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I'm working on trying to build a "tracking" type ADC from individual components. Shown below is my schematic using the parts available in Multisim. enter image description here

The design is based on the 74LS191N 4-bit Up/Down Binary Synchronous Counter.

Observing the logic chart in the datasheet below: enter image description here

In order to switch from "Up" to "Down" counting you must have the positive slope of the "Up/Down" occur during a "High" pulse on the Enable, all taking place during a high clock pulse (as also briefly described in the introduction of the datasheet).

Finally, shown below are the results of my transient analysis of the schematic where:

  • Red = Clock
  • Teal = DAC output
  • Pink = Analog Signal
  • Navy Blue = Enable pin
  • White = Up/Down pin

enter image description here

I have achieved getting a positive-slope transition while the enable pin is high, during a positive clock pulse but the counter still refuses to count down. Any help?

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