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Communication protocols
Modbus > Basics Modbus
6.4 Modbus
6.4.1 Basics Modbus
Overview
Master-Slave-Communica-
tion
Telegram-structure
Start ID
Slave address
Broadcast with slave
address = 0
ASCII-, RTU Modus
48
The Modbus protocol is a communication protocol that defines a hierarchic structure
between a master and several slaves. Physically, Modbus transmits via a serial half-
duplex connection as point-to-point connection with RS232 or as multi-point connection
with RS485.
There are no bus conflicts for the master, because the master can only communicate with
one slave at a time. After the master requested a telegram, it waits for an answer until an
adjustable wait period has expired. During the latency the communication with another
slaves is not possible.
The request telegrams of the master and the respond telegrams of a slave have the
same structure:
Function code
A request may be addressed to a certain slave or sent as broadcast telegram to all
slaves. For identifying a broadcast telegram, the slave address 0 is set. Only write com-
mands may be sent as broadcast.
Modbus supports two different transmission modes:
n
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:
n
–
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 watchdog.
The mode selection is made at the parameterization.
Data
Flow control
HB130 | CP | 341-1CH01 | en | 18-40
VIPA System 300S
End ID
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