SCAM selection

Edward A. Gardner 522-2247 13-Jul-1994 1555 gardner at ssag.enet.dec.com
Wed Jul 13 15:09:57 PDT 1994


>>How does a SCAM master device initiating a SCAM selection determine that
>>no other device has responded to the selection attempt?  Is there such a
>>thing as a time-out of a SCAM selection attempt or does the SCAM master
>>figure out no one else is out there from the subsequent SCAM transfer
>>cycles (i.e., he plays out the transfer cycles all by himself before
>>deciding there's no one else out there)?

You have it essentially right in the last sentence, a SCAM master
does transfer cycles all by himself until he decides there's
noone else out there.

Specifically, suppose a SCAM master is the only SCAM device on
the bus (or all the slaves have been assigned IDs already).  The
sequence is:

1.  master does SCAM selection.

2.  after maintaining SCAM selection for at least a SCAM
selection response time [250 ms], SCAM master transitions to
transfer cycles.

3.  first iteration should be dominant master contention, about
256 transfer cycles, call it 3ms.

4.  second iteration (if there is one, sometimes the master goes
bus free instead) should be Isolate (formerly called Assign ID). 
Master sends function code, reads back first bit of
identification string, discovers noone is out there, goes bus
free.

Going through the transfer cycles before the master discovers
there's noone out there really only takes a few milliseconds,
This is much, much shorter than the time to do SCAM selection. 
It didn't seem worth any additional complexity to optimize this.

Ed Gardner




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