VACON CX Modbus user’s manual Page 5
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3. MODBUS
3.1 General
The MODBUS protocol is an industrial communications and distributed control system to integrate PLCs,
computers, terminals, and other monitoring, sensing, and control devices. MODBUS is a Master-Slave
communications protocol. The Master controls all serial activity by selectively polling one or more slave
devices. The protocol provides for one master device and up to 247 slave devices on a common line.
Each device is assigned an address to distinguish it from all other connected devices.
The MODBUS protocol uses the master-slave technique, in which only one device (the master) can initiate
a transaction. The other devices (the slaves) respond by supplying the request data to the master, or by
taking the action requested in the query. The master can address individual slaves or initiate a broadcast
message to all slaves. Slaves return a message (‘response’) to queries that are addressed to them
individually. Responses are not returned to broadcast queries from the master.
A transaction comprises a single query and single response frame or a single broadcast frame. The
transaction frames are defined below.
Query message from master
Device address Device address
Function code Function code
Eight-Bit Eight-Bit
Data Bytes Data Bytes
Error Check Error Check
Response message from slave
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