Re: [CAD_CAM_EDM_DRO] Twisted Pairs

> Hi,
>
> I was talking to someone yesterday who was using a 10' twisted pair,
> sheilded cable for his encoder. He had used one pair for channels A
> and B, the other for +5VDC and ground. He called to say things
> weren't working very well.
>
> I suggested one pair be channel A and +5VDC, the other channel B and
> ground. He rewired it and called back to say everything was now
> running very well.
>
> The moral of the story is only one wire of a twisted pair should
> carry a signal; the other wire should be ground or supply. Otherwise
> cross-talk will get you.

Pairing signal and power lines can result in coupling signal onto the
power supply, which could also cause problems.

I'd much rather see 3 pairs used:

One for power and ground
One for channel A signal and connected to ground at the encoder
One for channel B signal and ground at the encoder

The extra grounds can either be left floating at the receiver end, or
in noisy environments used as the -signal inputs to differential line
receiver. Differential signalling is mostly a function of the
receiver, not the transmitter, so you can get a lot of the benefit
with a single-ended encoder wired in this way.

> This is true also if twisted pairs are used for step and direction,
> limit switches or any other I/O use. It can be the cause of many
> mysterious malfunctions.

I'd agree, with the possible exception of limit switches. These tend
to be low frequency signals, and a pairing them with eachother or a
power rail should not be a problem.

Chris

--
Christopher C. Stratton, stratton@...
Instrument Maker, Horn Player & Engineer
22 Adrian Street, Somerville, MA 02143
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