Fluke 8842a instruction manual Analog Control Signals

Maintenance 6

TROUBLESHOOTING

6-56. Hex Inverter (U221)

Check that U221-5 is the same as STROBE ZERO and that U221-6 is STROBE ZERO inverted.

6-57. Quad OR Gate (U211)

Check U211-6 for 0.2 us pulses, normally high, in two groups of 3 and 15, group widths: 50 and 100 us, group spacing: 10 ms (in fast reading rate).

Check U211-8 for 0.4 us pulses, normally high, groups of hundreds, group widths: 3.5-4

ms, group spacing: about 8 ms (variable).

6-58. Keyboard Wiring

Table 6-19 indicates which waveforms in Figure 6-11 are seen on keyboard inputs to U212 when each front panel button is pressed and held. For example, if the SRQ button is pressed and held, an inverted version of STROBE ZERO waveform is applied to U212-

38.If range button "20" is pressed then inverted STROBE ONE is applied to U212-1, and so forth. Note that these waveforms are very noisy with many spikes. That is normal. Compare these waveforms with normal STROBE ZERO at U215-8.

6-59. ANALOG CONTROL SIGNALS

Table 6-19. Keyboard Wiring

SIGNAL NAME

RL0

RL1

RL2

RL3

RL4

RL5

RL6

PIN

U212-38

U212-39

U212-1

U212-2

U212-5

U212-6

U212-7

BUTTONS THAT

PRODUCE

INVERTED

(STROBE ZERO)

SRQ

LOCAL

RATE

OFFSET

20MΩ TRIG EX TRIG

BUTTONS THAT

PRODUCE

INVERTED

(STROBE ONE)

20Ω/mV

200Ω/mV

2

20

200

2000

Not used

BUTTONS THAT

PRODUCE

INVERTED

(STROBE TWO)

VDC

VAC

2 WIRE kΩ

4 WIRE KΩ

mA DC mA AC AUTO

Table 6-20 is useful for determining whether the correct digital signals are being applied to the analog control devices indicated. Since most of these devices (the quad analog switches in particular) have no digital outputs, it cannot be determined directly whether the correct pattern is being latched. That determination must be made indirectly by analog means. Nevertheless, it is valuable to know whether the correct digital signals are reaching those devices.

6-51