isolated bus interfaces - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: February 12, 2021, 06:36:14 pm
content: I am wondering if using an isolated RS485 or CAN transceiver is worthwhile on a vehicle. Looking at transceivers I see that they accept common mode voltages of up to 12V despite being powered at 3.3-5V. OK, I do tend to work with 24V but if you have a ground float of 12V you have some wiring that is so seriously overloaded that fuses popped long ago apart from the fact that a 12V ground difference means a droop in supply of 12V as well bringing actual supply down to 0V anyway.My other thought is that if I have subsystems pulling similar currents on similar wiring the ground float will be about the same....If I was to play safe and use an isolated transceiver loosing the DC/DC would be an advantage whilst avoiding having to have a power bus as well running with the comms. What if I pulled a trick from high voltage differential scope probes and ran the "isolated" side from another voltage regulator with a resistor between the two grounds that would allow the two grounds to float apart but still deliver power. I guess the issue here is the power rating of the resistor. It looks like the driver side of these isolated drivers pulls about 60 mA so to allow say a 5V float. 5V at 100 mA is 0.5W. Of course as each interface would also be doing this they would be meeting half way so a bit more than 5V of float would be possible.

author: coromonadalix
author_rank: Super Contributor
timestamp: February 12, 2021, 08:15:23 pm
content: I would not do your double ground trick, if you have current consumption changes, the floating reference(s) may vary, i work in some cars, every computer interface has an usb isolator (they dont cost much and provide at least 1kv isolation)For the equipment supply, i use some smps psu's who accept 9vdc to 19-20vdc or linear based regulator/supply who goes up to 30vdcI've seen some alternator circuits going up to 14vdc and see some 12vdc stuff goes BAM lolllWith a good ground, i never had any problems ??

author: ajb
author_rank: Super Contributor
timestamp: February 12, 2021, 08:28:34 pm
content: You have to consider the kinds of faults you're attempting to protect against, in terms of common mode and differential mode voltages and available currents and the likelihood of the various mishaps might cause them.RS485 transceivers can be found with bus pins rated for +/-18V (actually even up to +/-60V, although those may be prohibitively expensive, idk), so there's really no way to apply "12V"/14.4V of the battery supply differentially to the bus interface in a way that would violate the ratings of the transceiver (except via transients, which deserve special handling). Now if you're working in a vehicle with 24V system, that does get a bit harder. At that point maybe it is better to pay for a +/-60V tolerant transceiver--the higher voltage tolerance will also make protecting against transients easier as a bonus.The primary concern addressed by isolation in a low voltage/high current situation like a vehicle is going to be break the signal ground (which RS485 requires as a reference for the differential signals) to prevent it from carrying load/fault current in lieu of the power ground line. If you're running separate twisted pair + ground/shield cable between a bunch of high current devices, then yeah, isolation so that if the power ground becomes disconnected you don't end up with a bunch of load current flowing through the signal harness and cooking it makes sense. But in a situation like a vehicle where you have a moderate number of nodes all close together and connected to a common low voltage supply, maybe you don't even need the separate signal ground, and you can just use the power ground as the signal reference. The transceiver won't care either way as long as their common mode range is respected, and there are some other LVDC+RS485 systems that do this.If the RS485 goes off the vehicle, then I would say you definitely want isolation, because who knows what happens if someone hooks up a shitty battery tender while your bus is connected to a mains-supplied device or something.I'm not as familiar with CAN, but a similar set of considerations will apply to any differential bus.

author: Simon
author_rank: Unknown
timestamp: February 12, 2021, 09:32:40 pm
content: Quote from: ajb on February 12, 2021, 08:28:34 pmThis is the scenario, a master ECU in one subsystem needs to communicate to it's slave ECU's in other systems. So each subsystem is powered on a pair of wires as there is no chassis in this case. So for every negative wire there is a positive. So 12V of withstanding is pretty much a 24V of withstanding. Nothing's ground reference can exceed 50% supply because as much as the local ground rises the positive supply also falls to meet at 50% supply with 0V across the system so even with a 28V system that is 4V and it's all shut down by then anyway.If I used my "fake" floating ground I would use a sensible sized output capacitor on the "isolated" regulator so that it would deal with fast stuff and just be moved up and down by the current draw of the "big" stuff causing the whole subsystems ground to move up and down. No signals are going off vehicle, those would be isolated when they do.

