Digital Communication Via VCC and GND - Page 1
Forum section: electronics / beginners

author: NickyYTSRB
author_rank: Newbie
timestamp: September 18, 2018, 02:55:21 pm
content: How can I establish digital communication between microcontrollers like powerline but on smaller voltage, for example: I have 2 wires that will caryy 24VDC as supply voltage and data. On transmitting side data has to be injected into 24VDC line and on receiving side data has to be extracted and that same 24VDC must power the ic which has to read that data. My goal is to establish communication between 3.3V or 5V mcu with one wire protocol(One way UART for example).I found this on internet:Is this enough or i have to add more stuff to this circuit. My goal is UART at 112.5kbit.

author: Paul Rose
author_rank: Regular Contributor
timestamp: September 18, 2018, 03:15:22 pm
content: The idea is sound, you may need to play with component values.Figure the lowest/highest frequency component of you signal and compute the inductive and capacative reactances of your L and C components.You want a larger inductor ( higher XL ) and larger capacitor ( lower XC ) for lower frequency signals.You want to pay attention to the DC resistance and current handling of you inductors to make sure you can supply the power for the DC load.Look up "bias tee" for ideas, but these are ususally for higher frequency signals ( RF ).

author: Paul Rose
author_rank: Regular Contributor
timestamp: September 18, 2018, 03:49:12 pm
content: You also might need to do something to re-establish DC level on the receive side signal output. The signal will appear to vary about 0v, which might not be what your UART expects.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 18, 2018, 03:54:50 pm
content: EIA232 is not DC free, so that's not going to work.If you want to stay in the baseband, you need at least a DC free coding; much better would be a modulated transmission

author: Paul Rose
author_rank: Regular Contributor
timestamp: September 18, 2018, 04:30:58 pm
content: Quote from: HB9EVI on September 18, 2018, 03:54:50 pmOf courseSorry about that.The "lowest frequency" component of 232 is esentially DC, which you can't separate from your power.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 18, 2018, 06:30:52 pm
content: Then show me the bias tee which will has the right properties for 112k uart signal

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 19, 2018, 04:34:44 am
content: Also, use balanced signalling for EMI reduction (downside: needs twice as many components), or route your power+data through coax.Direct serial would seem unfriendly, given the low cutoff frequency required. You'd probably choose a DC balanced line coding, like Manchester.Which, in turn, would probably be easiest to generate with an SPI channel at twice the data rate. That is, in software, encode the input data (e.g., 8 bit bytes) into two bytes with Manchester coding.A modulated (ASK, FSK, BPSK, QAM...) carrier may be better, and certainly easier to filter (both for transmit, receive and EMI purposes). Downside, the circuit gets more complex, and you'd want to look for an integrated solution instead, but I don't know offhand what would cover that...Tim

author: ogden
author_rank: Super Contributor
timestamp: September 19, 2018, 06:05:34 am
content: Quote from: NickyYTSRB on September 18, 2018, 02:55:21 pm24VDC is still power line. Modems used are the same, just coupling and protection circuits can be simpler than those used on AC mains PLC. Appnote about DC PLC:http://www.ti.com/lit/ug/tidu160/tidu160.pdfOne of many PLC IC's you can use:https://www.nxp.com/products/analog/interfaces/other-interfaces/home-automation-modem:TDA5051AT

author: radioactive
author_rank: Regular Contributor
timestamp: September 19, 2018, 08:14:51 am
content: I found this idea interesting... been playing around with it in LTSpice. I think it might work without modulation. I added some dc-offset tracking and comparator. Changed the coupling caps to 33nF for 115k operation. The receive side continues to look good with around 30dB of loss. You might be able to just add some basic error correction. Thoughts?

author: ogden
author_rank: Super Contributor
timestamp: September 19, 2018, 08:35:43 am
content: Quote from: radioactive on September 19, 2018, 08:14:51 amAre you absolutely sure there will be no noise on that bus? No switching regulator, no transients no nothing - pristine DC?

author: radioactive
author_rank: Regular Contributor
timestamp: September 19, 2018, 11:07:32 am
content: Quote from: ogden on September 19, 2018, 08:35:43 amGood point. I wasn't too concerned about the 24VDC supply since it is filtered quite good. Updated the sim with 2V @ 280KHz noise on the 24VDC supply (gets filtered out well), added 200mV noise @ 1 MHz (also tested at 200 KHz) to both the 3v3 supply to the comparator and the TX side to simulate some pretty insane ripple from supplies. Still looking good. Should be a pretty good start in any case.

author: tggzzz
author_rank: Super Contributor
timestamp: September 19, 2018, 11:33:00 am
content: If the wires are long, and particularly if they are outside, don't forget to consider the non-communication aspects of such as system.This is not a complete list, but think of different mains phases, different potential between ground at each end of the wire (especially w.r.t. lightning strikes), EMI and EMC.......

author: ogden
author_rank: Super Contributor
timestamp: September 19, 2018, 12:30:40 pm
content: Quote from: radioactive on September 19, 2018, 11:07:32 amSeems like good start - if simplicity is paramount to stability. FEC can give big improvement. It does not cost much - just codingOne of options:http://www.ti.com/lit/an/swra113a/swra113a.pdfIf you are interested - TI made their CC-radios FEC source code public. Just search for it.In case you see that unmodulated transmission does not work, try following. Obviously replace IR diodes with proper coupling to your DC bus):https://www.maximintegrated.com/en/app-notes/index.mvp/id/4622 Many microcontrollers have IR modulators built-in.