TM 55-4920-429-13
(3) With dc inputs (no logic ground), the lowest voltage range is +0.1999-volt dc. With K1
deenergized (voltage dividers R23, R25, R26, and R27 are not in the circuit), U19Y selects a Vref of
0.20 volt through voltage divider R4, R5, and R6, and U19Y. R11 is also selected by U19X when Vref
is 0.20 volt, providing a 0.2 volt reference to A/D converter U20.
(4) For the 0.2 to 1.999 volt range, 2.0 volt range is selected through U19Y (Y1) and U19X (X1).
With K1 deenergized, voltage divider R23, R25, R26 and R27 are not in the circuit. The 2.0 volt
reference provides ±1.999 volts full scale and is the basic mode and range for CCA A2.
(5) The 2 to 19.99 and 20 to 199.9 volt ranges provide the same 2.0 volt reference. Relay K1 be-
comes energized allowing the input voltages to be divided by 10 or 100. This increases the range of the
meter to ±19.99 volts or ±199.9 volts full scale, while the input to the A/D converter U20 is 2.0 volts.
Relay K8 will cause VDC lamp to come on.
(6) Ac voltage is measured as phase-sensitive polarity, + for in-phase and - for 180° out-of-phase.
The reference phase AC REF+ and AC REF- is applied to comparator U25, which controls analog mux
U19Z. The ac input signal is buffered by op-amp U26. The signal is switched through U19Z to the+ or
– input of ac averaging circuit U7-13, 12. The rectified average value is scaled to the rms value through
U7-2, 3. It is applied to analog meter M2 through U27. For ac inputs, all relays and switches operate
the same as on dc, except that relay K6 is energized by a logic ground (AC SEL–). Relay K7 lights the
VAC lamp.
(7) For K-ohms measurements, a constant current is applied to the unknown resistance (RX) and
the voltage is measured as dc. Constant current is provided by VR2 and applied through resistor R35,
R18, R19 or R21. The constant current for each range is given in table 1-3.
(8) The external meter driver is made up of U27, R28, and R30. R28 and R30 are switched by K4.
(9) Voltage clamp protection is provided for U26. U7-7 and U7-8 provide +3.5 volts and -3.5
volts. Diodes CR6 and CR7 provide the clamping action when the voltage at U26-3 is about ±4.2 volts
dc.
b. Digital. The multiplexed binary-coded-decimal (BCD) data output of A/D converter U20 is first
buffered (U21 and U23), then strobed into a set of quad latches U6,U12,U11, and U5.
(1) Digit 1 data (D IA) at U5 is encoded with sign and ±10 digit overflow and underflow bits.
The overflow/underflow conditions increment/decrement auto range counter U3 through direction
pulse generator U4-8/6, U10-2, U8-6, and U16-4.
(2) The direction pulse generator is started by an end of conversion (eoc) from U20, which sets
U10-5 to 1. This is anded with DS2 from U20 to produce a pulse only during the first DS2 after eoc.
This pulse is anded (U4) with outputs of U5-2, 14 and an inhibit signal from U9-3. The inhibit signal
prevents counting up from the highest range or down from the lowest range.
(3) Auto range counter U3 is a binary up/down counter. Only two bits, A and B, are used.
(4) U9 is a BCD-to-decimal decoder with only two inputs used.
(5) Decimal point controller U15 is a data selector that selects the proper decimal point for the
volts or ohms mode. For K-ohms mode, the decimal is shifted 1 place to the right.
(6) Over-range decode logic U8-9, 10, 11 detects an over-range condition on the highest range
available. An over-range condition causes square wave at U10-8 to be gated onto the over-range decode
logic. The output signal BLANK+ causes the digital display to flash for any out-of-range condition.
1-25. Digital Display CCA A1.
(See fig. 1-4 and FO-8.) The digital display card decodes BCD data from dmm card A2 and displays
3½ digits, the sign, and decimal point. Out of range conditions are indicated by flashing display.
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