analogue loop

Theoretically, the distance is limited only by how much voltage you have available to drive the 20mA through the cables and the load resistor. Practically, this will probably be fixed at 24V, so you can do the sums.

See the discussion in this thread (or search CR4 for "20mA") for more details.

Yup, here are two more threads (from my own list of discussions) that asked the same question:

http://cr4.globalspec.com/thread/12395/4-20mA-Signal-from-Level-Transmiter

http://cr4.globalspec.com/thread/21144#newcomments

If you want to calculate, first find out what your PLC's input resistance is. Then calculate how much voltage will be dropped across your input when the current is at 20mA. Finally, subtract that value from your power supply's voltage. That figure will give you the maximum voltage drop across your wires. Since you have two wires (I'm making assumptions), you need to divide your calculation by two.

Okay, so now you know how much your voltage drop is across your wires, your next step is to divide that number by 0.02 (20 mA). That gives you the maximum resistance your wire length can have.

Finally, get your cable's electrical data and find out what its resistance-per-meter or resistance-per-foot (whatever) is. Divide your maximum resistance by the resistance-per-meter and you get the theoretical maximum length of wire that you can use. To be on the safe side, divide that figure by two.

1.- Check your PLC I/O card specifications. Verify maximum load resistance allowed: RloadMAX.

2.- Check your field instrument specifications. Verify load resistance: Rload

3.- Check your wire specifications. Verify resistance per 100 yards for twisted pair: Rper100y.

4.- Maximum allowed wire resistance is:

RwireMAX = RloadMAX - Rload,

Maximum wire length is:

Max Wire length = (RwireMAX/Rper100y) yards. (1)

This is for twisted pair. If you use single conductor, then Rper100y is for one conductor, and result of equation (1) above must be divided by two to obtain maximum distance between PLC I/O card and trnasmitter or whatever load you are using.

Regards,

Sorry, I forgot to login before answering. I am GUEST.

Have a look here:

http://cr4.globalspec.com/thread/18535#comment193058

regards Uwe