Measuring 0-5V Directly
The PCU uses on-board 10-bit analog-to-digital converters (ADCs). The reference voltage is 5V.
A voltage between 0-5V volts can be directly applied to the analog input, the reading (0 to 1024) can be converted back to voltage by multiplying by 4.88 mV.
Measuring Higher Positive Voltages
Higher voltages can be measured by using a voltage divider. Recent PCU/GCU boards have a 100 kOhm resistor on one side of the divider pre-installed, and an appropriately sized resistor can be soldered into the other side to scale the voltage down to the native 0-5V input.
Measuring +/-30V
Phidgets sells a voltage measurement board (Precision Voltage Sensor - 1135) that can read +/- 30V.