GE P485 instruction manual Communication model, Plc

GE Consumer & Industrial

P485 Modbus to Profibus

Converter

Chapter 5: Communication model

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.

PLC P485 Modbus devices

Fieldbus slave

Fieldbus Masterand Modbus master

Query

Response

Broadcast

Query

Response

Query

Response

Node 1 (Modbus Slave)

Node 2

(Modbus Slave)

Figure 5-1: Master mode overview

P485 MODBUS TO PROFIBUS CONVERTER – USER GUIDE 5–1