What is Modbus? Simply put, it is a method used to transmit serial information between electronic devices. Modbus is an open protocol that is free to use for commercial applications. It was developed by Modicon to control their programmable logic controllers (PLCs). The protocol is based on master-slave architecture.
In Modbus communication, the master broadcasts requests to the slave devices which return a response. Slave devices each have a distinct identifying address. The master can send messages and requests to individual slaves or to all of them simultaneously. When a response is required, the message is usually sent to a specific slave device. The slave address is the first byte in a request so the slaves know whether to listen or ignore the message.
Many industries use the Modbus protocol to establish serial communication between devices. It supports the TCP/IP and RS232/RS433/RS485 protocols. Also supported but less often seen is Modbus over UDP.
Table of contents:
- Modbus monitoring software
- Why split Modbus data?
- Share Modbus over Ethernet
- Third-party utilities for Modbus protocol
Modbus monitoring software
The serial implementations of Modbus are the ones most used in industry. There are two different variations of serial Modbus:
Modbus ASCII - uses ASCII characters for more human-readable communication. Checksums are derived using a longitudinal redundancy check (LRC). Messages begin with a colon (“:") and end with a newline character (CR/LF).
Modbus RTU is the variation most often employed. It presents data in a compact, binary format. A message is comprised of consecutive bytes with no spaces between them. Error checking is performed by appending a cyclic redundancy check (CRC) checksum to commands. Modbus RTU software identifies the end of a message by a 3.5 character space that acts as a delimiter.
If you are a software developer working with either Modbus ASCII or RTU, you could greatly benefit by using professional-grade Modbus software that can help you monitor and analyze data sent with the protocol.