Instrument Cable for Digital Signal

Any cable can work at 24 volts, but only thing that need to be seen is what is the status of cable when 24 Volts is not there, Does it come down at ground level or is it just floating as an open circuit,

You need to ground this cable with a reasonable size of resistance if the pressure switch is just like an on off switch,

Good point. If the PLC input is high impedance even with the switch open the input could float up to 24v, especially if there are other 24v lines in a cable bundle. I've seen this happen before and it's difficult to troubleshoot. The solution was a 10k resistor across the PLC inputs. If the input is optically isolated there should be no problem.

GA.
The important thing is to ensure there is some protection on the input to the instrument to stop any nasty pick up or vampires. A couple of back to back zenners, a varistor, a load resistor, 0.1uF capacitor, clove of garlic ...the usual.
It's cheaper than using expensive screened cable.
Del

Follow the Cabling Standards in use at the installation.

Is there a length of run limitation with 24V? question address openingly

Not really if the supply and termination circuit are suitable.
You only need to drive say 10mA through it to give plenty of noise immunity, and if you don't mind dropping the odd volt or so, which is no prob if you are interfacing to 5v logic you have tons of spare thrunge to mess about with.
Del

Thanks

I routinely use 24 AWG Cat. 3 twisted pair telephone wire for this type of discrete signal from remote sites; it works well up to ~10 miles (16 km). We've been doing this since the late 1970's to provide alarm & indication from remote electrical substations. The only problems encountered have been due to lightning strikes on the aerial cable. At generation facilities, where the distance is relatively short (<500m), I use 16 AWG twisted shielded pair due to the higher electromagnetic fields around operating machinery.

As others have indicated, one side of the 24V should be solidly grounded at the source; the optical isolation and dropping resistor options mentioned by Stevem also work well.