hager HTG410H, Installation Manual

Page: 8 / 84

8 Technical changes reserved
5. Modbus and RS485 specific wiring rules
Modbus protocol:
The Modbus over Serial Line Specification and Implementation
Guide, published on www.modbus.org, defines the characteristics
of the Modbus protocol over serial line.
Modbus is a serial communications protocol originally published
by Modicon in 1979 for use with its programmable logic controllers
(PLCs). Simple and robust, it has since become a de facto standard
communication protocol. The development and update of Modbus
protocols is managed by the Modbus Organization which is an
association of users and suppliers of Modbus compliant devices.
The Modbus serial line protocol is a Master-Slaves protocol. Only
one master at the same time is connected to the bus, and one or
several (247 maximum number) slaves are also connected to the
same serial bus.
A communication is always initiated by the master. The slave will
never transmit data without receiving a request from the master.
The slaves will never communicate with each other. The master
initiates only one transaction at the same time.
The master issues a request to the slave in two modes:
- In unicast mode, the master addresses an individual slave. After
receiving and processing the request, the slave returns a message
to the master. In that mode, a transaction consists of 2 messages:
a request from the master, and a reply from the slave. Each slave
must have a unique address (from 1 to 247) so that it can be
addressed independently from other slaves.
- In broadcast mode, the master can send a message to all slaves.
No response is returned to broadcast requests sent by the master.
The broadcast requests are necessarily writing commands. All
devices must accept the broadcast for writing function. The
address 0 is reserved to identify a broadcast exchange.
HTG410H
Modbus and RS485 specific wiring rules