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Chapter 7. Programming SRTP Channel Commands
132 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual GFK-2224Q
SRTP Client Channels provides features that help the user preserve TCP connections. These include a period
time where one can establish an SRTP Channel and specify the channel to run at a given interval, or run as fast
as possible. One can also specify a number of iterations, or run forever. Additionally, SRTP Channels allows
channel re-tasking of an active channel to the same remote device, where the parameters of an active
channel, such as changing the channel command type (Read/Write), number of repetitions, time periods, local
memory 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 it
takes to create a channel.
SRTP Application Timeouts
The application timeouts within SRTP Channels also include the time needed to establish and maintain the
underlying 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 the
time needed for TCP connection establishment or maintenance exceeds the user-specified channel application
timeout values, an application timeout will occur. Channel application timeouts are temporary errors; the
channel 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 overhead
2. Ignore the timeout error on the first transfer
3. Use a two-step setup approach where the first COMMREQ has a timeout large enough to account for the
connection overhead and then Re-Task the channel to the normal operating timeouts.
7.4 Monitoring Channel Status
The COMMREQ Status word is returned from the Ethernet Interface to the PLC CPU immediately if the
Command Block contains a syntax error or if the command is local. For remote commands with no syntax
error, it is returned either after the channel is established successfully and the first transfer has completed or if
there is an error establishing the channel. The location of the COMMREQ status word is defined in the
Command Block for the COMMREQ function.
Format of the COMMREQ Status Word
Hex Format
High
Low
00 00
Minor Error Codes (high byte)
Success and Major Error Codes (low byte)
COMMREQ Status Word
Figure 52: Interpreting COMMREQ Status Word
It is critical to monitor the COMMREQ status word for each COMMREQ function. Zero the associated COMMREQ
status 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, the
Command Block has been sent to the Ethernet Interface, but no status has been returned. If this condition
persists, check the PLC Fault Table for information.