Whats the mininum # of connections to com. between 2 devices with seperate power - Page 1
Forum section: electronics / beginners

author: Beamin
author_rank: Super Contributor
timestamp: November 15, 2020, 04:11:41 am
content: If you have two devices that you want to communicate to what is the minimum number of wires needed to connect them, that have two different power supplies ie dont share a common ground?Three- tx, rx, and clockfour tx, rx, clk, gndtwo tx/rx same line , clockOnewith clock signal being sent with alternating data tx and rx between clock signals.Could you do this with fiber having just one line?

author: ataradov
author_rank: Super Contributor
timestamp: November 15, 2020, 04:36:11 am
content: Depends on the speed. For relatively low speed communications two is enough - ground and data. Use bidirectional half-duplex UART for the protocol.1Wire interface manages to do data and power over two wires.Of course you can use fiber. Or infra red communication and no wires at all.

author: I wanted a rude username
author_rank: Frequent Contributor
timestamp: November 15, 2020, 09:22:59 am
content: You could use a single fibre if you had a beamsplitter at each end. This would be a custom solution though ... not an off-the-shelf SFP+ module.Edit: wow, I've really got out of touch with that world.Sticking to (electrically) wired solutions, how about a half-duplex differential pair without ground? Not sure what kind of bias differential receivers can tolerate in practice.

author: pwlps
author_rank: Frequent Contributor
timestamp: November 15, 2020, 10:50:19 am
content: Quote from: I wanted a rude username on November 15, 2020, 09:22:59 amWith Manchester-style coding you can use ac coupling, then no dc-offset problem.

author: Yansi
author_rank: Super Contributor
timestamp: November 15, 2020, 10:55:08 am
content: Quote from: I wanted a rude username on November 15, 2020, 09:22:59 amWhat? Single-fiber transceivers SFPs are pretty much off-the-shelf standard. But there ain't just no regular beam splitter, it is a WDM (wavelength division multiplex).

author: mikerj
author_rank: Super Contributor
timestamp: November 15, 2020, 12:34:14 pm
content: Two wires can give you half duplex comms that can work across devices with significantly different ground potentials. Look at isolated CAN bus for example.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: November 15, 2020, 04:24:26 pm
content: Your TV and Remote control do this. Yes some of them communicate both ways.

author: Microdoser
author_rank: Frequent Contributor
timestamp: November 15, 2020, 05:25:04 pm
content: Quote from: JustMeHere on November 15, 2020, 04:24:26 pmZero then

author: mariush
author_rank: Super Contributor
timestamp: November 15, 2020, 06:37:09 pm
content: two wires is enough, one for clock and one for data.Think of how TI86 calculators or Nintendo Gameboy used to communicateYou could also do optical stuff, like two toslink cables between devices (one receive sensor and one transmit led on each device) and two toslink cables. (or make your own custom connector with both receiver and transmitter)

author: mariush
author_rank: Super Contributor
timestamp: November 15, 2020, 06:39:58 pm
content: Also see Ben Eater's videos which are quite informative :(in the first video he's using two wires between two arduino boards to send data)

author: Yansi
author_rank: Super Contributor
timestamp: November 15, 2020, 06:55:36 pm
content: A lot of times, what is not being given enough importance, is that it is not enough to have just the differential pair itself.A real world receiver, does not have infinite input common mode range, nor infinite HF common mode rejection.So, unless the interfaces are extremely well balanced and isolated/floating (which is quite rare and difficult to achieve tbh), a third reference ground wire isrequiredfor it to reliably function, even for isolated interfaces - their isolated grounds should be tied together, so that the common mode signal stays within limits of the receiver!I always hate seeing cheap RS485 or CAN bus shit made with just two wire terminals.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: November 15, 2020, 07:08:07 pm
content: You can actually do full duplex with just two wires.0v = no one transmitted5v = one side transmitted10v = both sides transmittedFor the 5v part, one side knows it's not transmittingNot sure how to do the 10v part electronically, but I'm sure its possible.Ofcourse voltages don't have to be 5 and 10.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: November 15, 2020, 07:22:22 pm
content: Quote from: JustMeHere on November 15, 2020, 07:08:07 pmI guess +5, 0, -5 would work too.One side gets + and the other gets -.If the voltage is 0 and you're transmitting then the other side did too. The other states are straight forward.

