Peritek
5-66
Programming On-board Devices and Memories
5.13 SCSI Port
The Small Computer Systems Interface (SCSI) port is an optionally
installed eight bit parallel interface designed to allow the VCT-V or VCU-
V to communicate with other intelligent devices. The standard SCSI
supports up to eight devices, which can be processors, disk controllers, etc.
The 5380 chip, which is as the interface to the SCSI, contains registers and
bus interface logic. It itself is not intelligent, and it requires a fair amount
of handholding by the CPU.
The 5380 controller is designed to accommodate the standard 8-bit parallel
SCSI as defined by ANSI X3T9.2. It operates in both initiator and target
roles, and can therefore be used in host adapter, host port, and formatter
environments. The 5380 supports arbitration, including reselection. It has
special high-current open collector output drivers, capable of sinking 48
mA at .5 V, for direct connection to the SCSI bus. No other external logic
is required to support the chip except for the on-board terminators.
Note
As mentioned in Chapter 2, the SCSI and the High Speed Port (HSP)
share some signals on the P2 connector, thus precluding simultaneous
installation of both options on a given board.
The 5380 contains several registers which facilitate control of the SCSI
bus and as input buffers which allow the direct readback of all SCSI
signals. The 5380 provides the actual electrical interface, but doesn't have
any intelligence built in. The 34020 or the VMEbus host must do the
actual bit manipulation and handshaking required to implement the SCSI
protocol.
The NCR 5380 design manual, MAXTOR XT8000S OEM Manual, and
the Adaptec ACB-4000A OEM manual are useful references. For the sake
of clarity, relative to the ACB-4000A, the 5380 functions as the initiator,
which means that the ACB-4000A is the target and controls the SCSI bus.
Also, in the 5380 manual, the signals are referred to in the register tables
as negative true signals, but in fact are programmed as normal positive
true. Using the ACB-4000A as an example, we shall go through the SCSI
protocol. While there certainly are other ways to use the SCSI port, this is
a common application.
As mentioned above, the VCT/VCU SCSI port functions as the SCSI
initiator, and the disk controller functions as the target. The 5380 has a
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