Active Low Work Around

Some radios are not quite able to get the active low COS signal low enough to trigger the circuit on the PI-REPEATER controllers. This process describes a work-around that so far has fixed the issues we have heard about.

  1. Toggle the selector switch for the COS to active high mode.
  2. In the Open Repeater software, Interfaces tab, click the gear icon, and change the COS gpio to Active High input.

What is this actually accomplishing? The circuitry for the active low detector has a diode in series with the circuit which adds voltage to the signal being detected on the COS input. This is why the requirement for a low signal on the controllers is below 0.6V. Above this threshold, the circuit is in a non-deterministic voltage range and behavior cannot be certain.

By switching to the active high inputs, the circuitry is different and the thresholds are respectively different. By only changing the first setting, the logic would be inverted so inactive would become active and the repeater would be stuck in the on position.

By also toggling the configuration in software as well, we are then performing a second inversion so the logic makes sense again. The only byproduct of this process is the COS LED is inverted and will constantly consume a few additional mA.

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