Inverted Power
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To be able to communicate with each other, digital devices must share a common ground reference voltage. The VWCDPIC must communicate with the car's head unit, so it must use the same ground reference as the car. If you want your laptop to be able to communicate with the VWCDPIC, your laptop must use the same ground reference as the VWCDPIC/car.

Many laptop car charging systems provide inverted voltage and make the laptop's ground reference a negative voltage relative to the car's ground. This fouls up communication with the VWCDPIC because now the laptop will be expecting inverted voltages on its signal lines, which the VWCDPIC can not produce.

Go to VWCDPIC.