Modbus

Modbus is an open request-response industrial protocol, first published by Modicon in 1979, that lets a client read and write coils and registers in a server device over serial lines (Modbus RTU and ASCII) or TCP/IP networks (Modbus TCP).

Modbus defines a simple data model of four tables: coils (read-write bits), discrete inputs (read-only bits), input registers (read-only 16-bit words) and holding registers (read-write 16-bit words). A client, historically called the master, sends a function code and address range, and the addressed server, historically the slave, replies with data or an exception code. Modbus RTU sends compact binary frames with a CRC over RS-485 or RS-232, while Modbus TCP carries the same protocol data unit over TCP/IP, conventionally on port 502.

Because the specification is freely available and simple to implement, Modbus is supported by PLCs, drives, energy meters, temperature controllers, gateways and SCADA software from many vendors. It is often the common denominator for integrating equipment of different makes and for retrofitting monitoring onto older machines.

Classic Modbus has no authentication or encryption, so networks carrying it should be segmented and protected; a Modbus/TCP Security specification that adds TLS has been published. Register maps differ by device and must be taken from the manufacturer's documentation, including data types, scaling, byte order and whether addresses are zero- or one-based. Polling many devices on one serial bus limits update rates.

Key points

Data model: coils, discrete inputs, input registers and holding registers
Modbus RTU runs over serial links such as RS-485; Modbus TCP uses TCP port 502
Client-server polling with function codes and exception responses
Classic Modbus has no built-in authentication or encryption
Register maps are device-specific and come from manufacturer documentation

Where AiVibe comes in

AiAmbA IoT lists Delta Modbus among its supported protocols, with driver availability confirmed per installation and writes to machines deny-by-default.

Explore AiVibe’s work in PLC & Industrial Control →

Modbus

Key points

Where AiVibe comes in

Related terms

Terms that refer to Modbus