Memo to:       J. Lohmeyer                             X3T9.2/88-108 Rev 2
               Chairman, X3T9.2
                                
Memo from:     R. Snively
               Adaptec 691 South Milpitas Blvd  Milpitas, CA 95035
               (408) 945-8600

Date:          August 31, 1988
               October 6, 1988
               October 16, 1988

Subject:       Clarification of B Cable Data Transfer Requirements

The revision 1 document was modified to clarify one consideration in section 3 
of the proposal.

The following paragraphs should be placed at the end of section 5.1.5.3, Page 
5-8, of the SCSI-2 Working Draft, X3T9.2/86-109 Rev 5 to clarify and define 
the requirements on the sequencing of the REQB and ACKB lines during wide data 
transfer.

To assure proper data integrity, certain sequence requirements must be met 
among REQB/ACKB and REQ/ACK.

1)      REQB and ACKB shall only be asserted during a data phase after wide 
data transfer has successfully been negotiated by the initiator and target.  
They shall never be asserted during phases other than the DATA IN and DATA OUT 
phases or if wide data transfer has not been successfully negotiated.

2)      REQB and ACKB shall operate in the same information transfer mode 
(asynchronous or synchronous) as was negotiated for REQ and ACK.  If in 
synchronous mode, the same offset and transfer period shall be used by REQB 
and ACKB as were negotiated for REQ and ACK.

3)      In both synchronous and asynchronous modes of data transfer, there is 
implicit agreement between the Target and the Initiator that REQ or REQB will 
not be asserted unless a unit of data transfer is ready for transfer and there 
is a known source or destination location for the unit to be transferred.  The 
transfer may be a DATA IN or a DATA OUT transfer.  Similarly, ACK or ACKB will 
not be asserted unless a unit of transfer was successfully transferred and a 
known source or destination is available for the next unit to be transferred.  
Normally, no other synchronization is required between REQ/ACK and REQB/ACKB 
sequences.    If a target implementation requires limitations on REQ/ACK and 
REQB/ACKB relationships, it must enforce the limitations by managing REQ and 
REQB accordingly.  If an initiator requires limitations on REQ/ACK and 
REQB/ACKB relationships, it must enforce the limitations by managing ACK and 
ACKB accordingly.

4)      The number of REQ/ACK sequences and the number of REQB/ACKB sequences 
in a phase shall be equal before the phase changes.  The phase cannot change 
until ACK for the last REQ/ACK sequence and ACKB for the last REQB/ACKB 
sequence of the phase have both been negated as described in section 5.1.5.

Implementor's Note:    If any violations of these rules are detected, the SCSI 
devices are assumed to have lost control of normal sequencing.  If such 
violations are detected by the target, the target may attempt to end the data 
transfer phase and provide CHECK CONDITION status.  If it is impossible to 
correctly terminate the phase, the target may abnormally terminate the 
connection by releasing BSY and entering the BUS FREE phase unexpectedly.  If 
such violations are detected by the initiator, the initiator may attempt to 
transmit a message out with Initiator Detected Error indicated.  If the 
initiator is unable to abnormally terminate the connection, it may assume that 
the operation was not completed correctly.  The initiator may have to 
establish the RESET condition to terminate the operation.