Wanting to send/receive data over DC power pair

[jonnyGURU]

Hard to categorize this one. It's power related... umm... not so much.

I'm wanting to do something that seems simple on the surface, but I can't figure out why its so dang hard to find information on when I'm looking for it. I don't know what IC to use, what kind of components... I don't know where to even start....

It's not hard to find information on data over AC (Home PNA, for example). But what I'm trying to do is send serial data (like to/from a USB device) over a +5VDC pair. My conduit actually handles two voltages (+19VDC and +5VDC as well as data), so if I can't figure out how to do data over DC power, I'm going to end up with five conductors instead of three.

So now I'm going to explain why I want this because I know the first post is going to be "what's the big deal about adding one or two more 32awg wires, blah blah blah..."

The issue I'm trying to address is not the wire, but the connection. What I'm wanting to avoid is having a 5-pin connection between the two devices. The connection is modular and I'm wanting the simplest connector on each end. Otherwise, I could easily use a mini-DIN (like an S-Video has) which has four pins and the shield is ground, but the issue I'm trying to address is how easy/difficult the connector is to plug in. I'm sure all of you are familiar with having to orient a mini-DIN correctly to get it to plug in. Pain in the ass. I've found that aviation headsets use a 5-conductor jack, but those jacks are a good 1/4" diameter!

So back to what I want to accomplish.... serial data over DC. Any ideas? I know it can be done... take digital desksets, for example. There's enough voltage to power a logic board and backlight an LCD and amplify audio in the earpiece, but there's also data going through the same pair of wires. And DSL... there you have an analog signal, a digital signal AND DC voltage going down the same pair of wires.

[jonnyGURU]

Re: Wanting to send/receive data over DC power pair

Oh.. here some important variables that might make or break this project....

I need to be able to supply 10 to 15W of power (5VDC @ 2 to 3A). And the data could be as high as 3 to 5 Gb per second. Think USB 3.0. Essentially, I'm trying to make a powered USB hub that can provide its own power when plugged into the wall, or pull power from the PC when the hub is not plugged in.

[Th3_uN1Qu3]

Re: Wanting to send/receive data over DC power pair

Power over AC is possible because the difference between the power voltage and the signaling voltage is large - the AC powered device doesn't give a darn about that TTL signal appearing on the line. On the other hand, sending 3.3v level data on a 5v line isn't going to end up too pretty for the device being powered by that 5v line. Same goes for DSL - remember, the line must still provide enough voltage to ring those old phones that use bells.

Your best bet is to use the 19v line and AC couple the desired signal at both ends. I'm quite sure it can't be done with 100% off-the-shelf ICs, you're going to need a few extra transistors and capacitors.

[kikkoman]

Re: Wanting to send/receive data over DC power pair

oh, I see. I thought you were talking about a few kbauds.
at those speeds though... good luck with that. there's a reason why USB3.0 uses TWO differential pairs and a separate shielding.
and even the current generation powerline modems do 'only' about 200 Mbps.

[Th3_uN1Qu3]

Re: Wanting to send/receive data over DC power pair

I'm trying to make a powered USB hub that can provide its own power when plugged into the wall, or pull power from the PC when the hub is not plugged in.

USB 2.0 and earlier uses four wires. USB 3.0 uses 9 wires. Where is the 2 wires thing coming from? Besides, like kikkoman said, USB uses differential signaling which needs a minimum of 3 wires. USB 2.0 could probably be made to work on three. Higher, i don't think so. And certainly not on just two... unless it's for like a meter or less.

[b700029]

Re: Wanting to send/receive data over DC power pair

Look at how Power Over Ethernet (PoE) works.

[jonnyGURU]

Re: Wanting to send/receive data over DC power pair

USB 2.0 and earlier uses four wires. USB 3.0 uses 9 wires. Where is the 2 wires thing coming from? Besides, like kikkoman said, USB uses differential signaling which needs a minimum of 3 wires. USB 2.0 could probably be made to work on three. Higher, i don't think so. And certainly not on just two... unless it's for like a meter or less.

