Model 8901B
Service
m4
r5v
L
FF
oc
INSTRUMENT
BUSPULLUP
RESISTOR
Figure
8B-2.
Example
Showing
Audio
Overvoltage Detector Readback
J
$lC
ENABLE
At
this point in the discussion, a more detailed description of the Instrument Bus
is
needed. Data
(d)
is read out from the
1/0
port of the Controller to the Instrument Bus through buffers
(TTL
inverters). However, data is read back
to
the
1/0
port directly, bypassing the buffers. An
1/0
port
outputs a low by actively pulling the line to ground.
It
outputs a high by allowing the output to
be passively pulled up by
an
external pull-up resistor. When a Controller
1/0
port inputs data
from other circuits of the Modulation Analyzer, these circuits must operate against the passive
pull-up resistor.
Readback devices that are read out to the data lines, such
as
U12C, are similarly configured. U21C
has an open-collector TTL output. When not outputting data, its output device is
off,
pulling
it
to a high-impedance (inactive) state. When it outpits
data,
a low is produced by switching the
output device to ground.
A
high is produced by switching the device
off
and allowing the output
to be passively pulled up. The readback lines are low true (that
is,
r
=1
when the line
is
low).
When U12D is disabled (enable is high), its output is low. Therefore, U21C is high (inactive) and
has
no
effect on the data line. U12D is enabled by Direct Control Special hnction 0.15d. The
value of
d
is
arbitrary to enable U21D, but the least-significant bit must be
0
(that is,
d
must be
even) to switch
off
the output device of the
1/0
data
port.
To clarify this concept, suppose that U19D has not been reset. If Direct Control Special Function
0,152
is
entered, the display will show 000010.0010. (0.15d also controls
FM
squelch. Using 0.152
deactivates squelch.) The four digits following the decimal are 0010 because
d
=2
was received by
the Controller from the keyboard and issued to the Instrument Bus. The set flip-flop (U19D) puts
a high on the input of U21D and an inactive high on the least-significant data line. This
is
read
by the Controller as
r
=O
and thus is the same as the bit issued. The other three data readback
lines are unaffected by the readback command and remain 001. Therefore, the
d
read back is 0010.
If U19D is reset, U21D puts a low on the least-significant data bit
(r
=l),
and the data read
back is 0011. The display is therefore 000011.0010. (Note that
rrrr
is different from
wwww.)
If
d
is keyed as a hexadecimal
F,
the display is 001
11
1.1
11
1
regardless of the state
of
U21D. This is
because all output devices on the data
1/0
port of the Controller are on (logical
1).
One final note, after a Direct Control Special Function is entered, it is periodically issued to the
Instrument Bus. If a fault causes
rrrr
to indicate a malfunction, the display will begin to read
correctly as soon as the fault is removed.
LEAST
SlGNlFl
CANT OATA
BIT
09
8B-4
Special Functions
rev.22SEP87
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