Chapter 2 System Components Connecting to a PNP PLC PNP PLC inputs are normally pulled to low inside the PLC. They need to be fed a voltage to make the signal go high. This is done through the use of external pull-up resistors, typically 3K Ohm. When the camera opto is open, the PLC input signal is pulled up to high through the external pull-up resistors. When the camera opto is closed, the current flows through camera instead of to the PLC inputs, making the PLC input go low. The camera outputs should be set to Active Low so that when camera output is active (opto open), the PLC input is pulled high via the external pull up resistors. 2-26 HAWK MV-4000 Smart Camera Guide