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IOF Problem Resolution
Problem E03
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Description The "AT" command gets an "ALLOCATE" or other initiation error attempting to start an APPC session on a remote CPU.
"AT" commands may work properly on some CPU's but not on others. The problem could also be limited to specific destination CPU's.


One or more of the APPC or VTAM parms has been incorrectly defined on one of more of the connected systems.

The IOF "COMTRACE" command can be used to get additional information about the error. This additional information should point to the parm(s) that are incorrectly or incompletely defined. To use:

  • Enter the "COMTRACE ON" command to enable communications tracing.
  • Enter the "AT" command.
  • If an APPC session is actually started, end it and return to the original IOF session.
  • Enter the "COMTRACE OFF" command to disable communications tracing.
  • The trace is recorded in the Job Console Log of your TSO session, and in the system log (SYSLOG or OPERLOG).
  • Select your TSO session, and then select data set number 1, your Job Console Log.
The communications trace may give one or more IBM error messages. Lookup the IBM error messages, and correct the problem described.


IBM also provides an APPC trace facility that may provide additional useful information. To Use:

1) Allocate a PS dataset. DCB characteristics don't matter unless you specify a BLKSIZE. If you do, make it 4096. The other DCB characteristics will be overwritten. Best way to do this is to issue these commands from TSO READY or ISPF Option 6:


This will allocate a dataset with no DCB characteristics. Be sure to specify enough space for the trace. There is no point in specifying secondary space. APPC will only use the primary space allocation wrapping back to the front when it hits the end of the primary space.

2) Make sure that the client address space and the server address space have UPDATE access to the trace dataset. The client is the TSO session that initiates the IOF session. The server is the transaction processor address space which is initiated when the AT command is issued. Both should normally have the same userid environment, so as long as the TSO session can update the dataset, the server should be ok also.

Additionally, the APPC address space also needs access to the dataset. If this is a RACF system, then this should occur automatically. If not RACF, then you might have to do something to set this up.

3) Issue the Rexx exec ATBTRACE with the following parms:

ATBTRACE START DA(Step #1 DSN) LU(partner_LU) TP(APPC_TP_name)

Basically, the partner_LU is the LU name defined in the B67 option member and APPCTP_name is the transaction processor name define in B67. You can also add the USERID parameter to filter the tracing down to APPC sessions that involve that userid. The syntax of the USERID parameter is as follows:


ATBTRACE causes APPC to trace the side of the conversation that is on this CPU. If both sides of the conversation are on the same CPU, then you will see both sides of the conversation.

Also, it appears that to capture a trace, you need to issue the ATBTRACE command prior to initiating the APPC session. You can't get into an APPC session and then start tracing.

4) Run the APPC function to be traced.

5) Issue the Rexx exec ATBTRACE with the following parms:


6) Read the dataset, it is in text format.


More Help Click here to EMAIL a problem report to IOF Technical Support for additional assistance.


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Triangle Systems, Inc. PO Box 12752, Research Triangle Park, NC 27709
(919) 544-0090

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