Isolated rs485 - logic analyzer - Page 1
Forum section: electronics / beginners

author: pacioc193
author_rank: Regular Contributor
timestamp: September 07, 2023, 07:38:23 pm
content: I’ve a working setup where a master and slave communicate troughs isolated 485.I’ve not the possibility to get ground from any of the two devices.Is there any way, using oscilloscope or better a logic analyzer to sniff data on this bus?

author: langwadt
author_rank: Super Contributor
timestamp: September 07, 2023, 07:49:45 pm
content: if it is really isolated use one line a "ground" the other as signal

author: artag
author_rank: Super Contributor
timestamp: September 07, 2023, 07:51:20 pm
content: A differential line receiver powerd from the oscilloscope or another isolated supply

author: pacioc193
author_rank: Regular Contributor
timestamp: September 07, 2023, 08:01:13 pm
content: Quote from: artag on September 07, 2023, 07:51:20 pmWhat could be use as differential line receiver?Quote from: langwadt on September 07, 2023, 07:49:45 pmCould you make an example??

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 07, 2023, 08:09:59 pm
content: Quote from: pacioc193 on September 07, 2023, 08:01:13 pmSN75176And add a few optocouplers. Or just buy an extra isolated RS485 transceiver module and stick it in your PC.

author: pacioc193
author_rank: Regular Contributor
timestamp: September 07, 2023, 08:15:36 pm
content: Quote from: Doctorandus_P on September 07, 2023, 08:09:59 pmI will explain my problem :I've connected an ADM2852E transceiver to the system.It can read the bus but digital signal generated from this transceiver is different from the one from other device.Could be possible that this transceiver is not good to translate the bus? Is possible to check what is the real signal on the bus trought a logic analyzer without using a transceiver?

author: langwadt
author_rank: Super Contributor
timestamp: September 07, 2023, 08:20:03 pm
content: Quote from: Doctorandus_P on September 07, 2023, 08:09:59 pmas long as the common mode is reasonable it doesn't even have to be isolated, these are not isolated but the do work (consideriing the price) with only two connectionshttps://www.ebay.com/itm/184580728698

author: pacioc193
author_rank: Regular Contributor
timestamp: September 07, 2023, 08:25:27 pm
content: Quote from: langwadt on September 07, 2023, 08:20:03 pmI can't have a common mode in this application.Ground of 1 device is not present anywhere.The 485 communication is complete isolated on eache side.

author: langwadt
author_rank: Super Contributor
timestamp: September 07, 2023, 08:47:55 pm
content: Quote from: pacioc193 on September 07, 2023, 08:25:27 pmthen that ebay thing will work just fine

author: pacioc193
author_rank: Regular Contributor
timestamp: September 07, 2023, 08:53:48 pm
content: Quote from: langwadt on September 07, 2023, 08:47:55 pmCommunication is not a standard async communication, Is a custom spi-like communication on two different bus ( CLK + DATA ) on RS485. I think the ebay stuff is just a usb-to-RS485 Async converter

author: langwadt
author_rank: Super Contributor
timestamp: September 07, 2023, 09:04:19 pm
content: Quote from: pacioc193 on September 07, 2023, 08:53:48 pmthen just a use any old rs485 receiver

author: pacioc193
author_rank: Regular Contributor
timestamp: September 07, 2023, 09:15:11 pm
content: Quote from: langwadt on September 07, 2023, 09:04:19 pmI don’t have ground connection… where I attach my ground cable?

author: langwadt
author_rank: Super Contributor
timestamp: September 07, 2023, 09:32:56 pm
content: Quote from: pacioc193 on September 07, 2023, 09:15:11 pmyou don't, look at that ebay thing, it only has A/B no ground

author: pacioc193
author_rank: Regular Contributor
timestamp: September 07, 2023, 09:47:38 pm
content: Again… the one on eBay could be an isolated one. I want to see on the oscilloscope/logic analyzer the signal before transceiver convert it to ttl. That’s it

author: langwadt
author_rank: Super Contributor
timestamp: September 07, 2023, 09:50:41 pm
content: Quote from: pacioc193 on September 07, 2023, 09:47:38 pmthe ebay one is not isolated, I have onesince everything else is isolated you don't need isolation, a regular rs485 transceiver will do

