So I am kind of doing something a bit strange it seems, as I have been trying to get help on each piece as each problem comes up. But this is what I am looking at and I might have to change my approach or setup.
That can be okay, but reductionism doesn't take you far in system design. You have to consider the whole of the system, including nearby noise sources, radio transmitters, etc.
Not that you can have perfect knowledge about those sorts of things, but you can make SWAGs. That's really all the regulatory standards are; 3V (conducted) or 3V/m (radiated) susceptibility (typically used for CE marked equipment) is sort of an "average worst" case. It's expected to get worse than that in some conditions, but those are rare enough not to really care. Equipment for tougher conditions (industrial, military, lab, special purpose) will need more (for which there are other standards that can be used, as well).
I am trying to fit the following connections down a 5-wire cable (a 5-din midi cable)
1.) + audio (balanced pair)
2.) - audio (balanced pair)
3.) 24 V dc power
4.) Ground
5.) RX from one microprocessor to the receiving one
Within those
1.) The ground wire is the shield
2.) Cable resistance of each conductor is 3 ohm, capacitance is 1nF
3.) Audio balanced pair produces fundamentals from 20Hz to 3KHz, with prominent harmonics up to 10KHz
4.) 24 V power is a switching supply with noise in the 64Khz range and associated harmonics above up to 2MHz
5.) The receiving end powers 3 independent PWM led drivers that operate at 600KHz, with current pulses at 500Hz which is seen in the 24V line.
Ok, that sounds pretty good. But, are those the frequencies that you're filtering them at (at either end, ideally both), or merely
what you intend to be there?If the two answers are different, you should probably add the filter to fix that.
In addition, the setup I had before had single ended audio and instead had a TWI (SCL/SDA) setup for communication between the microprocessors and communication worked fine, except the single ended audio caught all the noise and I couldn't filter it all out sufficiently.
You'll still have that problem with the uncontrolled impedances in the cable, unless you've very specifically obtained a cable that has the pair twisted independent from the rest.
Controlling slew rate and filtering the signals will be the biggest help. If you didn't have any filtering, and the audio signal was high impedance, and the signal path included bipolar op-amps or something like that, all the high frequency noise will be detected by the amps and converted to audio.
Using a
smaller digital signal level will obviously help, too. Naked RS-422 wouldn't help you much here due to the high slew rate, but some RLC filtering can knock that down to quite reasonable levels. And then you won't need to run as slow as 9600, though you still can.
The main downside is you'd need to add a sixth pin to the cable and connectors...
(Or you can do something quirky like multiplex it with the supply pin, which is possible with, say, Manchester encoded data and a transformer -- ala PoE -- but now we're talking lots of extra hardware.)
Tim