC Unix timestamp. Versioned GET uses "operation": "get", "references": ["holding_reg01"]. The response includes the request ID and a result for each reference: confirmed, rejected, failed, not_attempted, or unknown when a write acknowledgement was lost. Repeating an identical request returns a cached response while its ID remains in the bounded process-local cache. This protection does not survive process restarts. An ID reused with different content is rejected. See the operational and migration guide for limits and failure semantics.

--once polls once, flushes MQTT publications, and exits with 0 on success, 1 on runtime/poll/publication failure, or 2 for invalid configuration/options. It does not execute commands. Invalid configurations previously skipped, numeric wraparound, fractional unscaled integer writes, and oversized UTF-8 strings now fail explicitly. See CHANGELOG.md for the complete migration list.

Examples

# Modbus TCP
modpoll --tcp 192.168.1.10 --config examples/modsim.csv

# Modbus serial
modpoll --serial /dev/ttyUSB0 --serial-baud 9600 --config contrib/eniwise/scpms6.csv

# MQTT + JSON export
modpoll --tcp 192.168.1.10 --mqtt-host localhost --export data.json --config examples/modsim.csv

# Multiple config files
modpoll --tcp 192.168.1.10 --config examples/modsim.csv examples/modsim2.csv

See examples/ and contrib/ for more device configurations.

Credits

This project builds on:

• modpoll — Ying Shaodong (MIT)
• modbus2mqtt — Oliver Wagner (MIT)
• spicierModbus2mqtt — Max Brueggemann (MIT)

License

MIT — see LICENSE.