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Circuit Description
CPUA40
U101
is
the front-panel processor
(FPP)
and monitors the front-panel con-
trols.
It
is
a single chip microprocessor with built-in
RAM,
ROM,
A-to-D
converter,
programmable timer, and serial communications interface.
The
front-panel processor reports any changes
in
state
of
the front-panel
controls
to the
U201
on the
A5
CPU
board via the serial communication
interface.
The
programmable timer
TCMP1
(U101
pin
2)
produces a 0 to
5V,
1 kHz
square wave
signal (CAlSIG) that is used for probe compensation. TCMP2
(U101
pin
1)
produces a 0 to
5V,
1 kHz signal that
is
converted to
ECl
level
(DITHER)
by the resistive divider R503-R505. DITHER
is
used by the Ana-
log Signal Processor on the analog board.
FPP Pots & Probes
A4<Y0
The pot/probe scanner working with the A-to-D converter internal to the
front-panel processor digitizes the inputs and reports the amount a pot has
turned and the type of probe used.
Control lines
to
the analog multiplexers U420 and
U421
determine whether
to scan for pots or probes.
If pots are to
be
scanned, one of 10 possible pot
inputs are read from either
multiplexer U420 or U421. The voltage at the
wiper of the pot
selected
is
applied to the front-panel processor and digi-
tized. The amount and direction of change from the previous stored
value
is
calculated and sent to the host processor
68331
on the processor board.
The intenSity and readout intenSity pots on the front
bezel are stopped pots
and represent one input each on the
analog multiplexer U420. The other 8
pot inputs on
U420 and
U421
represent the 4 continuous rotation pots on
the front
panel which are made of two wipers separated by 180 degrees and
contact a
single resistive arc. Each continuous rotation pot represents two
inputs
to
the multiplexers.
The analog multiplexer U420 also doubles for selecting one of the 4 possible
probe inputs. The probe code resistance from the P6139 probe is converted
to a
voltage by the
pull-up
resistors R411-R414. This voltage is read by
the pot/probe scanner and the probe type
is
determined and sent to the
host processor.
LEOs & Driver
A4
~
There are 5 LEDs (light emitting diodes) on the front panel which are con-
nected between the outputs of
an
8 bit
LED
latch U202 and a pull-up resistor
to
+5
V.
When a particular
lED
needs to
be
turned on or off, the front-panel
processor converts the
lED
identification received from the host processor
to the correct
lED
address on the latch (U202) and changes the state
of
the
lED.
TAS
475 and TAS 485 Service Manual
3-7
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