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Home Assistant as a Modbus TCP slave

Turns Home Assistant into a Modbus TCP device. Pick an entity and a register address, and any PLC, SCADA or energy manager can read it. Set up entirely in the UI, no YAML. Open source.

View on GitHub ↗ Install via HACS

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Latest version: v1.1.0 · · Changelog

// features

What it does

Everything Home Assistant knows becomes available as Modbus registers. Your PLC reads the outdoor temperature from a Zigbee sensor, your energy manager reads the car charger's power, and if you allow it, the PLC switches a light or sets a thermostat.

  • Simple setup

    Slave ID and port (default 502), and Home Assistant answers Modbus TCP requests.

  • Map any entity

    Pick an entity and a start address. The register follows the entity's state right away, no polling.

  • Automatic data types

    On/off becomes a bit, whole numbers uint16 or int16, everything else float32. Override with uint32, int32 or scaled16 (value × 10/100/1000 in one register).

  • Works with every master

    Coils and discrete inputs return the same bits, holding and input registers the same registers, so masters that only support FC3 still see everything.

  • Optional write-back

    Tick Writable and the master can switch entities or set numbers, with proper Modbus exception codes on invalid writes.

  • Allowed masters

    Only accept connections from listed IP addresses or ranges, and optionally listen on one interface, such as your PLC VLAN.

  • Behaves like a real device

    Answers only its own slave ID, like a device behind a gateway. Requests for other unit IDs get no reply.

  • Safe on a live site

    Read-only by default. Writes only go to switchable and settable entities, within their min/max, and never half a 32-bit value.

Modbus TCP has no authentication. Fill in Allowed masters, keep the port on a trusted network or VLAN and never forward it to the internet.

// screenshots

What it looks like

Demo with values from a P1 smart meter (power, energy, current) mapped to registers.

  • Setup: slave ID, port, listen address and allowed masters
    Setup: slave ID, port, listen address and allowed masters
  • Add mapping: pick any entity
    Add mapping: pick any entity
  • Detected data type, ready to confirm or change
    Detected data type, ready to confirm or change
  • All mappings with their registers
    All mappings with their registers

// installation

Install via HACS

Open HACS repository on My Home Assistant

  • 01Add the repository

    Open HACS, choose ⋮ → Custom repositories, paste https://github.com/CervezaStallone/HA-Modbus-Slave and pick category Integration.

  • 02Download

    Download Modbus Slave and restart Home Assistant.

  • 03Add the integration

    Go to Settings → Devices & services → Add integration, search for Modbus Slave and set the slave ID and port.

  • 04First mapping

    Click Configure → Add mapping, pick an entity and a start address. The master can read it right away.

Port 502 works on Home Assistant OS and Supervised. On Container or Core running as non-root, pick a port from 1024 up, for example 5020.

// links

Open source

Free to use under the GPL-3.0 licence. Like the integration? A coffee helps further development.

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Connecting Modbus to your PLC, BMS or energy manager? Get in touch.