author: pacioc193
author_rank: Regular Contributor
timestamp: September 07, 2023, 10:05:30 pm
content: Quote from: langwadt on September 07, 2023, 09:50:41 pmI’ve used multiple transceiver not isolated like :https://www.ebay.it/itm/184550877233?chn=ps&norover=1&mkevt=1&mkrid=724-128315-5854-1&mkcid=2&mkscid=101&itemid=184550877233&targetid=1827389197184&device=m&mktype=pla&googleloc=20569&poi=&campaignid=17955385566&mkgroupid=142692416071&rlsatarget=pla-1827389197184&abcId=9301109&merchantid=118621252&gbraid=0AAAAACadmUmwjxLoldac54wBzfHSjicAF&gclid=CjwKCAjw6eWnBhAKEiwADpnw9jgA6uGCeubtfx0LD275lUi2Bbn2eazVka4HdXVWtOjoxvugLiZQxhoC5UkQAvD_BwEAnd no signal was picked up because no common ground was shared… the only way to pickup some signal was using isolated 485 ones

author: langwadt
author_rank: Super Contributor
timestamp: September 07, 2023, 10:12:55 pm
content: Quote from: pacioc193 on September 07, 2023, 10:05:30 pmhow did you wire it up?if everything is isolated ground is what you say it is, in the case of using non-isolated receiver on a fully isolated busits ground is the ground

author: pacioc193
author_rank: Regular Contributor
timestamp: September 08, 2023, 05:11:09 am
content: Quote from: langwadt on September 07, 2023, 10:12:55 pmA classic 485 common ground transceiver was connected in the same way of isolated one without common ground and no signal was present on the line. I lost multiple hour to test the situation, ground to microcontroller, ground to earth ecc ecc but the transceiver was not capable to convert data until I try with a isolated one. Signal are converted but seems not in the correct ways. For this reason I want to try to read directly the differential bus and not the converted signal.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 08, 2023, 09:47:12 am
content: Quote from: pacioc193 on September 07, 2023, 08:15:36 pmWhen there is any doubt about signal integrity, then a logic analyzer is mostly useless, and you need an oscilloscope to look at the waveforms themselves.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 08, 2023, 09:57:56 am
content: Quote from: pacioc193 on September 07, 2023, 08:25:27 pmRS485 simply does not work that way.RS485 (just as CAN) needs a GND wire to ensure the common mode of the bus signals stays within limits. For RS485 this is from -7V to +12V, and there is no guarantee to stay within these limitations without a third wire.You can use isolated bus drivers, so the nodes work on another GND lever then the bus itself, but the bus itselfALWAYSneeds a GND wire on itself. If the bus does not have a GND wire, then any voltage shifts due to leakage or capacitive coupling are taken out of the data wires, and this will distort the bus signals themselves.The cheap RS485 transceivers from Ali and other sources often have just two wires, and this is a bit misleading. They rely on the earthing of your PC, ans assume the nodes on the bus are earthed too. The GND wire in the bus is often omitted as a way to break GND loops, but this can only be done to break the loop itself. There still has to be another GND path nearby.Isolating the bus is not a magic wand that solves all problems.

author: langwadt
author_rank: Super Contributor
timestamp: September 08, 2023, 06:08:26 pm
content: Quote from: Doctorandus_P on September 08, 2023, 09:57:56 amwith an isolated transceiver and a bit of bias the signal lines should be in the allowed CM even without any shared groundfor receive only there's also IC's like IL610

author: pacioc193
author_rank: Regular Contributor
timestamp: September 08, 2023, 08:30:12 pm
content: Quote from: Doctorandus_P on September 08, 2023, 09:57:56 amUsing an isolated transceiver the MASTER ground will not affect the transmission line because an isolated GND is created.In this case, is not need to use a common ground mode because the isolated GND beetween all nodes will be in spec for all of them.Sharing ground is not always possible cause different voltage potential ( 220VAC system VS 5VDC system ).In my specific case the communication beetween nodes doesn't share ground. For this reason i need to use isolated transceiver.Quote from: langwadt on September 08, 2023, 06:08:26 pmCorrect!Quote from: Doctorandus_P on September 08, 2023, 09:47:12 amYes you right... I never used an oscilloscope without ground lead. After having search online oscilloscope with MATH function is the correct instrument for me.CH1 RS485 - ACH2 RS485 - BOscilloscope use the GND from the battery of laptopRS485 isolated but use the GND of a powerbankGND lead probe are disconnected. Only A and B signal are used. NO GND is shared beetween the two systemThe signal oscillate beetween -550mv + 840mvIn this case, no logic analyzer can read this signal... At least my chinese one.If i connect the GND from laptop and isolated bus, the signal oscillate from 0V to 5V

