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HP 27130B
SID
NO.
SID
MNEMONIC
CTSA-
(INPUT)
Ta ble 3 - 6. SIO Special Functions (Continued)
MUX CARD
MNEMONIC
FUNCTION
Both the single-ended and differential
drivers should be disabled during the
internal loop back test.
This will avoid
unintentional data transmission to the
other systems through the frontplane
drivers, thus causing unpredictable results.
During the internal loop-back test, all
the receivers are automatically disabled.
Thus, the card is completely isolated from
the frontplane receivers.
To assert the LOOP signal, a 1 must be
written into bit 7 of SID
0 channel A,
register 5.
On power up (reset) LOOP- is
unasserted, i.e., no loop back.
If the diagnostic hood is not installed,
the HOOD_ON- signal is pulled to +5V by a
3.3K ohm resistor on the MUX card.
If the diagnostic hood is installed, the
state of HOOD_ON- is the complement of the
state of the HLED- signal (i.e., 0-->1,
1-->0, HLED- --> HOOD-ON-).
Figure 3-4 shows the circuit used to sense
the diagnostic hood.
This circuit is also used
to turn the hood LED on and off.
The state of the HOOD_ON- signal can be read from
bit 5 of SIO
0 channel 1, register O.
A 0
indicates that HOOD_ON- is being pulled to +5V.
A 1 indicates that HOOD_DN- is being pulled to
ground.
It is recommended that a string of Os and 1s
be written to the HLED- output, and read back
through HOOD_ON- during self-test to verify
that the diagnostic hood actually is installed.
3-16
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