Peritek VCD-V, User Manual

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Peritek
5-68 Programming On-board Devices and Memories
Table 5-40 SCSI Protocol
The basic order of the SCSI protocol is arbitration (optional - required
only for multiple initiators), selection, command transfer, data transfer
(optional), and termination.
Selection is achieved as follows:
Monitor the SCSI signals BSY, RST, and SEL.
When they are clear, load SCSI ID#, SCSI data bus on, and SEL.
Test for BSY.
When BSY is detected, clear SEL and procede to command phase.
The Command transfers either 6 bytes or 10 bytes of command data:
Set the TCR for command phase and verify phase match.
Load data byte in output register.
Wait for ACK. When it is received, check REQ and phase.
If phase is the same, load more data, check ACK, etc.
Keep sending until phase changes. The ACB-4000A knows when
to stop.
Now, the controller will want to transfer data or send termination status:
Check the CSBS register to find out what is needed. If data is to be
transferred, call the 34020 routine to transfer data. Using pseudo
DMA mode will speed data transfers. If a message is to be sent,
read it in.
The Termination consists of a status byte and a message byte:
The Status byte indicates to the driver whether any problems have
been encountered. The bits are decoded as follows:
Bit 0,4,5,6,7 reserved
Bit 1 ERROR - If set, execute Request Sense command
Bit 2 Equal - Search condition satisfied. (not used).
Bit 3 Controller Busy - re-run the current command.
The ACB-4000A always sends a zero for the message byte, but the
data must accepted anyway, using the REQ/ACK interaction. Once the
message phase has been terminated, the controller is ready for another
command.