VIPA CPU 21 Series, Manual

Page: 264 / 394

Chapter 9 Deployment CPU 21xSER-1 Manual VIPA CPU 21x
9-6 HB103E - Rev. 05/45
The Modbus protocol is a communication protocol that fixes a hierarchic
structure with one master and several slaves.
Physically, Modbus works with a serial half-duplex connection.
There are no bus conflicts occurring, because the master can only
communicate with one slave at a time. After a request from the master, this
waits for a preset delay time for an answer of the slave. During the delay
time, communication with other slaves is not possible.
After a send command, the acknowledgement telegram must be read by a
call of the SFC 218 SER_RCV.
The request telegrams send by the master and the respond telegrams of a
slave have the following structure:
Start
sign Slave
address Function
Code Data Flow
control
End
sign
A request can be directed to a special slave or at all slaves as broadcast
message. To mark a broadcast message, the slave address 0 is used.
In opposite to a "normal" send command, the broadcast does not require a
telegram evaluation via SFC 218 SER_RCV.
Only write commands may be send as broadcast.
Modbus offers 2 different transfer modes:
•
ASCII mode: Every Byte is transferred in the 2 sign ASCII code. The
data are marked with a start and an end sign. This causes a transparent
but slow transfer.
•
RTU mode: Every Byte is transferred as one character. This enables a
higher data pass through as the ASCII mode. Instead of start and end
sign, a time control is used.
The mode selection happens during runtime by using the SFC 216 SER_CFG.
Modbus
Broadcast with
slave address = 0
ASCII, RTU mode