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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

Response

Modbus devicesPLC

Fieldbus Master

Broadcast

P485

Fieldbus slave

and

Modbus master

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Ge P485 Manuel d'utilisateur Page 37