Generally PLC input point must have a photo-coupler to do isolation, then uses the clear signal at the inside of PLC to do logic processing.
So to trigger a digital input point of VB / VH uses DC 24V through its current limiting resistor and photo-coupler to make this point is interpreted as ON.
Whole digital inputs in the VB series (main unit, expansion unit or expansion module) are equipped with bidirectional photo-couplers. If want to trigger those input points by the negative logic (Sinking), please use the positive (+) of DC 24V connects to the S/S terminal (it is to one side of the photo-couplers), then current by an X input terminal through a switch (dry contact or NPN transistor) to the negative (-) that became a circuit. On the contrary, the positive logic (Sourcing) that S/S point is connected to the negative (-) of DC 24V, while positive (+) through a switch of dry contact or PNP transistor, and then trigger the X input.
However, the VH Series PLC is equipped with single directional photo-couplers only (Sinking). The positive of photo-couplers have connected with the DC 24V (+), and the COM terminal has connected with the DC 24V (-) in a VH main unit or an expansion unit. So just use the COM point through a dry contact or NPN transistor to trigger the X input.
Note: Please do not connect an exterior DC 24V's positive (+) at the COM terminal of the VH series main unit or expansion unit. But at the expansion unit of VH, you need to input a DC 24V (+) to the 24+ terminal for supplying the positive of internal photo-couplers, then use the DC 24V (-) through a switch to trigger an X point.