RE_240-1CA20

Chapter 3 Deployment Manual VIPA System 200V
3-36 HB97E - CP - RE_240-1CA20 - Rev. 12/42
Modbus - Basics
The Modbus protocol is a communication protocol that defines a hierarchic
structure between a master and several slaves.
There are no bus conflicts for the master is only able to communicate with
one slave at a time. After the master requested a message, it waits for an
answer until an adjustable wait period has expired. During waiting is no
other communication possible.
The request telegrams of the master and the respond telegrams of a slave
has the same structure:
Start ID Slave
address
Function
code
Data Flow
control
End ID
A request may be addressed to a certain slave or send as broadcast
message to all slaves. For identifying a broadcast message, the slave
address 0 is set.
Only write commands may be sent as broadcast.
Modbus supports two different transmission modes:
• ASCII mode: Every Byte is transferred in 2-character ASCII code. A start
and an end ID mark the data. This enables high control at the
transmission but needs time.
• RTU mode: Every Byte is transferred as character. Thus enables a
higher data throughput than the ASCII mode. Instead of start and end
ID, RTU uses a time watcher.
The mode selection is at parameterization.
Overview
Master-Slave-
Communication
Telegram-
structure
Broadcast with
slave address = 0
ASCII-, RTU-
Modus