Monitoring the communications channel, Sequencing communications requests – FANUC Robotics America GFK-1541B User Manual
Page 149
ries 90™ PLC User's Manual
–
May 2002
GFK-1541B
4
Monitoring the Communications Channel
Once you have a working ladder program, you can use the status data to monitor your
communications activity and take the desired action upon certain events.
Monitoring the COMMREQ Status Word
It is critical to monitor the CRS word for each COMMREQ function you initiate. First, zero the
associated CRS word before executing the COMMREQ function. Then when the CRS word
becomes non-zero, you know the Ethernet Interface has updated it. If the CRS word is updated to
a one (1), then the Command Block was processed successfully by the Ethernet Interface. If the
CRS word is updated to a value other than 1, then an error has occurred in processing the
Command Block.
Do not use data received from a server until the CRS word for that channel is 1 or the Data
Transfer bit goes to 1.
Monitoring the Channel Open Bit
This bit is 1 when a Channel has successfully established a connection with a remote server, and
is 0 when a Channel has been closed.. The Channel Open Bit is meaningful whenever the PLC
CPU is in run mode and the particular channel is being used by Modbus/TCP. The Channel
Open Bit is set at the same time the successful status is returned to the CRS word for the Open
Modbus/TCP Connection COMMREQ.
Sequencing Communications Requests
If the Ethernet Interface receives Command Blocks from the PLC CPU faster than the Interface
can process them, the Interface will log an exception event 08, Entry 2=0024H and will log the
PLC Fault Table entry:
“Backplane Communications with PLC Fault; Lost Request”
Only one COMMREQ function per channel can be pending at one time. A COMMREQ function
is pending from the time it is initiated in the ladder program until its CRS word has been updated
to a non-zero value by the Ethernet Interface.
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