Torch SASI/SCSI Interface
                  =========================

+------+--------------------+------------------------+
|      |      Write         |         Read           |
+======+====================+========================+
| FDF0 |                  Data                       |
+------+--------------------+------------------------+
| FDF1 |       Reset        |        Status          |
|      |  Data irrelevant   | b7: BUSY               |
|      |                    | b6: MSG                |
|      |                    | b5: 0=Command, 1=Data  |
|      |                    | b4: 0=Input, 1=Output  |
|      |                    | b3: SEL                |
|      |                    | b2: RST                |           
|      |                    | b1: ACK                |
|      |                    | b0: REQ                |
+------+--------------------+------------------------+
| FDF2 |      Select 1      | SASI board: Set ACK    |
|      |  Data irrelevant   | SCSI board: -          |
+------+--------------------+------------------------+
| FDF3 |      Select 0      |           -            |
|      |  Data irrelevant   |                        |
+------+--------------------+------------------------+