Technical data

SCSI Controller
Overview
The SCSI interface is a single-ended, bi-directional, 8-bit-wide bus
to which up to eight devices can be attached. The KA49 system
module is one of those devices, allowing the attachment of up
to seven additional devices. Devices may play one of two roles:
initiator or target. An initiator originates an operation by sending
a command to a specific target. A target performs an operation
that was requested by an initiator. The KA49 module is always
the initiator and all other SCSI devices attached to it are targets.
Connecting
Devices
Each device attached to the SCSI bus is identified by a unique
device ID number in the range 0 through 7. During the
arbitration, selection, and reselection bus phases in which an
initiator and a target establish a connection, the device IDs
of the initiator and target are both placed on the data bus by
asserting the data bits corresponding to the device ID numbers.
By convention, the ID number of the VAXstation Model 90 system
is six (this is controlled by the programs that drive the SCSI
interface; it is not fixed in Model 90 hardware).
The electrical interface consists of 18 signal lines. Some of these
lines are driven only by initiators, some only by targets, and some
by either. The SCSI bus is always terminated at each end. The
bus is permanently terminated at the controller (near end). Far
end termination can take place in one of two locations:
At the expansion connector on the rear of the system
enclosure
At the second expansion connector on a storage expansion
unit
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