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 09, 2023, 02:02:25 pm
content: Quote from: pacioc193 on September 08, 2023, 08:30:12 pmNow it's getting silly.The two signal wires form an electrical circuit between all nodes (whether they have extra isolation or not) and a GND wire is simply mandatory third wire. There simply is no bullshitting around that. Without a GND wire you loose signal integrity. You can't talk your way out of physics. You are making the false assumption that a "perfect isolation" is possible. In reality there is all kind of interference. From long wires picking up stuff as antenna's, and capacitive coupling to the rest of the world. When you ignore those things exist, you get yourself into trouble.But start with some hard data. Draw a circuit diagram that shows how your nodes are connected to the common RS485 bus, and how those nodes are powered. Drawing that overview might already make it clear for you. If not, then post that schematic and we can talk further.

author: langwadt
author_rank: Super Contributor
timestamp: September 09, 2023, 02:12:32 pm
content: Quote from: Doctorandus_P on September 09, 2023, 02:02:25 pmethernet works just fine without a third wire

author: pacioc193
author_rank: Regular Contributor
timestamp: September 09, 2023, 05:12:42 pm
content: Quote from: Doctorandus_P on September 09, 2023, 02:02:25 pmI cannot understand why you cannot admit that a connection without shared ground cable could work. Abs the impedance between the two ground is relative high.Rs-485 can works without 3rd cable.As shown by my photo , the ground was not connected.The system works because the system have high tolerance as differential input. As you see on the oscilloscope the the system is able to sum signal and determinate a correct convert signal.The original Signal oscillating from 0-5 volt. The oscilloscope can read it with a low voltage span. But this low voltage span is enough to convert signal.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 09, 2023, 08:08:44 pm
content: Quote from: pacioc193 on September 09, 2023, 05:12:42 pmBecause it is a hit-and-miss situation and you will not reliable results even if it "seems" to work.Once you start loading the RS485 data wires by abusing them for equalizing the "GND" level of the nodes you start distorting the RS485 data. There simply is no way around the physics unless you are a magician from another world.If yo don't believe me, then go buy an oscilloscope and measure what is really happening with your data lines.Or at least draw the schematic, complete with how all the nodes get their power to make it possible to have a sensible conversation.If that is too much work for you, well then good luck to you, but I'm not wasting any more time on you.

author: pacioc193
author_rank: Regular Contributor
timestamp: September 09, 2023, 11:51:18 pm
content: I don’t want to argue on this topic cause I’m here to ask suggestion.From my try communication was pretty stable and I need to sniff it for just a short periodo of time.I’ve just a DB9 where I can find :Pin 1 : Data BPin 2 : Data APin 3 : Clock BPin 4 : Clock AI’ve tested to read signal using oscilloscope with power supply ground but signal is not clear. For this reason I think it use an isolated transceiver.I’ve tested the method present in this YouTube video :https://youtu.be/VWodjUSkYVE?si=IgGqxHjJ1kwlV8Aaand I was able to pickup signal. Now I’m searching the festers way to decode signal instead of using manual method ( using a printer and a pen… )

author: tooki
author_rank: Super Contributor
timestamp: September 17, 2023, 08:33:13 am
content: Quote from: pacioc193 on September 09, 2023, 05:12:42 pmBut just to be clear, all one can say is that "you can often get away with it". The RS-485 standard unequivocally says to use a ground connection. Unfortunately, the common belief that ground isn't necessary has led some manufacturers to sell equipment with no ground in their RS-485 connectors. That doesn't mean it's a good idea.If your ground potentials "drift away" from each other, you can end up frying the transceiver when its common mode voltage is exceeded.