Bradley 233 - Page 19

4.1. 2 Schmitt Trigger
The action of IClOb is as follows:
As the input to R16 goes high, with pin 6 low, pin 7 reaches the logic threshold. Positive
feedback from pin 6 to pin 7 then causes pin 6 to snap into the high state.
Note that the output at pin 6 is in phase with the input to R16.
4.1.3 Inverter
IClOd inverts the Schmitt output so that it is in phase with the starting edge of the PERIOD
multivibrator lOd in the gated mode.
The triggering edge from ICIO is negative going, as required by the differentiators at the
input to the delay circuits.
IC20a inverts the signal so that the external width signal and the internal square-wave are in
phase with the A and B widths at the input of the output circuits.
4.1.4 Non-inverting Buffer
IC60a and 70a prevent capacitive loading on signals used for other purposes.
4.1. 5 Multivibrator (Waveforms : Fig. 3)
Pins 3 and 15 of ICIO perform a 'wired OR' function, a high state overpowering a low state.
Thus when pin 6 is high, the output of pin 3 has no effect on the output at pin 15. IClOc then
becomes a free-running relaxation oscillator. The output at pin 15 is always anti phase to
that at pin 9 so that the current through Rll discharges Oil until the voltage at pin 13 reaches
the threshold. Positive feedback through Gil causes the signal at pin 9 to snap into the other
state, taking pin 13 one logic swing away from the threshold. It then proceeds to drift back
towards the threshold.
When the Schmitt output pin 3 forces pin 15 into the high state oscillations stop,with pin 13
and pin 9 also in the high state, without curtailing the negative half cycle at pin 9.
When the Schmitt allows pin 15 to return to the low state, pin 13 drifts down to the threshold.
There is a delay of about a quarter period before the first negative active edge at pin 9.
This delay does, however, ensure that the first period is equal to succeeding ones.
IC 10 PIN 6
.. PIN 13
— PIN 15
OUTPUT
PIN 9
FIG. 3 MULTIVIBRATOR WAVEFORMS
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