Monitoring The Communications Channel - GE RX3i User Manual

Chapter 9. Modbus/TCP Client

Monitoring the Communications Channel

The status data can be used to monitor communications and take action after certain events.
Monitoring the COMMREQ Status Word
It is critical to monitor the CRS word for each COMMREQ function. First, zero the associated CRS word before
executing the COMMREQ function. When the CRS word becomes non-zero, the Ethernet interface has updated
it. If the CRS word is updated to a one (1), the Command Block was processed successfully by the Ethernet
interface. If the CRS word is updated to a value other than 1, an error occurred in processing the Command
Block.
Do not use data received from a server until the CRS word for that channel is 1. In addition, do not initiate any
additional commands to a channel until the CRS word has been updated. The exception to this rule is when
you want to terminate a command by using the Close Modbus/TCP Connection command.
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 when the 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 CPU faster than it can process them, the Ethernet
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.
170 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual GFK-2224Q