connecting 8ft of cable to digital logic (OUCH)

You should put the opto at the 74hc193 end of the wire, the series resistor at the pic end, that way you have a good chance of success.

Something else I am curious about is: lets say I connect the GND wire as you say, so basically the PIC's GND has a 12 foot long piece of wire attached to it. Could this somehow couple noise to the PIC's GND and cause problems? I can see that your method prevents the COUNTER GND from seeing noise, since the opto is on its side.

Is optical isolation absolutely necessary? If not, use RS232 for this as it is designed to transmit digital signals up to 100m distance at up to 115kHz. The common MAX232 will do the job nicely. You could also use RS485 which is designed for cable runs of 1km or more. The MAX481 is even easier to use as you won't need external caps.

If you must do optical isolation, look around the net for opto-isolated RS232 schematics/products.

RS232 and RS485 solves the noise problem differently. RS232 simply transmits at a high voltage (+/-12V) to drown out noise. RS485 uses a low voltage differential scheme in which the noise on + signal and - signal cancel each other out. RS485 should be used with twisted pair wires for best result.

Move the optocoupler to the receiving end and you have a system that resembles the current loops from the 70's used to connect terminals across several 100 feet of wire to mainframes, running at 9600 or 19200 baud. It's called "current loop" and is very insensitive to noise.

Meindert

That doesn't seem to be much worse than any other audio signal on speaker wire. Speaking from the depths of my ignorance, what application would be so sensitive to something so seemingly small and uncomplicated?

Agreed.

Doing it the OP's way results in a high-Z condition when the opto is 'off' and the 'wire' may be subject to any manner of interfering signals that it can't reject.

Graham

Very unlikely.

It's about keeping the impedance low on the cable actually.

Graham

Perhaps drive push-pull, like this.

5v +--[R]-+-[R]--+0v Remote V+ | | | +---+ | +--[C]-+-[C]--+ | _|_ |/ | | \\_/ -|Opto1 +--------------+ | |\\ Line | + | PIC->--[R2]-------------------+ +-->4KHz sq wave. | + | | _|_ |/ | \\_/ -|Opto2 | | |\\ +---+ | Remote 0v

This keeps the line low-impedance all the time, with only a +/- V-Led transitions on it.. bearing in mind that 12 ft of untwisted cabling is a transmitting aerial as well.

The two [C]'s deliver the Led current and the [R]'s need only be low enough to keep the centre tap at near Vcc/2 if the sq wave is not 1:1. Led current is 2.5V/R2.

--
Tony Williams.

Put the optoisolator at the destination end.

Good Luck! Rich

And if you can't twist them, at least twirl the plug a couple of times.

Good Luck! Rich

No, that's what the optical isolation is for.

Lessee if I remember - that's the one providing the signal, right? Since it's an output, it shouldn't hurt, as long as you've followed all of the standard grounding practices.

Have Fun! Rich