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SID
NO.
SID
MNEMONIC
Table 3-6. SIO Special Functions (Continued)
MUX CARD
MNEMONIC
FUNCTION
HP 27130B
1 - - - + - - - - - - - - - - - - - - - - - · · - - - - - - - - - - - - - - - - - - - - - - 1 '
o
1
o
RTSB-
(OUTPUT>
RTSB-
(OUTPUT>
DTRA-
(OUTPUT>
LED
LoOP-
Active HIGH. When asserted, the
MUX card LED is turned on.
To assert the LED signal, a 0 must
be written into bit
1
of SID
0
channel B, register
5
and SID
1
channel B, register 5.
On power up (reset), LED is asserted,
i.e., the LED on the MUX card is on.
Because a minimum of
6
mA is needed to
turn on the LED, a 422 ohm resistor is
used to supply the current.
Two SID
modem control signals are used in parallel
to shunt the current and turn off LED.
The control circuit of the LED is shown
in figure
3-3.
In order to avoid a large current being
sunk by only one of the two SID
control signals for a long period of time,
thus damaging one of the SIOs, the time
between programming the two SID signals
should be kept as short as possible.
Active LOW.
When asserted, the
self-test loop-back circuits are
activated.
The output of
TXDs (transmit data> of each SID
is fed back to the input of RXDs
(receive data> of the same channel,
e.g., the data is sent from TXDA to
RXDA of the same SID.
All eight channels are controlled by
one loop-back circuit, therefore, it
is not possible to loop back test only
one channel while the other channels are
still operating.
3-15
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