voltage drop

A few km?

If it's a worry, consider putting a local outstation near the pressure switch, and sending the switch position as a data signal down the PLC's remote I/O facilities via a network cable.

If transmission speed becomes a concern remember that if a signal in the real world plant changes faster than once per second, it ain't real.

If you're going to detect whether the switch is open or closed, the input impedaance of the PLC should be high enough to run it as far as you need. In other words, there should be little to no voltage drop.

If the pressure switch is close enough to the PLC so that the cost of the cable doesnt outweigh the cost of the PLC and pressure switch you wont have a problem. If you are going to spend $k on cable then using an existing comms cable or telephone network may be a better option.

Dear all,

Voltage drop will not be a problem as many have already mentioned.

However, in case of AC, cable core to core capacitance will allow some current back into the PLC input even the contact is open. If the cable is long, PLC always detect the high signal even the contact is still open. if the cable length is 500 m or greater, use DC is better.

Regards

Controller,

The specifications for the PLC's input module should include two vital items: minimum voltage for it to turn on, and current flow through the input. Your good old Ohm's law will then be used: R=E/I = (voltage difference)/(current when on). This will tell you the absolute maximum value the resistance of the circuit can be. Then, given a table of cable resistances, you can determine the maximum length of cable. Since it is a two-way trip, this length needs to be divided by two to give the maximum possible distance to the switch. Finally, you need an engineer's margin of safety, such as a factor of 2 or even 3. Take this maximum possible distance and again divide it by the margin of safety. This will give you the design criteria for your job (such as "cable with cores not less than XX diameter, and length not greater than ZZ meters", using units and wording appropriate for your area).

As an example, assuming 15mA current flow and minimum voltage of 85 volts, R = 25/0.015 = 1667Ω. With a wire of resistance 45Ω/1000' and a safety factor of 3, the permissible distance is: D = (1667/45)*1000/2/3 = 6,200'.

OK? --JMM

thanks guys for all your comments :)