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Theory of
Operation
The inputs of the HP 10343B are received by two banks of analog
comparitors (AM26LS32) on each line of the SCSI Bus. One bank of
comparitors is used to receive single-ended signals and the other bank is
used to recieve differential signals. U24 generates a 1.6 volt threshold
level for single-ended
signals.
When operating in the single-ended mode,
all SCSI devices ground the odd numbered pins. However, these pins are
not grounded in the HP 10343B. Instead, the odd numbered pins are tied
through the HP 10343B to the other SCSI connectors on the HP 10343B.
The two comparitors in parellel look like 5k ohms tied to a 2.5 volt source.
A current of 420 uA is drawn from each line when the line is at a TTL
Low of 0.4 volts. This is the active state of a line for a Single-Ended bus.
The Differential bus will see a smaller load because its signals move
between 2 and 3 volts.
Note
The term "active" and "released? are terms used in the SCSI
Standard. "Active" is when the SCSI Bus goes true. In the
Single-Ended Mode, this is when a line goes low on the SCSI
Bus. Internally, after the input comparators on the HP 10343B
interface board, "active" means a TTL high state. "Released? is a
false or a higli state on the SCSI bus, but on the interface board it
is TTL low. This section uses the terms "active" and "released" in
reference to the TTL levels on the interface board.
The selection of single-ended or differential bus is accomplished by
enabling the output of one of the banks of comparitors with switch SI.
The outputs of the comparitors are tristate. The SCSI Bus is low true, but
the outputs of the input comparitors are positive true.
Additional Information
A-5
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