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Appendi x A 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.
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