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60
VII-2.3.
PM 2522
If the input becomes logical "0" the measuring cycle is started at the first positive edge
supplied by the input flip-flop, so that jitter and stalling of the digital part of the ADC are
prevented.
- a polarity output circuit.
If the output is logical "0", the polarity is positive, if it is logical "1", the polarity is
negative. (In this case logical "0" is <- 9 V; logical "1" >-1.5V).
- a pulse output circuit.
During the measuring time, the ADC pulses are supplied to the display circuit.
Remark: The scanning output supplies a signal with a frequency of 500 Hz and a duty cycle of
about
1:15 at a clock frequency of 8 kHz.
This 500 Hz pulse feeds the scanning system of the display.
Clock oscillator
(Fig. 17)
The clock pulse is derived from the programmable U. J.T.
Т5106.
The oscillator frequency is adjustable with R156. This frequency should be about 8 kHz.
By this the frequency of the scanning output of IC104 will be 500 Hz.
УН-2.4. Display
The display is divided in the following blocks:
- 4-decade counter/register
(multiplexed output)
- decoder switches
- overrange detector
4 - decade counter/register
This counter is a monolitic (P-channel-MOS) integrated circuit and contains the following
circuits:
- four cascaded decade counters. The supplied pulses are converted into a BCD code per
decade.
- four buffer-registers which store the digital information from the decade counters.
Only at the end of a measuring cyclus the buffer-registers will follow the decade counters.
- a multiplexer which converts the BCD parallel information into a serial form and transfers
the digital information of each decade to the input of the decoder driver synchronously with
the relevant anode switch.
- a scaler of four which converts the scan frequency together with a decoder into а | out of
4 code to drive the anode switches.
Decoder driver
The decoder driver (IC301) converts the supplied BCD information into a sevensegments-code.
It offers active-low, high-sink-current outputs to drive the indicators directly
Anode switches
(Fig. 18)
To convert the open drain current scan outputs of IC105 into a T. T.L. information, TS307 up
to TS311 are connected between IC105 and IC302.
IC302 is a hex inverter.
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