Pulsing to circumvent noise?

1 yes. 2 no.

tv remote controls sometimes use an optical burst of band limited signal near 50khz, with a matched filter at the receiving end. A similar plan could help your signaling to reject all except that unique signal.

What does your receiving side look like? If it's noise at the rising edge that's giving you problems, have you tried using a comparator with a lot of hysterisis? Possibly also have the comparator fire a retriggerable one-shot for a belt and suspenders approach.

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Rich Webb     Norfolk, VA

A simple rc network on each line, followed by a Schmitt trigger would do nicely.

A few more details might help. Is 1 us timing accuracy good enough?

What's the noise look like on the signal?

Why is there more noise at the transistions? Do you have ringing? Maybe cross-talk between the numerous signal lines?

How fast is the rise time of the square wave?

Can you ground the cable shield at both ends?

Is the cable shield have braided copper, aluminum foil, or both?

George H.

Typically, in industrial settings, which are horrendously noisy, they use a 4-20 mA current loop with a twisted pair, and even shielded twisted pair isn't out of the question.

Of course, that's assuming that it's not too late to redesign your device.

Good Luck! Rich

Yes, that'd be possible. You could capacitor-couple and the square wave would become alternating positive and negative pulses, then (at the receive end) a Schmitt trigger (NE555 actually is a pretty good one for this application) will regenerate the square wave. This kind of operation (bandwidth limiting the sent signal, then using a receiver that's insensitive in a wide noise-margin range) is common.

The 'short pulse' low-to-high is no more important than the alternate high-to-low pulse, in this scheme. The '555 needs both a SET and a RESET event in each cycle.

Use a twisted balanced pair of wires.. a differential TX and RX circuit or a current loop which will regulate the current to maintain it on the other end. If you are using shielded wire, connect the shield to ground only on one end.

If you want to keep with your current method, then lower the R value of the load on each end to the lowest R and gain the most load the transmitter can handle and use hysteresis type logic on the RX end, like a Schmitt Gate,inverter or comparator with a hysteresis window in it.

Jamie

Thanks for the answers.

The output is from a 339 comparator with a 3K pull-up resistor. It's a quad comparator and each circuit only uses three of them, so there is one that is wasted. Is there any way to take advantage of the extra comparator?

Don

How are you using the comparator? One really must have it set up with positive feedback to achieve enough hysterisis. If it's just wired with a fixed reference versus your input, yes it will go nuts at transitions.

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Rich Webb     Norfolk, VA

I meant the circuit output before the long lines. The comparator is generating the square wave. When the circuit is operated alone, it produces a beautiful, crisp square wave. When I add the other circuits I get crosstalk between signals and noise from the coils.

Don

s .

I've never done any industrail stuff. So 4-20 mA means I get to hang a resistor on the end (impedance of twisited pair is ... 100 ohms?) and measure 400mV as 'zero' and 2 Volts as 'one'?

George H.

I haven't dealt with much cross talk. The brute force approach, is separate shields for each line. I'd try and keep everything grounded if it's possible. (both ends!)

Sometimes it's good to slow the transitions down, less dV/dt. A bit of C on the output of a comparator can slow it down..... but what's the C of 6 feet of twisted pair?

Oh, have you looked at your power supply when everythings on?

George H.

Yes, and the impedance or even resistance of the wires doesn't make any difference because the 4-20 mA supply has voltage compliance, i.e., it will develop however many volts as it takes to drive the current.

It has the added benefit, or maybe just a bonus - if there's 0 current, that indicates "fault."

Some of these might be useful:

Cheers! Rich

Are you using the 339s as your line drivers? If so, not a good plan. They're not really designed to drive long lines in a noisy environment.

Try shifting your interconnects to using differential signaling over twisted pairs. Line drivers/receivers labeled for RS-422/485 are easy to find (and typically multiple-sourced) and would probably help a lot. Common mode interference (a transient that affects both signals) will, for the most part, disappear.

Common CAT5 cable would probably be sufficient for the twisted pair. If you're in an extreme environment, shielded twisted pair may be needed, but I'll bet that just moving to the differential drivers/receivers clears up most, if not all, of the problems.

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Rich Webb     Norfolk, VA

Shouldn't be *too* bad if you drive a current loop and detect with another '339. They are a little weak, though.

They aren't cheap and suck a lot of power. A '339 is less than $.10. An RS422 drivers and receivers go closer to a buck. '339s come in four-packs. Use two (half). ;-) I agree, though. RS-422 is the way to go if the budget allows.

CAT-5 will almost always be good enough. We use it for runs specified up to

1500' (with RS-422 from DC to 1Mb) and have never had a problem reported.

I was going to ask if you were channeling Joerg when I realized he would have opted for a couple of one penny discretes and some fractional-cent passives. ;-)

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Rich Webb     Norfolk, VA

I'm learning. ;-)

To drive a line with that comparator, your best bet is to hit

110 ohms output impedance, with a 220 ohm pullup to +5 and a 330 ohm pulldown to GND. That makes the drive a good match to CAT-5 wiring (or most twisted pair wiring) while maintaining TTL signal levels. To receive the signal, an old MC1489A is suitable, or any logic gate with hysteresis. Best, though, is a differential receiver with hysteresis and good common-mode rejection. Like RS-422 serial receivers.

Twisted pair wiring and shielded-pair wiring and differential drive all reduce pickup from nearby machinery.

What do you guys think about fiber optic lines?

Don