Chapter 7. Programming SRTP Channel Commands132 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual GFK-2224QSRTP Client Channels provides features that help the user preserve TCP connections. These include a periodtime where one can establish an SRTP Channel and specify the channel to run at a given interval, or run as fastas possible. One can also specify a number of iterations, or run forever. Additionally, SRTP Channels allowschannel re-tasking of an active channel to the same remote device, where the parameters of an activechannel, such as changing the channel command type (Read/Write), number of repetitions, time periods, localmemory address, remote memory address, etc. can be changed. SRTP Channels also allows channel re-tasking of an active channel to a different remote device (changing the remote device’s IP address, etc.).However, re-tasking to a different remote device will neither conserve TCP connections, nor save on the time ittakes to create a channel.SRTP Application TimeoutsThe application timeouts within SRTP Channels also include the time needed to establish and maintain theunderlying network and SRTP connection. Examples are establishing the TCP connection for a new channel,establishing communication with the remote device, and TCP retransmissions during Channel operations. If thetime needed for TCP connection establishment or maintenance exceeds the user-specified channel applicationtimeout values, an application timeout will occur. Channel application timeouts are temporary errors; thechannel continues to run when the expected response is received.If the application is seeing timeouts during channel startup, there are a few different options:1. Increase timeout value to account for Channel connection overhead2. Ignore the timeout error on the first transfer3. Use a two-step setup approach where the first COMMREQ has a timeout large enough to account for theconnection overhead and then Re-Task the channel to the normal operating timeouts.7.4 Monitoring Channel StatusThe COMMREQ Status word is returned from the Ethernet Interface to the PLC CPU immediately if theCommand Block contains a syntax error or if the command is local. For remote commands with no syntaxerror, it is returned either after the channel is established successfully and the first transfer has completed or ifthere is an error establishing the channel. The location of the COMMREQ status word is defined in theCommand Block for the COMMREQ function.Format of the COMMREQ Status WordHex FormatHigh Low00 00Minor Error Codes (high byte)Success and Major Error Codes (low byte)COMMREQ Status WordFigure 52: Interpreting COMMREQ Status WordIt is critical to monitor the COMMREQ status word for each COMMREQ function. Zero the associated COMMREQstatus word before executing the COMMREQ function. When the COMMREQ status word becomes non-zero,the Ethernet Interface has updated it.If after executing a COMMREQ function, the COMMREQ status word is zero (0) and the FT Output is OFF, theCommand Block has been sent to the Ethernet Interface, but no status has been returned. If this conditionpersists, check the PLC Fault Table for information.
