Chapter 9. Modbus/TCP Client170 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual GFK-2224QMonitoring the Communications ChannelThe status data can be used to monitor communications and take action after certain events.Monitoring the COMMREQ Status WordIt is critical to monitor the CRS word for each COMMREQ function. First, zero the associated CRS word beforeexecuting the COMMREQ function. When the CRS word becomes non-zero, the Ethernet interface has updatedit. If the CRS word is updated to a one (1), the Command Block was processed successfully by the Ethernetinterface. If the CRS word is updated to a value other than 1, an error occurred in processing the CommandBlock.Do not use data received from a server until the CRS word for that channel is 1. In addition, do not initiate anyadditional commands to a channel until the CRS word has been updated. The exception to this rule is whenyou want to terminate a command by using the Close Modbus/TCP Connection command.Monitoring the Channel Open BitThis bit is 1 when a Channel has successfully established a connection with a remote server, and is 0 when aChannel has been closed.. The Channel Open Bit is meaningful when the CPU is in Run mode and theparticular channel is being used by Modbus/TCP. The Channel Open Bit is set at the same time the successfulstatus is returned to the CRS word for the Open Modbus/TCP Connection COMMREQ.Sequencing Communications RequestsIf the Ethernet interface receives Command Blocks from the CPU faster than it can process them, the Ethernetinterface 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 fromthe time it is initiated in the ladder program until its CRS word has been updated to a non-zero value by theEthernet interface.
