Service Model
8903B
pulled up by an external pull-up resistor. When a Controller
1/0
port inputs data from other circuits of
the Audio
Analyzer,
these circuits must operate against the pull-up resistor.
Readback devices that are read out
to
the
data
lines, such as
A7U15B,
are similarly configured.
A7U15B
has an open-collector TTL output. When
it
outputs 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=l
when the line
is
low).
When
A7U15B
is
disabled (enable is low), the output is high (inactive) and has no effect on the data
line.
A7U15B
is enabled by Direct Control Special Function
0.70d.
The value of
d
is
arbitrary to enable
A7U15B,
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.
--1
PI0 A2_1N_PUT
AMPLIFIER-ASSEMBLY
--
PROGRAMMABLE
GAIN AMPLIFIER
I
TO HP AND
WEIGHTING BP
I
f
I
LTERS
I
1
FROM DIFFERENTIAL-
I
TO-S
I
NGLE-ENDED
-)
I
AMPLIFIER
I
INPUT
I
DETECTOR INPUT OVERLOAD
DETECTOR
A
1'1
PI0
A7
LATCH
ASSEMBLY
-
--
r-
I
II
+5v
FF -TO GAIN
I
I
SIGNIFICANT
CONTROLLER
PIR
U14
,
RESET LATCHES
""*
LQ
ENABLE
Figure
8-5.
Example Showing Input Overload Detector Readback
To
clarify this, suppose that
A7U14
has not been reset.
If
Direct Control Special Function
0.700
is
entered, the displays will show
0000
0000.
The second four digits are
0000
because
d=O
was
received by
the Controller from the keyboard and issued to the Instrument Bus. The un-reset flip-flop puts a
low
on
the input of
A7U15B
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
000.
Thus the
d
read back
is
0000.
If
A7U14
is
reset,
A7U15B
puts
a
low on the least-significant
data
bit
(r=l),
and the data read back is
0001.
The displays are therefore
0001
0000.
(Note that
TTTT
is
different from
wwww.)
If
d
is
keyed in as
a
hexadecimal
F,
the displays are
1111 1111
regardless of the state
of
A7U15B.
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 re-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
8-10
reu.20
JUN8
7
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