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4-4 TCP/IP Ethernet Communications for the Series 90™ PLC User's Manual
–
May 2002 GFK-1541B
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.
Client
Series 90-30
PLC CPU
Ethernet
Interface
Server
Ethernet Interface
LAN
Power flows to
COMMREQ in ladder
program
Command Block sent to
Interface
Verify
Command Block
and set up channel
to server
Read Request
Data
Data
Client
COMMREQ
Status Word
Return COMMREQ
Status (CRS) Word
to CPU
Data
Data
Domain of a channel
Domain of a TCP connection
Domain of a remote server
Server
CPU
Interface
Read Request
Data
Backplane
PLC
Server
Power flows to Open
ConnectionCOMMREQ in
ladder program
Command Block sent to
Interface
Verify Command
Block and set up
channel to server
COMMREQ
Status Word
Return COMMREQ
Status (CRS) Word
to CPU
Set Channel Open Bit
Channel Open Bit is
set to 1
Power flows to
Connection COMMREQ in
ladder program
Command Block sent to
Interface
Verify
Command Block
and close channel
to server
COMMREQ
Status Word
Return COMMREQ
Status (CRS) Word
to CPU
Clear Channel Open Bit
Channel Open Bit is
set to 0
This sequence must
be repeated for each
read or write request
Accept
connection
Send connection
acknowlegement
Send disconnect
acknowlegement
Receive
Disconnect
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..