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HP 27130B
'Vhen the single-ended drivers are enabled and the differential drivers are disabled, the receivers
will be driven by the single-ended drivers. When a SPACE
(>=
+4 volts) is sent through SD, the
D 1 diode in the hood will be forward biased. The transmitted signal will pass through the hood
to the receiver input (with a 0.7 volt drop). When a MARK (
<= -
4 volts) is sent through SD, the D 1
diode will be reverse biased. The transmitted signal will be blocked. However, the MUX card fail-
safe circuit will pull the input of the receiver (RD(A)) low, so that the transmitted data will be
interpreted correctly. Because the differential drivers are disabled, the outputs of these drivers are
in a high-impedance state, and they will not affect the single-ended drivers.
"-'hen the single-ended drivers are disabled, all the SD lines will be In a MARK condition (
<=
-4
volts) and the Dl diodes will be reverse biased.
The reverse-biased Dl
will isolate
the single-
ended drivers and let the differential drivers drive the receivers.
The 511-ohm resistors are used to protect the U9636 single-ended drivers during the power-up tran-
sient. During the power-up transient, it is possible to have the outputs of the U9836s at the HIGH
state and the outputs of the 751 74s at the LOW state. If the diagnostic test hood has been connected
to the card during this period,
D
1 will be forward biased. Because the 7517 4s can sink a large amount
of current, the 511 ohm resistors are used to limit the current that can be sourced by the U9636
drivers and protect them from being damaged.
3-33
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