[Schmitty!]

I believe the interface is a little more complex than just measuring a voltage. I believe it's trying to measure an impedance (not just a resistance). The OI inputs use a multiplexed sample-and-hold circuit to measure the impedance.

There is a current sink internally on the OI that sinks a known current from whatever is connected to the OI input. At some point in time later it samples the voltage present on the input. Based on the measured voltage it assigns an appropriate 0-255 value. This method also allows the OI to determine if the input is an 'open' and for safety reasons assigns an 'open' measurement to a value of 127. This is also the reason why if you drive the input lower than 50mV it will register it as a reading of 127.

The resistance range of 0 to 100KOhms is what is typical for these inputs and I believe this is an industry standard for most remote control inputs. So if you use a non-standard value such as a 10K resistor you will only be able to use a small portion of the total range.

I saw a presentation at one time explaining how this standard interface operates, but I haven't been able to find it lately. If someone can find this, I think it would be very beneficial for anybody trying to use non-standard inputs on the OI.

I also contacted IFI last year about more details about this interface and the only response I got was that we should only use 100K pots. We are responsible for any other items we attach to these inputs.