I suspect what you are looking for is the rated voltage for a PLC discrete input. Inputs have a range of voltages that will trigger the "ON" status of the input. For example a DC input might be rated for 24 VDC nominal. It respond to a range of voltages at the input module terminals of 10.2 VDC to 26.4 VDC. When a voltage is present above 10.2 volts at the terminals it will change the status in the PLC memory for that point to on. Voltage above 26.4 VDC may cause damage to the module.