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8842A
Instruction Manual
6-48
16, -18, -21, -25 are the same as Waveform B. The waveforms should be interrupted
every 1.5 sec for 0.2 sec due to interrupts from the watchdog timer. (Note: XU222 pins
refer to a 28-pin socket.)
6-45. Calibration Memory (U220)
Sync on U219-3. Verify that U220-1, -3, -5, -7, -10, -13, -15, -17, -22 are the same as
Waveform A (U219-3). Verify that U220-2, -4, -6, -8, -9, -11, -14, -16, -19, -23 are the
same as Waveform B. The waveforms should be interrupted every 1.5 sec for 0.2 sec due
to interrupts from the watchdog timer.
6-46. Relay Buffer (U201)
At this point it is necessary to return the 8842A to the normal operating mode by turning
the power switch off, removing the short from TP205 (U202-38), and installing the True
RMS AC option, if present, so that U201-14, -15 may be checked. Power up the
instrument. Unlike the previous checks, outputs are steady state and therefore do not
require a sync signal. Logic "1" is approximately 4.3V dc.
Check that U201-14 is high (4.3V) for mA AC and all ranges of VAC, and low for all
other functions.
Check that U201-15 is high (4.3V) for mA AC and 200V ac and 700V ac ranges, and low
for all other functions.
Check that U201-16 is high (4.3V) for all 2-wire and 4-wire ohms ranges, and low for all
other functions.
Check that U201-17 is high (4.3V) for the lowest three VDC ranges and all 2-wire and 4-
wire ohms ranges, and low for all other functions.
6-47. 3-to-8 Chip Select Decoder (U208)
Make the following checks in the normal operating mode using the fast reading rate and
any function and range. These sequences begin 5.5 ms after the A/D trigger, which is the
falling edge at U202-40.
Check U208-13 for 0.2 µs pulses, normally high, groups of 1, pulse spacing: 10 ms.
Check U208-12 for 0.2 µs pulses, normally high, groups of 1, pulse spacing: 10 ms (10
µs after pin 13).
Check U208-11 for 0.2 µs pulses, normally high, groups of 1, pulse spacing: 10 ms (46
us after pin 13).
Check U208-15 for 0.2 µs pulses, normally high, groups of 13, group width: 100 µs,
group spacing: 10 ms (230 µs after pin 13).
Check U208-7 for 0.6 µs pulses, normally high, groups of 5, group width: 50 µs, group
spacing: 10 ms (380 µs after pin 13).
This concludes testing of the basic µC system. If the keyboard or display is still suspect at
this point, proceed to Display System, below. Otherwise proceed to Analog Control
Signals, below.
6-48. DISPLAY SYSTEM
The display/keyboard system is operated by a complex LSI IC (U212). Generally, this IC
is checked indirectly by observing behavior of the simpler logic devices which it drives.
If the keyboard is working at all, the 8842A display should be "frozen" to make the
following tests. This places the 8842A in a special display test configuration. If it is not