Appendix B

Typical Bus Phase Sequence

This appendix is included to provide an example of the SCSI bus phase sequence 
for a typical READ command (Tables B1 and B2).  In this example, the target 
does not disconnect from the SCSI bus prior to completing the command.

                                   Table B1
                     Typical READ Command Phase Sequence

==============================================================================
                                   Signals
               ---------------------------------------------------------------
               B  S  A  M  C  I  R  A  R    D    D
               S  E  T  S  /  /  E  C  S    B    B
Bus Phase      Y  L  N  G  D  O  Q  K  T  (7-0) (P)    Comment
------------------------------------------------------------------------------
BUS FREE       -  -  -  -  -  -  -  -  -    -    -     SCSI bus is available. 

ARBITRATION    1  -  -  -  -  -  -  -  -    ID   X     Initiator tries to get 
     "            1                                    the SCSI bus.

SELECTION      1  1  1  -  -  -  -  0  -  ID I,T V     Initiator has SCSI bus 
     "         -  1                       ID I,T V     and selects a target.
     "         1  1                       ID I,T V     ATN is on. 
     "         1  -                         X    X

MESSAGE OUT    1  -  1  1  1  0  0  0  -    X    X     Target has control of
     "               1           1  0       X    X     the bus and gets the 
     "               1           1  0       X    X     IDENTIFY message from
     "               1           1  1    Message V     the initiator. 
     "               1           0  1       X    X
     "               0           0  0       X    X

COMMAND        1  -  0  0  1  0  0  0  -    X    X     Target gets a command 
     "                           1  0       X    X     from the initiator. 
     "                           1  1    Command V     (This phase is repeated 
     "                           0  1       X    X     for each byte.)
     "                           0  0       X    X
==============================================================================














                                   Table B2
               Typical READ Command Phase Sequence (Continued)

==============================================================================
                                   Signals
               ---------------------------------------------------------------
               B  S  A  M  C  I  R  A  R    D    D
               S  E  T  S  /  /  E  C  S    B    B
Bus Phase      Y  L  N  G  D  O  Q  K  T  (7-0) (P)    Comment
------------------------------------------------------------------------------
DATA IN        1  -  0  0  0  1  0  0  -    X    X     Target sends data to 
     "                           1  0  Read Data V     the initiator.  (This
     "                           1  1       X    X     phase is repeated for
     "                           0  1       X    X     each byte.)
     "                           0  0       X    X

STATUS         1  -  0  0  1  1  0  0  -    X    X     Target sends status to 
     "                           1  0    Status  V     the initiator. 
     "                           1  1       X    X
     "                           0  1       X    X
     "                           0  0       X    X

MESSAGE IN     1  -  0  1  1  1  0  0  -    X    X     Target sends a COMMAND 
     "                           1  0    Message V     COMPLETE message to the
     "                           1  1       X    X     initiator. 
     "                           0  1       X    X
     "                           0  0       X    X


BUS FREE       -  -  -  -  -  -  -  -  -    -    -     SCSI bus is available. 
==============================================================================

Key:  -       =  Signal driver is passive.
      0       =  Signal is false.
      1       =  Signal is true.
      "Blank" =  Signal state is the same as the previous line.
      ID      =  SCSI ID for arbitration.
      ID I,T  =  SCSI ID of initiator and target.
      V       =  Parity is valid.
      X       =  The signal is not guaranteed to be in a known state.