I know USB 2.0 uses four wires, but two of those wires are DC power and ground: +5VDC, Ground, data + and data -. USB 2.0 operates with two wires if you remotely power the device. For example: When we did Phobos at BFG, the iPod dock on top was only wired up to the USB controller with two wires, D+ and D-. The port's power came from the +5VSB of the PSU. This allowed the docked iPod or iPhone to continue getting charged even when the PC is "off" and in stand by.

I was not aware USB 3.0 used 9 wires. In fact, I'm not sure what USB 3.0 application uses 9 wires, but I'll take your word for it. I have a USB 3.0 add on card and a USB 3.0 drive and it still uses the standard 4-pin USB connector (+5V, Ground, D+, D-) and speeds are easily double USB 2.0 speeds.

But yeah... if there's going to be signal issues at those speeds, I might be barking up the wrong tree. Good news is, the +19V goes one way and the +5V goes the other way so I think I figured out a way to use one wire for both of those. At least I'm down to four conductors. At least that's a smaller connector than five! LOL!

Look at how Power Over Ethernet (PoE) works.

PoE, IIRC, uses at unused pair. You have a pair for TX and a pair for RX, which leaves two more pair for DC+ and - if you're not also running telecom on the same cable. When I was an old Avaya hack, it was blue for deskset, green and orange for TX and RX and brown for PoE.

[jonnyGURU]

Re: Wanting to send/receive data over DC power pair

Ok... I looked at the USB 3.0 pin out and it looks like it uses the standard +5V, gnd, D+, D- like USB 2.0, and better speeds can be accomplished due to the better controller, but there's also the differential pins, 2 x RX and 2 x TX to obtain even better speeds. So it certainly seems like full USB 3.0 speds are out of the question.

And certainly, since the data isn't going to be shielded from power, I'm only going to see 1.5Mps speeds at the highest, which is pretty pathetic.

That sort of speed sacrafice isn't going to be worth it. That, and it seems like the components required to make this work are going to take up more space than the devices I'm trying to relocate!

Arrgh... oh well.

[MixMasta]

Re: Wanting to send/receive data over DC power pair

PoE, IIRC, uses at unused pair. You have a pair for TX and a pair for RX, which leaves two more pair for DC+ and - if you're not also running telecom on the same cable. When I was an old Avaya hack, it was blue for deskset, green and orange for TX and RX and brown for PoE.

Except this is only one mode. The other mode is phantom power mode, which works with data pairs, and of course supports 1000base-tx which as you surely know uses all the pairs in a cat5 cable. But the reason it works is because Ethernet signals are ac and the power injectors are supplying dc power.

[jonnyGURU]

Re: Wanting to send/receive data over DC power pair

But I've never used that, nor do I have an immediate use for it so.....

I think I'm stuck in the mud on this one. Might have to count my blessings with four conductors and USB 2.0 speeds.

I used to think DC power was superior, but now I'm wishing for some more AC in my life. Oh well!

[kikkoman]

Re: Wanting to send/receive data over DC power pair

Except this is only one mode. The other mode is phantom power mode, which works with data pairs, and of course supports 1000base-tx which as you surely know uses all the pairs in a cat5 cable. But the reason it works is because Ethernet signals are ac and the power injectors are supplying dc power.

they aren't exactly AC, they're a differential signal.
it isn't any more difficult to impose a signal on a DC power line (to the contrary I believe) but you can't get away without the differential signaling, twisted pairs and separate shielding for each pair at those speeds.

btw, USB3.0 uses two pairs for 3.0 signals and one pair for 2.0 signals (for backwards compatibility I guess). hence the number of wires.

[kikkoman]

Re: Wanting to send/receive data over DC power pair

of course you could lose the +5V line and step the 19V down to 5V in each device but I doubt it's worth it over one wire less.