Introduction

The basic MODBUS protocol

MODBUS is a communications protocol originally developed by Modicon in 1979. It is currently managed by the Modbus Organisation. It can seem quite a primitive protocol (i.e. Modbus messages lack basic features such as terminators) but it is very popular within the industrial automation industry, and many Omron devices support it.

A basic MODBUS message consists of a single byte function code, followed by a variable length data segment. The length of the data depends on the function code. The MODBUS protocol includes a basic memory model for devices - each device is seen as consisting of a sequence of coils (boolean flags) and registers (2 byte words). Registers may be either read-only (input registers) or read-write (holding registers). When a device receives a MODBUS message, it is responsible for mapping the MODBUS message to an actual memory location or change in device behaviour. For example, when a device receives a request to write a value of true to coil 1, it may interpret this as a "run" command, whilst a value of false may be interpreted as "stop".

The specification for the base MODBUS protocol is here.

To learn more about the benefits of Modbus integration on a real pack stacking machine, explore the customer reference in the packaging solutions area.

MODBUS serial communications

For RS232 and RS485 MODBUS communications, a wrapper is added to the base protocol. The wrapper consists of

• Slave address: 1 byte - identifies the target device
• Modbus message: variable length
• CRC: 2 bytes
• Slave address Function code Data CRC

The CRC is a checksum calculated according to a standard mathematical algorithm (CRC-16-IBM).

MODBUS serial communications support 2 transmission modes:

• RTU: 11 bit format. Default mode for all devices and mandatory support.
• ASCII: Optional mode for 10 bit format requiring 2 bytes per character.

The specification for MODBUS over serial can be found here.

MODBUS over TCP/IP

When used over TCP/IP, a 7 byte header is added to the start of every MODBUS message. This header consists of the following fields:

• Transaction ID: 2 bytes - this is used to link requests to responses
• Protocol ID: 2 bytes - always set to 0
• Length: 2 bytes - Length of the rest of the message (including unit ID)
• Unit identifier: 1 byte - identifies the target device. This may be translated into a slave ID when gatewaying to serial MODBUS devices.

The specification for MODBUS TCP/IP can be found here.

Links to Samples