Conference 7.286::fddi

1300.0. "DEMFA UCHANGE Error" by OFOS01::SCEPPA (Nick @ OFO) Wed Mar 30 1994 13:07

     Hi.

     I seem to have found an undocumented "feature" of the DEMFA card.

     I am programming the DEFMA directly at the card-level (i.e., I am NOT
     using a Digital-supplied device driver).

     According to the DEC FDDIcontroller 400 Technical Description Manual,
     "the format [and therefore, the length] of the [USTOP] command is the
     same as the USTART command." [p. 4-17]

     Also in the same manual [p. E-2], Table E-3 lists the minimum and maximum
     buffer size (in bytes) for each type of command. For this discussion, 
     the key parts of the table indicate:

     Command     Min Size     Max Size
                (decimal)    (decimal)
     -----------------------------------
     USTART        168          168
     UCHANGE       168          168

     I first tried to issue a USTART command to the DEMFA.

     The USTART command worked successfully. (The first longword of the [only]
     Command Ring Entry contained a value of 0x80015000 after completion.)
     
     NOTE: Bit 30 of this longword represents an Error Summary bit. Since it's
     clear in this case, it indicates no error.

     NOTE: Bits 9:21 of this longword represents the length of the command 
     issued to the DEMFA.

     Then I tried to issue a UCHANGE command to the DEMFA.

     The UCHANGE command failed. (The first longword of the [only] Command Ring
     Entry contained a value of 0xC8015000 after completion, which, according
     to the same documentation referenced above, indicates a "Buffer Length
     Error". 

     And so, the question I have is ...

     Other than an invalid buffer length, what causes the DEMFA [firmware] to
     return a "Buffer Length Error" in the Error Code field of a Command Ring
     Entry when the DEMFA [firmware] is issued a UCHANGE command ?

     In order to answer this question, someone will probably need to examine
     the DEMFA firmware's source code. I'm willing to do this if no one else is.

     Thanks.

     Nick Sceppa.
                        

Well....From looking at the code it appears that it is possible to
have a return status of 1 for the invalid buffer length as well as
for problems while processing the USTOP and USTART commands. ( UCHANGE
is exactly a USTOP followed by a USTART )  

Since you didn't have problems with the USTART I will assume that the
errors are from the USTOP.  So some possible reasons to return a code of 1
for a USTOP failure are....

		1) Multiple FC classes or invalid FC class specified.
		2) If a FC class has more than eight users.

Can you post the USTART/UCHANGE command?   Also could you try doing the
USTOP/USTART sequence instead of the UCHANGE to verify that it is the USTOP 
that is failing?

Maureen