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5.59
The 8421 weighted BCD outputs are taken via pins 7, 9, 8 and 6 to the Control
Circuit Assembly and thence to the display package.
5.60
The BCD outputs assume logic '1' or '0' states as the count progresses from 0 to 9
as indicated in Fig.2.3.
5.61
The output of the high speed decade is taken from pin 8 of IC5b via pin 10 of the
p.c.b. to the next decade divider on the display assembly.
Overflow Indication Fig.4.5
5.62
At the end of a measurement cycle the negative reset pulse to pin 5 of IC7d on the
control Assembly ensures a logic '1' to input pin 9 of IC12b. Pin 10, 12 and 13
arenormally at logic '1' therefore the output of IC12b pin 8 is always at logic '0' after the
reset pulse. JC7b and JC12b remain in this condition unless an output pulse from the most
significant decade of the Display Assembly appears at the OVERFLOW input to the Control
Assembly, indicating that the display counter chain has overflowed.
5.63
This pulse is differentiated by C14 and R35, the positive spike is clamped by 06
.
and the negative input to IC12b causes pin 8 of IC12b to go to logic '1'. Pins 4
and 5 of IC7b are both now at logic '1' since the collector of Q10 is normally high, thus pin
9 of 1C12b goes to logic '0'. IC{'b and IC12b remain in this condition unless a reset pulse
appears at pin 5 of IC7b. IC12b thus applies logic '1' to pin 2 of IC13a from the time an
overflow occurs to the next reset pulse.
5.64
The logic '1' applied to pin 2 of IC13a after overflow, is fTansferred to the Q
output pin
5
of IC13a when the transfer pulse is generated at the end of the gate
time. This logic '1' switches on Q11 and lights the lamp.
FREQUENCY STANDARD
5.65
This is provided by a 5 MHz Racal fast warm-up oscillator Model 9440 which has
a stability better than 1 part in 1o6 over the complete operating temperature range.
The oscillator which is of the plug-in type has a B7G base. For further details refer to para.
1 .6 of the Technical Specification.
POWER SUPPLIES
No
ins Pack
Ass_
e mbly 11-0758 Fig .4. 12
5.66
Single phase mains input is applied to the Noins Pack Assembly via the flying
lead to the rear panel. Adjustment is provided for inputs of 110V, 120V, 200V,
220V and 240V with protection on the live side provided by FS2 (110/120V- 315
mA:
200/240V - 1 00 mA).
5-9
9057/9059
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