Communication Model; Description - GE D485 Instruction Manual

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Grid Solutions

DESCRIPTION

D485 MODBUS TO DEVICENET CONVERTER – USER GUIDE
D485 Modbus to DeviceNet
Converter
Chapter 5: Communication model

Communication model

Introduction
In master mode, the D485 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 D485 and the sub-net nodes (Modbus slaves) is based on
transactions with a query/response architecture. The D485 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 D485, 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.
PLC
Fieldbus Master
D485
Query
Broadcast
Fieldbus slave
Query
and
Modbus master
Query
Figure 5-1: Master mode overview
Modbus devices
Node 1
(Modbus Slave)
Response
Node 2
Response
(Modbus Slave)
Response
5–1