P485 MODBUS TO PROFIBUS CONVERTER – USER GUIDE 5–1
GE Consumer & Industrial
P485 Modbus to Profibus
Converter
Chapter 5: Communication model
Communi cation mo del
Introduction
DESCRIPTION In master mode, the P485 is configured to run as a master on the Modbus sub-network,
using a scan-list for communication with the Modbus slave devices. The scan-list is created
using EnerVista P485/D485 Setup and can consist of multiple nodes with multiple
transactions.
Communications between the P485 and the sub-net nodes (Modbus slaves) is based on
transactions with a query/response architecture. The P485 sends out a query on the
Modbus sub-network, and the addressed node is expected to send a response to this
query. Slave nodes are not allowed to respond without first receiving a query.
An exception to this is broadcaster functionality. Most protocols offer some way of
accessing all network nodes. In the P485, this is called a ‘broadcaster’. The broadcaster can
transmit messages to all nodes on the sub-network without expecting a response.
In Modbus, it is possible to broadcast a message to all nodes by sending a message to
node address 0. The Modbus slaves will receive the message, but not Respond to it.
Figure 5-1: Master mode overview
Node 1
(Modbus Slave)
Node 2
(Modbus Slave)
Query
Query
Query
Response
Response
Response
Modbus devicesPLC
Fieldbus Master
Broadcast
P485
Fieldbus slave
and
Modbus master
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