Communication Model; Description - GE P485 Instruction Manual

GE Consumer & Industrial

DESCRIPTION

P485 MODBUS TO PROFIBUS CONVERTER – USER GUIDE
P485 Modbus to Profibus
Converter
Chapter 5: Communication model

Communication model

Introduction
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.
PLC
Fieldbus Master
P485
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