Introduction

Modbus is a relatively easy protocol to write, even from scratch, and as such has been prolific in industrial communications. Modbus comes in a few variations. In OMRON PLCs and Machine Controllers, Modbus is easily implemented. Often it is a Function block is used to perform the Modbus transactions.

Modbus RTU and Modbus ASCII

Modbus RTU is the most common form of the protocol and is used on any serial communications layer such as RS232, RS485 and RS422. It is a binary protocol. 

Modbus ASCII is similar to RTU in that it also is used on serial networks, however all values are encoded in ASCII characters. Rather than a value of say 12345 (integer) which on a binary protocol would require 2 bytes, in ASCII the ASCII characters "1", "2", "3", "4" & "5" would be sent, each using one byte. This is obviously less efficient. As such it is not a very commonly used variant of the Modbus protocol.

Modbus TCP

Modbus TCP is simply the same simple protocol, but embedded into a TCP Ethernet frame. In order to perform TCP communications from the user program, the PLC unit must support Socket Services. Some Ethernet units do not support this, such as the CJ2M-CPU3x CPU built in Ethernet/IP port or the CP1W-CIF41 ethernet unit.

The following table gives a summary of the PLCs and what units and software is used for Modbus  TCP and RTU. Click on the links to download the required function blocks and documentation.