Schneider Electric 174 CEV

ModbusPlustoEthernetBridge

1.2BridgeMessageTransactions and Paths

Twotypesofmessagescan be initiated toModbusPlusnodesusing Modbus

commands.Programming messagesare used forlogging into a node and

controlling it, asinload/record/verifyoperations.Data access messagesare used

toread orwrite data at the node,orto getstatistics fromthe node.

When a message transaction isinitiated,alogicalpathisestablished between the

originatingnode and destination node.The pathismaintained until the transaction

is completed.The type ofpaththatis setin each deviceisdetermined bythe

Modbus command embedded inthe message.

Table 1 summarizesthe fourpossibletypesofModbusPluspaths.The bridge

supportsup to eightpathsofeachtype.

Table 1 ModbusPlusPathTypes

PathTypeOpenedinNodeOpenedforModbusCommand

DataMaster(DM)OriginatorData access

DataSlave(DS)Destination Dataaccess

ProgramMaster (PM)OriginatorProgramming

ProgramSlave(PS)Destination Programming

The maximumamountofconcurrentTCP/IPconnectionsavailableinthe bridge is

between 8 and 16,depending on the usage ofpathsin hostapplications.

Forexample,up to 16 dataconnections can be active between TCP/IPhostsand

ModbusPlusnodes(amaximumof8 activeDMpathsand 8 activeDSpaths).Up

to 16 programming paths can be active(8PMand 8 PS).

If Modsoft softwareisbeing used,itsetsbothaPMand a DMpathinthe bridge

foreach activeconnection itmaintainstoaModbusPlusdestination node.

Modsoft couldberunningonasingle host, orsimultaneouslyon multiple hosts.

Thusup to eightTCP/IPconnections can be maintained withModsoft running on

one to eighthosts.

Table 2 summarizesthe Modbusfunction codeswhich open DMand DSpathsin

the respective nodes.Forcompleteinformation aboutModbus commands,see

the ModbusProtocolReferenceGuide,publication PI--MBUS--300.