What is Modbus?

Modbus lets one device, the client, ask another, the server, for the values it holds or tell it to change them. A meter might expose voltage, current and energy totals; an inverter its output power and a setpoint for its export limit; a generator its running state and fuel level.

The protocol says nothing about what a value means. Each manufacturer publishes a register map listing which address holds which quantity, in what format and scale, and every integration starts by reading that map.

Key functions

• Read holding and input registers
• Read and write coils and discrete inputs
• Write single or multiple registers
• Serial multi-drop on RS-485
• Ethernet access over TCP port 502
• Gateways between serial and TCP

Modbus is the common denominator of energy hardware. It is cheap to implement, it runs on small processors, and nearly every meter, inverter and battery controller supports it, which makes it the default way to reach equipment that has no cloud API.

Its simplicity has costs. There is no built-in security, no discovery and no standard data model, so each device needs its register map, and anything on the network can read or write it. Integrations put Modbus behind a gateway on a protected local network for that reason.

Data lives in four tables: coils (single-bit outputs), discrete inputs (single-bit inputs), input registers (16-bit read-only values) and holding registers (16-bit read and write values). Larger numbers span two or more registers, and devices differ on word order, so the register map has to say.

A client sends a request with a unit ID, a function code such as 03 for reading holding registers or 16 for writing several, a starting address and a count. The server replies with the values or an exception code. In Modbus RTU the frames travel on an RS-485 bus with a checksum; in Modbus TCP they travel inside TCP packets with a short header.

1. 01→

   Client

   Gateway or controller

2. 02→

   Request

   Unit ID, function code, address

3. 03→

   Server

   Meter, inverter, battery

4. 04

   Response

   Register values or exception

Voltra Edge runs a local Modbus driver, so meters, inverters, batteries and generators on a site’s network come into the same device model as chargers and building systems. Device profiles hold each model’s register map, so a supported device is mapped once and reused across sites.

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  Voltra Edge

  The on-site runtime with local protocol drivers.

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  Data acquisition

  Collect data from plant equipment over Modbus and other protocols.