Modbus Protocol Description; Master-Slave Principle - ECOLAB Versatronic Modbus Beschreibung

Modbus protocol description

3 Modbus protocol description
3.1

Master-slave principle

NOTICE!
The Versatronic can only be operated as a slave.
Communication between a master (e.g. a SCADA system or PLC) and a Versatronic as
slave via Modbus takes place on the basis of the masterslave principle in the form of a
data inquiry/instruction – response.
Depending on the transmission technology, the bus users (master and slaves) are
addressed as follows:
n For Modbus via a serial interface, all slaves are identified by their device address
(1 to 254). Master devices do not need an address.
Ä Chapter 6 'Modbus over a serial interface' on page 37
n For Modbus via Ethernet, users are identified by their IP address. Slave responses
are sent to the IP address for the master.
Ä Chapter 7 'Modbus over Ethernet' on page 39
The master controls data exchange by cyclically querying the slaves on the overall bus.
The slaves (e.g. Versatronic) have only a response function. The master has write and
read access to the slaves. This enables data to be communicated between master and
slave devices in real-time. Slaves cannot communicate with one another directly. In order
to transmit data from slave to slave, the master must extract data from one slave and
transfer it to the next.
As a rule, the master directs its queries to individual slaves. For this, it must communicate
with the individual slaves by means of their unicast address. Queries can be directed to all
slaves on the bus also in the form of a broadcast message. In this case, the broadcast
address '0' is sued as the slave address. Slaves do not respond to broadcast queries.
Data collisions would otherwise result in serial bus systems.
For this reason, use of broadcast addresses only makes sense with function codes for
writing data. Broadcasts cannot be used with function codes for reading data.
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