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4
4-5
GFK-1004B
Chapter 4 Programming Communications Requests
Operation of the Communications Request
The figure and text below explains how a Communications Request is executed. The
figure specifically illustrates the operation of an Establish Read Channel Command.
Client
Series 90–70
PLC CPU Backplane
Ethernet
Interface
Server
Series 90 PLC
LAN
Power flows to COMMREQ
in ladder program
Command Block sent to
Interface
Verify
Command Block
and set up channel
to server PLC
Read Request
Data
Data
Read Request
Data
Data
Client
Data
Data
COMMREQ
Status Word
Return COMMREQ
Status Word (CRSW)
to CPU
.
.
.
.
.
.
Read Request
Data
Data
Data
Pulse Data Transfer bit
Pulse received
Pulse Data Transfer bit
Pulse received
Pulse Data Transfer bit
Pulse received
Data
Domain of a channel
Domain of a TCP connection
Figure 4-2. Operation of the Communications Request for an Establish Read Channel-
Command
1. A Communications Request begins when there is power flow to a COMMREQ func-
tion in the client PLC. At this time, the Command Block data is sent from the PLC
CPU to the Ethernet Interface.
2. For the Establish Read Channel Channel command, 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 next significant event: upon failure to
establish a channel correctly and in a timely manner or upon the first successful trans-
fer of data.
Once the channel is successfully set up to the server PLC, the Ethernet Interface
performs the periodic reads as specified in the Command Block.