RS-485 Network: Do I need isolation if there is no earth connection? - Page 1
Forum section: electronics / projects

author: elepo
author_rank: Contributor
timestamp: November 04, 2021, 09:41:56 am
content: TL;DR: I am creating a network of RS-485 connected devices. We don't use earth grounds in my country. Do I still need to provide isolated power to each of the RS-485 ICs?Hi everyone.I'm designing a rather big system for monitoring the state of ~60 doors.My design is as follows:Every door has a controller.Every 20 controllers are daisy chained together and connected to a "hub". This hub is a master device that polls the 20 controllers under its supervision through RS-485 connection continuously. I use MAX485 ICs for RS-485 communication.There will be a total of 3 hubs that will monitor the state of the 60 doors.Each hub will have an Ethernet port and a web server running on it. A computer will connect to each hub via Ethernet cables and a network switch and continuously polls the hubs to monitor all of the 60 doors.A software will display the state of each door and store logs.I have never done anything with this complexity before so I did a bit of research on RS-485 standard and tried to design the network according to application notes and other resources.My main sources are these two:Ten Ways to Bulletproof RS-485 InterfacesRS-422 and RS-485 Applications EbookThis is what my design looks like:(XCVR is MAX485)The considerations I have taken into account are:1: Termination: resistors with a value of 120R (matching the wire impedance) at start and end of the line2: Failsafe: biasing pullup/pulldown resistors at the "master" node (value will be calculated based on the number of Unit Loads on the line as per Applications Ebook page 32)3: Transient Protection: Use of TVS diodes (as per Applications Ebook page 62)4: The configuration is daisy chained (as per TI checklist #3)5: The medium will be AWG24 wire or CAT5 Ethernet cable (will decide in the future)6: There will be a ground wire connecting the transcievers other than the A & B lines (as per TI checklist #9)7: The baudrate is not very important as the data is very small (the state of the door only). Most likely 9600 or similar8: The master will poll each node and wait for a response until polling the next node, hence avoiding contention9: Stubs are minimal in length (a few millimeters on PCB)10: Total length of the wire will not be more than 500 meters, and each node will not be further apart more than 10 metersIf you have any comment on any of these considerations please kindly share.And now to my main question:In almost every source I've read Ground Potential Differences are mentioned.As I understand it, if all of the nodes are connected toearth ground, there will be ground potential differences between the nodes due to potential differences in the underlying mains installment. Ground loops will also be created which could pick up electromagnetic noise. This can introduce error into the system or damage the chips if the GPD is too high. To avoid this it is suggested to isolate the RS-485 chips from the MCU with opto-isolation and toalso provide isolated power to each of the chipsas in the diagram below:But since there is no earth connection in my design I was wondering if I could do this like my own diagram above without the "supply isolation". i.e. Have one SMPS power all the MAX485 chips through a 4-wire cable, 2 wires for VCC and GND and 2 for data. And only isolate the chips from the MCU so that I won't have to connect all the grounds of the power supplies and MCUs together forming a very long wire that could pick up all kinds of noise. There is still a long wire connecting the ground of all MAX485 ICs together but I'm assuming that wouldn't be a problem since there is no earth connection and RS-485 is pretty robust against noise. Also I'm using the TVS diodes to protect against voltage spikes.I couldn't find any reference online that didn't use an earth ground, I suppose because it's such a common practice. But earth ground is rarely used where I live so I was wondering if I still need the kind of isolation in picture 2 even without an earth ground.(Thanks in advance and sorry for the long post and the hand drawn schematics.)

author: Berni
author_rank: Super Contributor
timestamp: November 04, 2021, 11:38:59 am
content: Yes this is fine to do.The reason for the isolation is to not connect together two devices that are powered from mains outlets in different parts of the building. Running your own isolated power along with your RS485 data lines is perfectly fine.The other way of doing it is to run the data lines trough a 1:1 transformer right before they go outside onto the bus, that way the transceiver does not need to float. This is how Ethernet solves isolation. But the downside of that is that only AC signals may be used, so instead of RS485 you need to use some other encoding scheme like Manchester.

