Valid SCAM Selection?

Tony DeLaCruz tdelacru at QNTM.COM
Sun Jun 15 22:20:05 PDT 1997

* From the SCSI Reflector (scsi at, posted by:
* tdelacru at (Tony DeLaCruz)
     To all,
     I was wondering if some could confirm that it is illegal to assert the 
     I/O during SCAM selection. The following is my interpretation of the 
     correct sequence of SCAM selection:
     Start of SCAM Protocol:
     1. Wait for Bus Free to occur.
     (Section 6.1.1 of SCSI 2 specification)
     a. Detection of both SEL and BSY false.
     b. All devices shall release all SCSI bus signals after detecting Bus 
     2. All SCSI bus signals are false at this point.
     (Section 6.1.2 of SCSI 2 specification)
     a. Host asserts the BSY signal and optionally asserts the Data Bus 
     signal representing it's ID.
     b. The Host asserts the SEL signal after winning arbitration.
     3. Only the BSY and SEL signals (and possibly a Data Bus signal) are 
     asserted at this.
     (Section B.4.1 of SPI Amendment r03)
     a. Host shall release all DATA BUS signals and assert the MSG signal.  
     b. Wait at least two deskew delays and release the BSY signal.
     4. Only the SEL and MSG signals should be asserted at this time. There 
     has been no reference to the assertion of the I/O signal yet. This is 
     the point which we sample the bus for a valid SCAM selection. We look 
     for SEL and MSG signals asserted and all other signals released for 
     confirmation. Is it safe to say that it would be an invalid SCAM 
     selection if any other Bus signal is asserted other than the SEL and 
     MSG signals?
     Tony De La Cruz
     Quantum Corporation
     500 McCarthy Blvd.
     Milpitas, CA 95035
     E-Mail: tdelacru at
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