Pull Up Resistors on PLC DCV inputs?

Does anyone use pullup resistors for PLC inputs? Is an open contact a reliable logic zero as opposed to a grounded input?

I only ask because I have seen some old solid state devices that would pick up stray voltage and actually toggle from nearby inputs with voltage (DGH modules). Using pullup resistors made a reliable input bacause the input was grounded through the resistor when the contact opened.

Check any 'equivalent circuit' diagrams for the inputs. Many input modules already have some method of ensuring that an open input is not suseptible to noise.

I would predict that any manufacturer would tell you that if a resistor was desirable at an input, that they already built it in. In my experience, that is not always the case, and I have often found that they improve the performance.

To be clear, a "pullup" is intended to raise the input well above the logic transistion voltage. This is often desirable when an open collector device like a prox sensor is in the off mode. This is sometimes called Negative True Logic, when the sensor is true, it will pull the input across the threshhold toward neutral and when off, the resistor will pull the input back across the threshhold toward the positive supply voltage.
It is wise to take into account the additional current that the sensor must conduct to oppose what ever the input provides and well as the contribution from the pullup resistor. If the sensor is stalled in the True condition for a period of time, the resistor also has got to be big enough to handle continuous the load.

Best Regards,

Bob A.