finding signal direction on a bidirectional wire - Page 1
Forum section: electronics / projects

author: bernie79
author_rank: Contributor
timestamp: November 15, 2021, 02:27:06 am
content: Hi,I'm not that expert in OPV-circuits, so I'm asking for advise.I try to sense the signal direction on a bidirectional used wire. I cannot open the master nor the slave of this connection.My idea: I put a resistor between master and slave and compare the voltages on either side. The transmitter surely has a higher voltage as the receiver. I'm not interested in the absolute voltage value or the measured current, only the direction of the signal is imporant. The signal will be sampled in another circuit. Btw some values: the signal mark and space can reach +-5Volts over ground, the driven current can be up to 30mA.But the more I search for the right OPV model, the more I'm lost. The OPV have to resist "blue smoking" if the measured voltages goes negative. I have tried to put all this in a schematic. My first choice was LM339comparator, but this doesnt will work because the LM339 doesnt like voltages less than the negative voltage.GreetingsBernie

author: Whales
author_rank: Super Contributor
timestamp: November 15, 2021, 02:45:35 am
content: There are ways of separating signals based off the direction they come from. Old-fashioned phones do this, to avoid you hearing your own voice when it gets send over the single phone line.I can't remember the terminology, sorryJust tried looking it up and couldn't find it.EDIT:hybrid telephone coil? Theexplanations seem a bit wordybut there are probably simpler ones.EDIT2:Hybrid coil!That's it.

author: bob91343
author_rank: Super Contributor
timestamp: November 15, 2021, 02:47:06 am
content: OPV?

author: Whales
author_rank: Super Contributor
timestamp: November 15, 2021, 02:58:30 am
content: Quote from: bernie79 on November 15, 2021, 02:27:06 amWhat negative voltage? Do your signals go below GND?Your 1K resistors might be enough to limit the effects of this. Depending on the frequencies you are listening to: it might be worth increasing these resistors to 10K.

author: fourfathom
author_rank: Super Contributor
timestamp: November 15, 2021, 03:06:52 am
content: Or just add a DC offset in the input section. Here's one that will allow an the inputs to range between roughly +7V and -2V. Replace the two 100-Ohm divider resistors with an opamp if you don't like the 25mA drain. This circuit will divide the input signal by two, so not knowing the current you are trying to sense I don't know if that's going to be a problem for you.Is there some reason you need the complimentary outputs? Without a deadband the no-signal outputs aren't going to be well-defined, just a function of opamp input offset.

author: bernie79
author_rank: Contributor
timestamp: November 15, 2021, 03:49:07 am
content: @bob81343: yes sorry, its the common abbreviation in my language. In english, its OPA, operational amplifier or OpAmp.@Whales: yes, I know what you mean.the transformer in the right lower corner...Unfortunately, it doesnt work in this case.It can be negative, because (In+) or (In-) can be more negative than ground in some cases (if the measured signal gets the same ground as the sample circuit as the signal value sample part - not shown here). Its a RS485 signal with +1, -1 and 0 (from the view of R1), but its up to 1MHz.@fourfathom:This is a good idea. The drain is something I cannot do, but using another opamp is something I haven't thought of.Yes there is a deadband.Looks like I have to warm up the soldering iron and play around

author: fourfathom
author_rank: Super Contributor
timestamp: November 15, 2021, 06:56:30 am
content: Quote from: bernie79 on November 15, 2021, 03:49:07 amThere isn't any deadband in the comparator circuit you posted, or in mine. When no current is flowing either comparator output can be high or low, depending on the input offset current and offset voltage. Deadband can be implemented with a couple more resistors though.What will you be doing with the comparator outputs? Might you be able to use a simple differential amplifier circuit that goes positive or negative proportional to the signal current value and direction?

author: MegaVolt
author_rank: Frequent Contributor
timestamp: November 15, 2021, 10:02:50 am
content: A bi-directional current monitor chip?https://www.ti.com/amplifier-circuit/current-sense/analog-output/products.html#p2192=Bi-directionalBut there are problems. For example, if we use an open collector, the current flows to the output although the signal is transmitted in the opposite direction.

author: nali
author_rank: Frequent Contributor
timestamp: November 15, 2021, 10:31:14 am
content: Do you mean the AC characteristics of the signal is up to 1MHz or the data rate is 1Mbit?Also, do you know the charateristics of the data? Many master/slave RS-485 systems use pretty standard protocols like MODBUS (or something derived from it) so it may be a lot easier to packet sniff to pick out the master / slave data, which should be pretty easy for a small microcontroller.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 15, 2021, 01:47:47 pm
content: Another way to check the direction of current flow is with some diodes. I used a 1A rectifier here but you will get better results with schottkeys. Note the voltages on the nodes for the two current directions. You can get bery high speed from this set up. Just add a comparator to test the volts on the reistors at the bottom side of the bridges.Dirt cheap.More refined [ Specified attachment is not available ]

author: ajb
author_rank: Super Contributor
timestamp: November 15, 2021, 04:17:47 pm
content: I've done a full RS485 directional transceiver using more-or-less the method you have here, but including all of the logic to decode data in both directions to two separate pins. I'll have to look for my notes to see what values I used, but because I wanted this to work on un-terminated lines and the loading of a single transceiver may not cause enough voltage drop across the series sense resistor, I added loading resistors across the line on both sides of the current sense resistors (I used two series resistors just to keep the line symmetrical as well). For the current sense measurement, I used dividers from both sides of the sense resistor to ensure that the comparator could handle the full common mode voltage range. Worked pretty well, except that differences in propagation delay could allow some glitches through to one decode line or the other. I had planned to build a real PCB with all of this integrated but the perfboard version solved my immediate problem so it hasn't been a priority to finish it yet.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 15, 2021, 04:51:29 pm
content: More simply?

author: fourfathom
author_rank: Super Contributor
timestamp: November 15, 2021, 07:13:37 pm
content: Quote from: Terry Bites on November 15, 2021, 04:51:29 pmThat chip won't take negative common mode voltages (below ground). I don't know why, but the OP was concerned about negative voltages.