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Modbus Overview
Introduction
Modbus is one of the most popular automation protocols in the world. It supports both serial and Ethernet
devices. Many industrial devices, such as PLCs, DCSs, HMIs, instruments, meters, motors, and drivers, use
Modbus as their communication standard.
Devices are Either Masters or Slaves
All Modbus devices are classified as either a master or a slave. Masters initiate all communication with slaves
and do not communicate to other masters. Slaves are completely passive and communicate only by sending a
response to a master’s request.
Slaves are Identified by ID
Each Modbus slave in a system is assigned a unique ID between 1 and 247. Whenever a master makes a
request, the request must include the ID of the intended recipient. Master devices themselves have no ID.
0
1~247
248~255
Broadcast address
Slave individual address
Reserved
Communication is by Request and Response
All Modbus communication is by request and response. A master sends a request and a slave sends a response.
The master will wait for the slave’s response before sending the next request. For broadcast commands, no
response is expected. This is illustrated by three scenarios as follows:
Normal
The master sends a request to the slave. The slave sends a response with the requested information.
