TTI 1604 Service Manual page 11

Frequency Measurement
Q8 buffers the signal at the Trms AC/DC converter input and drives comparator IC18A. The
microcontroller then measures the frequency. IC21A selects between EOC (end of conversion)
from the A to D or IC18A for frequency measurement.
The output of IC18A is ± 5V; R58 and IC10's input protection diodes limit the voltage below 0V.
Processor, Keyboard and Display
The microcontroller, IC10, controls the DMM IC, IC2, together with the display, keyboard and the
serial interface.
The drive to the six 7-segment display digits is multiplexed; the individual LEDs, LED1 to LED8
and LED9 to LED16, are treated as the segments of two 'digits' to give an overall multiplex
scheme of 1:8. For each digit in turn the segment data is written to IC8 by the microcontroller and
clocked by SEG-LAT; the digit enable is then written to IC14 and clocked by DIG-LAT.
IC9 provides the high current sink capability for the common-cathode digit arrangement.
Inter-digit segment blanking is provided by the SEG-EN signal applied to IC8.
The keyboard is arranged as a 2 x 7 matrix. The microcontroller lines ROW-1 and ROW-2 are
driven low alternately (the other being tri-stated). The status of the keys in each row is latched
into IC13 by KEY-PAD and read into the microcontroller via the B0 – B7 data bus.
The RS232 functions are internal to the CPU which is optically isolated by IC15 and IC16 from
the interface socket. The line side is powered from the DTR and RTS signals via D3 and
R52/R53. D4 and R54 provide input protection for the receive input.
IC19 and IC20 provide digital noise isolation of the B0-7 data bus during measurements. The
DMM circuitry configuration and control is achieved by the CPU control signals driving the
analogue switches IC1, IC3 and IC21 and the data latch IC7.
Buzzer
IC18B is a 2kHz square wave oscillator; Q9 enables the oscillator.
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