author: nctnico
author_rank: Super Contributor
timestamp: February 12, 2021, 10:04:55 pm
content: That sounds very complicated. Just stick to the basics. CAN and RS485 are designed to deal with some amount of voltage difference between grounds. In the end the differential signal is all what matters so as long as the signals are reference to more or less the same ground voltage everything is OK. CAN is probably your best bet. Some of the more ruggedised CAN transceivers (from NXP for example) will handle 60V (and some even more) to survive load dumping events.

author: WattsThat
author_rank: Frequent Contributor
timestamp: February 13, 2021, 01:20:30 am
content: While it may be a bit OT, I’ll just add that with cheap 485 transceivers (think 75176 and max485), adding isolation only saves the processor, the line drivers will still blow quickly when their rather low max input voltages are exceeded. The higher withstand voltage parts didn’t exist back when I was working, I wish they had as they probably would have saved me some significant pain.Parts like the MAX3430 has the added advantage of 80V withstand and they are only 1/4 load so they would support 128 devices per bus. The but is they are $4 each in quantity.

author: ajb
author_rank: Super Contributor
timestamp: February 13, 2021, 03:31:14 am
content: Quote from: Simon on February 12, 2021, 09:32:40 pmThis assumes you have no wiring problems, which is not an assumption that you should build your fault protection plan upon. A positive supply connection going open circuit probably isn't a problem (as long as it doesn't close a circuit with something else), but what if the negative goes open circuit? Does your signal wiring provide an alternative path for load/fault current in that case? Will your transceivers tolerate having a potential difference near the supply voltage between one node and the next on the bus lines?Again, you need to identify the potential problems, their respective likelihoods, and the costliness of various failure modes before you (or anyone else) can plan your protection out.

author: Simon
author_rank: Unknown
timestamp: February 13, 2021, 08:04:13 am
content: Well typically there is more than one negative and positive wire so there is some redundancy in wiring but yes you make a good point and if 24V or more tolerant parts are out there I'd probably use them anyway it's really just the principle I am looking at of isolation versus not. I am struggling to see why I would need isolation on a 24V system.

author: m k
author_rank: Super Contributor
timestamp: February 13, 2021, 02:19:49 pm
content: Back in the day when computers were bulky and terminals were far away many were making their bread and butter by swapping line drivers.I wouldn't overlook that driver thing.

author: coromonadalix
author_rank: Super Contributor
timestamp: February 13, 2021, 02:45:17 pm
content: You wont need isolation if you're sure the your ground level points are shared evenly with all the connected instruments ..... but when you deal with differentials signals / communication stuff etc etc ... not only in cars, you can have some surprisesI work in rf stuff, and i do need some isolation for my communication inputs or outputs, i found i had rogues signal / noises on the lines, even on the differential pairs when connected on my computer, i have good ground points, everything is fine for the psu's supplying all my stuff BUT, i have this phenomenon, and the rf board has an floating ground point in it, i output 600v peak of 27.xxx mhz signal from a mere 48v dc.Bought usb to rs232 / rs485 adapters with an supplemental ic for isolation, and all the noise disappeared, and i can have all my communication plugs stay connected while doing tests and calibrations.And my scope has an isolation transformer on the ac line.At one point in the past i had 78vac between the probe ground and the system "DUT" groundBeats me ?Some smps psu's are buck boost topology based, not providing the required isolation.And finally some of our stuff is medical graded, we push the tests to 4kv and they hold, at 4010 volts they break, the protection blows lollSimply put:Add some isolation if needed on you pieces of equipment what ever it is, it doesn't cost much and can avoid huge $$$ bills of broken stuffMy most expensive adapter is 60$ for an heavy duty usb to rs232 adapter (2.5kv), came from polland and its din rail mount ??I have 3 to 4k worth of stuff connected to my Devices under tests (DUT) and they are iso certified once in a year, i simply don't want to take chances.

author: Simon
author_rank: Unknown
timestamp: February 13, 2021, 03:03:08 pm
content: I'm talking about a vehicle system where it's all at the same potential and can hardly float. If I can get a transceiver that will work with it's negative shunted up at the positive supply then I struggle to see the problem.this is not just about the drive cost. If your serious about isolation that is another power loom for the comm's system and every single devices connector now needs another 4 pins for iso power in and out as well as the data in and out quadruple when the whole device has main power anyway.