author: AndyC_772
author_rank: Super Contributor
timestamp: November 15, 2020, 07:45:08 pm
content: It might be worth looking up how a conventional POTS (plain old telephone service) line works. Two wires provide power to the phone and simultaneous communication in both directions. Search terms you may find relevant include "2 to 4 wire hybrid", "longitudinal balance", "metallic balance" and "side tone".

author: chris_leyson
author_rank: Super Contributor
timestamp: November 15, 2020, 07:47:29 pm
content: @Beamin Two wires for asynchronous full duplex e.g. Serial, two wires for synchronous full duplex e.g. Manchester or modified Manchester or a single wire for half duplex and it's up to you how you translate between the two different grounds. You could of course do it all in the frequency domain with filters and diplexers and then you would only need one wire or no wires if you capacitively on inductively couple. You need to add some context to the question.@AndyC_722 Had forgotten about POTS hybrid transformers, and you get galvanic isolation, thank you.

author: Beamin
author_rank: Super Contributor
timestamp: November 15, 2020, 08:38:04 pm
content: I guess Im trying to figure out (not building a project just building knowledge) the holy grail which would be one wire connecting two different devices over a distance potentially long with both devices being able to tx and rx to each other.Could you do one wire and have two different ac signals or four that handle tx and rx much like radio? Have unit one tx at 100mhz rx at 110mhz and unit two transmit over 200 mhz and rx at 210 mhz? How would you handle the clock would you need a clock wireless devices certainly dont have a clock signal. Or could you send the clock at its own 5th frequency?I watched ben eaters video and was trying to figure out how to do things with isolated grounds and no clock. He uses basically three wires a tx/rx wire a clock wire and a common ground (virtual 3rd wire).How does my cable modem send over just one conductor, or is it two with the ground shield acting as the common ground? Is the inner conductor basically like an antenna ie sending out a radio signal just connecting a wire instead of RF ?

author: ataradov
author_rank: Super Contributor
timestamp: November 15, 2020, 08:40:36 pm
content: Quote from: Beamin on November 15, 2020, 08:38:04 pmThe shield is the common ground. The signal on the central conductor is referenced to the shield.The only reason that cable is coaxial is to preserve decent RF properties. Assuming your microcontrollers communicate at much slower frequencies, you can just use two wires.But modems do a lot of DSP. which is probably not practical for general purpose MCUs, especially given there are plenty of other ways to do the same thing.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 15, 2020, 09:09:56 pm
content: Why don't you use an isolated RS-422 full duplex? For its implementation, there are inexpensive chips with integrated isolated power supply and ESD protection. For example: ADM2682E (16Mbit) or ADM2687E (500kbit). There are similar transceivers for RS-232. You only need a few capacitors and couple resistors. For full duplex, use 4 wires (2 twisted pairs) and you will have an end-to-end UART-TTL channel, for half duplex, you can use 2 wires (one pair), but the Protocol for accessing the line will be need (turn the transmitter on and off).

author: mariush
author_rank: Super Contributor
timestamp: November 15, 2020, 09:28:52 pm
content: When I read your message, my mind went to ocean cables and the first cables used for telegraphs where they use earth as return and just some high voltage to send pulses. Seehttps://en.wikipedia.org/wiki/Earth-return_telegraphI guess you could boost your voltage to some high value to account for voltage drop in the long cable and then maybe have 2 or 3 voltages... ex 100 volts clock low , 150v clock high - data 0, 200 v - clock high, data is 1 and you make it a rule that voltage constantly flips between 100 and 150v or 200vYou'll need an opamp or something to reduce the voltage to safe levels and give your device the 0s and 1s...For short distances, a single cheap plastic fiber could be used... check Toshiba's offerings in this pdf, see pages 6...8 in particular :https://www.tme.eu/Document/3363e65f4c705941469014401686faf2/TOFC100-xx.pdf

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 15, 2020, 09:55:09 pm
content: Quote from: mariush on November 15, 2020, 09:28:52 pmWow, you have a lot of voltage.How and with which transients will you charge and discharge the transmission line with non-harmonic wave?