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D-4
Setting Data Transfer Rates
D.2.2 TI 75LBC976 Architecture
The 75LBC976 is made up of nine differential transceiver channels. Each
of these channels has a direction control pin and bidirectional data pin.
The direction control pin, labeled nDE/RE/ (where n is a value from 1
through 9), controls the direction of the transceiver. Pulling the signal
high enables the channel as an output to the SCSI bus, and low enables
the channel as an input from the SCSI bus. These direction control pins
will be connected to the DIR, IGS, and TGS pins of the Symbios SIOP.
The bidirectional data pins, labeled nA (n is a value from 1 through 9),
are configured as inputs or outputs depending upon the state of their
respective direction control (nDE/RE/) pins. When the direction control
pin is high, the data line is configured as an input; when the direction
control pin is low, the line is configured as an output. These data lines
are connected to the Symbios SIOP’s SCSI data and control pins.
The nA and nDE/RE/ pins are the only direct contact with the transceiver.
There are no separate pins for the input and output of the channel, nor
are there separate input and output enables. The signals from the inputs
and outputs of the transceiver are tied together internally to form the
bidirectional data pin (nA). This is also true of the enables for the input
and output of the transceiver channel. These two enables, one active
high and the other active low, are tied together internally to form the
direction control pin (nDE/RE/). An example transceiver channel is shown
in
Figure D.2
. There are five other control pins on the 75LBC976 labeled
CDE0, CDE1, CDE2, BSR, and CRE/. All of these lines are grounded in
the interface except for CDE0. This pin controls the functionality of all the
channels on the transceiver. When this pin is high, the channels are
configured as fully functional bidirectional transceivers. When it is low,
they are configured as input only transceivers. The CDE0 pin is
connected to the DIFFSENS SCSI signal, to protect the transceiver if a
SE device is plugged onto the SCSI bus.