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2-11. Diode Test Function
Q4 provides the source current for the diode test function. The input is sensed through
R2. R2 and R9 form a 10:1 divider for measuring the voltage in diode test. A single beep
sounds when the input drops below about 0.77 V; a continuous tone sounds for inputs
below about 60 mV.
2-12. RPM Function
The output signal pulses of the RPM80 Inductive Pickup are divided by input resistors
R40 and R41, then applied to comparator U4. Resistors R42 and R43 set the trigger level
for U4 (and the protection clamp voltage for Q12), while R44 and R45 provide
hysteresis. When Q13 is turned on, R47 parallels R43, resulting in the lower input trigger
level. Q13 off produces the higher input trigger level. The lower trigger level is
annunciated on the display as a 4 V range, while the higher trigger level is annunciated
as a 40 V range. The output of U4 drives the frequency counter in U1. To convert to
rpm, the frequency is multiplied by 60 for RPM1 (1 revolution/spark), or 120 for RPM2
(2 revolutions/spark). The 10 A input serves as a common for the RPM input. Fuse F1
must be intact for RPM to work.
2-13. Current Function
Input current through R11 develops a voltage that is proportional to the input. The dc
voltage is routed to the active filter and a/d converter inside U1. The ac voltage is routed
to the ac buffer, ac converter, active filter, and a/d converter.
2-14. Analog Section of Integrated Multimeter IC (U1)
The a/d converter, autorange switching, frequency comparator, and most of the
remaining analog circuitry are contained in the analog section of U1. Peripherals to this
U1 analog section include the crystal clock, the system reference voltage, the filter and
amplifier resistors, and capacitors.
U1 uses the dual-rate, dual-slope a/d converter circuit shown in Figure 2-4, A/D
Converter. For most measurements, the basic a/d conversion cycle is 25 ms, for a rate of
40 measurements per second. A single conversion at this rate is called a minor cycle
sample. Each minor cycle sample is used to provide updates at a rate of 40 per second
for the fast response bar graph display, and also provides fast autoranging.
Eight minor cycle samples are necessary to accumulate data for displaying a full-
resolution (4000-count full scale) measurement on the digital display. A 40-ms autozero
phase occurs following every eight-sample sequence. Therefore, each digital display
update requires 240 ms, approximating four updates per second.
Basic a/d conversion elements and waveforms are illustrated in Figure 2-4, A/D
Converter. A voltage level proportional to the unknown input signal charges (integrates)
integrator capacitor C12 for an exact amount of time. This capacitor is then discharged
by a reference voltage of opposite polarity. The discharge (read) time, which is
proportional to the level of the unknown input signal, is measured by the digital circuits
in U1 and sent to the display.
Basic timing for the a/d converter is defined as a series of eight integrate and read cycles,
followed by a 40-ms autozero phase. However, the 40-MJ, overload recovery, and
autoranging modes all require variations from the basic timing.
Theory of Operation
Circuit Descriptions
2
2-7
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