Digital signal + power supply through network cable - Page 1
Forum section: electronics / beginners

author: Chaos_Klaus
author_rank: Contributor
timestamp: January 10, 2016, 01:47:08 pm
content: Hi Forum,I want to connect two devices via a single cable. The cable would have to carry a symmetric power supply (+-9V at least, about 200mA) and a digital signal at 12.288MBit/s. It's an ADAT signal and the clocks are derived from the signal itself by the receiver. So it sould only take a single data line.I was thinking about using a network cable with "RJ45" (=8P8C) connectors. Connection will be no more then 10m.Now I have a few questions:1) Can I have the powerlines and the digital signal in the same cable like that?2) If so, how should I make use of the twisted pairs in the cable. Is it necessary to use a cable where the pairs are shielded against each other? Like U/STP or U/FTP?3) If I use a any of those cables with shielding, I guess I need a jack that has shielding aswell?4) Is it better to have the voltage regulators for the power supply before or after the cable. I guess the voltage would be more stable when the regulators are in the receiving device, but then I'd have power rails with considerable ripple in the cable.5) Do I need some kind of driving stage to transmit the digital signal or can I just go straight out of an optical reciever into that cable and place a buffer opamp on the recieving end? (That might be a really stupid question.)6) CAT3 goes up to 16Mhz. So that should be fine for my 12.288MHz signal, right? But since nobody uses CAT3 network cables anymore, maybe it is cheaper to use a more common cable such as CAT5 anyway?Maybe I's a stupid idea and I should use another cable all together. Are there alternatives?Thank you for your time.Cheers!

author: Mr.Elendig
author_rank: Newbie
timestamp: January 12, 2016, 12:20:36 am
content: Quote from: Chaos_Klaus on January 10, 2016, 01:47:08 pmyesuse standard unshielded solid or stranded copper(donotuse cca) cat5e or cat6. If you expect it to move, use stranded. Unshielded is preferred unless you are in a really noisy environment due to better electrical characteristicsAt least at one end, and actually connected to earth, if you want the shield to actually be useful.depends on how important the voltage level is, local regulation lets you basically ignore the voltage drop in the cableyour input and output stages should match the cable used, specially since cable lengths tend to increase well beyond what you first imagined :pI would use cat5e/cat6, almost as cheap, easier to find, larger diameter conductors, mechanically much more robust, easier to work with, much better electrical performance, more uniform quality. The other alternative is some form of signalling cable, but they are often just as expensive as cat5e, might not be standardized between manufacturers and so on.Thishas a nice quick overview over the challenges of doing 1-wire transmissions.For the power, you can push 0.5A trough a single cat5e/6 conductor, but due to voltage drop it is often common to eg use one pair for positive and one for negative, or take the common colour from two pairs as negative, and the two distinct colours for positive. The later also allowing you to easily use 3 pairs for power and one for signal.

author: Brumby
author_rank: Unknown
timestamp: January 12, 2016, 12:26:46 am
content: To reduce ripple in the cable, I'd put some decent filtering at the source end and have your regulation at the receiving end, with decoupling for any rubbish picked up along the way.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 12, 2016, 12:54:43 am
content: Quote from: Chaos_Klaus on January 10, 2016, 01:47:08 pmYes, should be no problem at all.QuoteOne pair for the ADAT signal (made into DIFFERENTIAL, of course!)One pair (BOTH wires together) for ground.One pair (BOTH wires together) for + voltageOne pair (BOTH wires together) for - voltage.DO NOT put +V and -V on two wires of the same pair.you run the risk of blowing up something if somebody plugs the wire into the wrong place.I have a bricked $700 streaming box that suffered that exact failure mode.Completely avoidable with proper design.QuoteThe properly differential data pair should not need shielding any more than the millions of meters of UTP network wiring around the planet.The power pairs should not need shielding as they can be very trivially filtered for any ambient grunge they may pick up.QuoteIf you use shielded network cable, then the male plugs on the cable AND the female jacks they plug into must all be metal/shielded.Using an ordinary plastic connector at any of the four points in the chain completely obviates any benefit of using shielded cable.QuoteIt is better to have the regulators at the far end (destination/load) of the cable.It is not AT ALL clear how that equates to "considerable ripple in the cable"QuoteOr at least unanswerable since we don't know (signal-wise) where you are coming from and where you are going.As a generic response, I would say that it seems very unlikely that you have a proper differential signal available that can drive a long cable, and equally unlikely that you have a proper differential receiver at the other end. But since you have revealed nothing about either the source or destination circuits, we can't even really guess.QuoteNot sure where you can even find Cat3 cable anymore. Cat5 is ubiquituous and has perhaps the highest bandwidth/cost ratio on the planet.QuoteNo I think it is an excellent idea. I was thinking of making a cheap TOSLINK to Cat5 adapter set (transmitter and receiver) just for running ADAT over long distances. Like a cheap, budget digital snake with 1~4 Behringer ADA8000 at each end.But it is not at all clear to us what you are proposing down at the far end of the cable that needs +/- 9V? So we don't know how to answer your power questions.

author: Chaos_Klaus
author_rank: Contributor
timestamp: January 22, 2016, 09:47:10 am
content: Hey,sorry for the delay. Thanks for your answers.All right, unshielded CAT5 it is.The application is something likethis. It's a personal monitor mixer intended for use in my studio. This obviously already exists but is hard to get where I live and it's bulky ... and I want to do it myself ... I know it's not cheaper, but I learn a lot designing it.It features a central hub with power supply, TOSLINK ADAT input. From here I want to run that ADAT signal through maybe 8 network cables to 8 devices. Those have an ADAT Decoder, DACs, a simple analog mixing section and a headphone amp.The idea is to give the musician as few knobs as possible to not distract him/her from performing. Many of the newer products do this completely wrong, I think. No musician wants to mix 16 channels with EQ. I want a relatively small device and the least amout of cables.QuoteThat is good advice. Thanks!QuoteYes. True. It obviously depends on the filtering of the supply rails. I was more concerned about other audio equipment in the vicinity picking up hum if I run these unshielded powerlines through my studio. But actually, most of the audio connections are symmetric (apart from guitar amps, ect. ). Hum from the v+ and v- rails should cancel out, anyways.QuoteI think differential pair is in order and both source (optical reciever) and receiver are single ended.On the transmitting side, I'd use two opamps. One as a buffer and one as an inverting amplifier to get the inverted signal.On the receiving side, I could use an opamp as a differential amplifier. Probably need a buffer before the inverting input to get higher input impedance, right?Can't I just use a comparator for the receicer? Sorry, this is probably basic. But I've never done this before.What opamp would be suitable? Signalclock is 12.288MHz.QuoteI think I saw something like that somewhere. :]Cheers!

author: SL4P
author_rank: Super Contributor
timestamp: January 22, 2016, 10:20:07 am
content: Quote from: Richard Crowley on January 12, 2016, 12:54:43 amNow there's a topic in itself !Re-use / misuse of standard connectors and cable pin-outs by equipment designers - without clear, alarming labels!!