Instrument cable

Hello,
I have a question regarding instrument cable. I know that for 4-20ma transmitters twisted pair cable should be used with the screen grounded at one end only. However for digital signals ( limit switches , push buttons ) can 2 core/ part of a multi core cable be used ?

Thanks

Yes.

Thanks for this. Am I correct in that noise will not impact a digital signal as much as an analogue and that is why cores can be used instead of twisted pairs.

Thanks again

Lc1 said:

Thanks for this. Am I correct in that noise will not impact a digital signal as much as an analogue and that is why cores can be used instead of twisted pairs.

Thanks again

Click to expand...

Of course, one can always find an exception to the rule, but generally speaking, that is correct. I little bit of electrical noise will not typically cause a false reading on a discrete input.

I would say that you often dont need twisted pairs

This is one of those "it really depends..." kinda answers.

In a reasonably low noise environment we do this often.. one 24V supply out to an instrument with multiple digitals, the inputs come back on various cores in the multicore. No problems.

But in a very noisy environment, you want to take advantage of the common mode cancelation provided by a twisted pair. Squirting a lot of noise back into your DC PSU isn't ideal. If your cable has to run close to large cables from noise generating things like VSDs, motors, switchmode power supplies etc... then use one twisted pair per digital.

If you use individually screened pairs then i have no issue with mixed analogs and digitals in the same cable.

Thanks for all the replies.

If you have vfd's be sure to use the correct cable for the vfd wiring. line and load filters may be required. we ran into this issue several years ago.
james

consider cable routing also so 4-20 signals are kept away from vfd and motor cables