GE GFK-1541B manual Domain of a TCP connection Domain of a channel

4

Operation of the Communications Request

The figure and text below explains how Communications Requests are executed to complete a data read from the remote Modbus/TCP device. The figure specifically illustrates the successful operation of a data read.

Domain of a TCP connection

Domain of a channel Domain of a remote server

Client

Series 90-30

PLC CPU

Power flows to Open ConnectionCOMMREQ in ladder program

	Client			Server
PLC	Ethernet		Server
Backplane	Interface	LAN	Ethernet Interface Interface

Server

CPU

Command Block sent to Interface

COMMREQ

Status Word

Channel Open Bit is set to 1

Power flows to COMMREQ in ladder

program

Command Block sent to Interface

Data

COMMREQ

Status Word

Power flows to Connection COMMREQ in

ladder program Command Block sent to

Interface

COMMREQ

Status Word

Channel Open Bit is set to 0

Verify Command Block and set up channel to server

Return COMMREQ Status (CRS) Word to CPU

Set Channel Open Bit

Verify Command Block and set up channel to server

Read Request

Data

Return COMMREQ Status (CRS) Word to CPU

Verify Command Block and close channel to server

Return COMMREQ Status (CRS) Word to CPU

Clear Channel Open Bit

Accept connection

Send connection acknowlegement

Read Request

Data

Receive

Disconnect

Send disconnect acknowlegement

This sequence must be repeated for each read or write request

Data

1.A Communications Request begins when there is power flow to a COMMREQ function in the client PLC. At this time, the Command Block data is sent from the PLC CPU to the Ethernet Interface.

2.The COMMREQ Status word (CRS word) is returned immediately if the Command Block is invalid. If the syntax is correct, then the CRS word is returned after the transfer of data..

4-4 TCP/IP Ethernet Communications for the Series 90™ PLC User's Manual– May 2002 GFK-1541B