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GFK-1004B
Chapter 4 Programming Communications Requests
Sequencing Communications Requests
If the Ethernet Interface receives Command Blocks from the PLC faster than the Inter-
face can process them, the Interface will log an exception event 08, Entry 2=0024 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.
Data Transfers with One Repetition
If your ladder program for issuing COMMREQs is constructed so it does all the follow-
ing, all available TCP connections can quickly be used up.
The number of repetitions (word 9 in an Establish Channel COMMREQ) is set to 1
and a new COMMREQ is issued immediately upon completion of the prior one.
Each successive COMMREQ is directed to the same target device (same IP address).
Each successive COMMREQ is directed to the same channel number.
To avoid using up all TCP connections, set the number of repetitions (COMMREQ word
9) to 2 and set the read/write period (COMMREQ words 10 and 11) to be very large, for
example, 60 seconds. With these parameters the ladder program will issue the first
COMMREQ, wait for the COMMREQ Status (CRS) word to turn to 1, then issue the next
COMMREQ, wait for the CRS word to turn to 1, and so forth.
By interrupting an active channel, you allow the reuse of an existing TCP connection,
while a repetition count of 1 started the time-consuming TCP connection teardown
immediately.