First, there are posts in this thread that are confusing the RC inputs with the OI. This thread is about the operator interface, not the Robot Controller, they are different designs.
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The wiring is currently: Negative on pot to pin 4 on OI (ground). Positive on pot to pin 1 on OI (+5 V), & signal from pot
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The Operator interface is designed to work nominally with a 100 K ohm variable resistance (potentiometer or switched resistors) with only two connections as shown in IF documentation and the 2007_Guidlines_Tips_Good_practices document. It probably will generate values throughout the range of a 120K pot due to various tolerances.
Disconnecting your ground should give you a range of values of around 128 values (I forget if it is 0-128 or 128-255).
By connecting the ground you are drawing more current than intended by IFI and thus messing up their current measuring circuit and software. Connecting a pot in this manner can also cause the OI input to look like a disconnected joystick which will generate a 127 as an output value at one end of the range.