If we need to connect an instrument to a PLC, what is the maximum cable distance that I can use as a rule of thumb? For example pressure switch (DI).

 

Be more specific when you ask questions to get precise answers.

If you are hard-wiring, the total length depends of the type of cable and its ohms/meter and the PLC input voltage rating, etc. If you are connecting over a network, again the distance varies depending on the media and the protocol.

 

Hi there,

It depends on what fieldbus the PLC is using and what baud rate. If you know what fieldbus, then go to: http://en.wikipedia.org/wiki/Wiki
and search for the fieldbus, most likely you will find the answer.

Best regards

/Pat

 

If the pressure switch has a dry contact relay that is switching 110 vac back to the PLC for a DI, you can most likely go the length of your facility. A 110 vac input card typically looks for 85-130 vac as a high which allows for very long lengths despite voltage drop. If the pressure switch is providing or switching a 24 vdc signal you could find yourself limited to as little as a 50 foot run depending on wire size and power supply size. Hope that helps.

 

"switching a 24 vdc signal you could find yourself limited to as little as a 50"

Sorry, I have to disagree with that. 24 VDC will be better for extreme distance. The input modules are high impedance. On the other hand 120 VAC is limited by the inter-conductor capacitance which will cause input to remain on if it is too high (cable too long). A few hundred feet is OK.

If you have solenoids sharing the same cable it's important to use a reversed biased diode (for DC) or MOV (for AC) across the coil otherwise the inductive spike on Turn-off is enough to trigger an input.

Roy