author: elepo
author_rank: Contributor
timestamp: November 04, 2021, 11:51:26 am
content: Quote from: Berni on November 04, 2021, 11:38:59 amThank you very much. I appreciate you reading the long post.One side question: Will TVS diodes only on the "master" device suffice? Or do I have to put them on every node?

author: Berni
author_rank: Super Contributor
timestamp: November 04, 2021, 12:06:52 pm
content: When it comes to TVS diodes, its more a question of if you have a reason NOT to use one, rather than coming up with a reason for using one.It is good practice to have TVS diodes on all signal lines going outside of your product since you have no idea when you might get something nasty coming in. From static discharges, to cables picking up a surge from lightning, to cables shorting out to something they shouldn't... etc. They don't cost much, but could save the product from death. Also having some resistance between the TVS diode and outside world also improves resilience since it limits the amount of current the TVS diode has to sink, helps burn the fault energy as heat and can go open circuit like a fuse in the event of a huge surge.I like to ask myself the question "How much voltage/current/power can i apply to this connector before my product blows up?"Then again improper TVS diodes can make a product more likely to fail. At work we have some CAN to USB converters fro SysWorks that have a 5V TVS diode on the CAN lines. This means that shorting a CAN line to 12V kills the device (Something that really easily happens since most CAN systems are powered by 12V). I opened up some of these dead ones (with a lot of cursing as they are designed to be tricky to open) and just swapped out the TVS diodes for 24V ones as that is what the CAN transceiver survives and it all works fine and survives 12V shorts now.

author: elepo
author_rank: Contributor
timestamp: November 07, 2021, 08:30:06 am
content: Quote from: Berni on November 04, 2021, 11:38:59 amLooking back at my setup, it seems to me that even if all the SMPS were connected to earth ground there would still not be any problem. Because the RS485 network of devices would still remain isolated. Am I correct?If yes then why isn't this setup recommended anywhere? In all application notes I read a setup like my second pic in my first post is suggested.

author: Berni
author_rank: Super Contributor
timestamp: November 08, 2021, 06:50:01 am
content: You can use the isolated SMPS for this too. The only thing is that they have capacitors between the mains and the isolated GND of the output for EMI reasons. So those capacitors can pick up noise from the mains and push it into your grounds. Still should work fine tho. You can help the noise immunity of RS485 transceivers by giving them a common mode choke between them and the bus.

author: Kjelt
author_rank: Super Contributor
timestamp: November 08, 2021, 07:15:48 am
content: Do check ESD-Protection.I believe the cheap max485 has not much ESD to,erance while the newer chips all got 15kV ESD-Protection.For long lines and human touched devices this might be wise.I remember some story of a factory with a few hundred nodes where they had to replace some drivers every month due to ESD issues.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 08, 2021, 11:07:45 am
content: I once bought a bag of "MAX485" chips from Ali.I've used a few, they tend to last about 2 weeks.I have not used enough to do a thorough investigation.About those TVS diodes.They are mostly for ESD protection at the moment that plugs get connected.If you use screw connectors, then always connect the GND wire firs, and unscrew the GND wire last. Some connector types (for example USB, SATA, uSD) have longer tabs for the GND (and Vcc) wires, so power gets connected before data.ESD events have a very short duration, and a TVS diode that is connected a few meters down some cable is not going the protect a driver on the other side of the cable.RS485 should be designed with signal levels between -7V and +12V, and TVS diodes should be matched to (approx) those voltages, There are bipolar TVS diodes available especially for RS485.A good way to add some extra protection is to put PPTC fuses between the TVS diodes and the connector. The TVS protect the RS485 driver, while the PPTC protects the TVS against DC overload (which can easily happen if you have for example a 24Vdc power supply through the same cable).

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 08, 2021, 12:08:31 pm
content: That's fine, but one thing you need to check is the voltage drop on your ground line due to the power draw of the nodes - this will effectively shift the common-mode level of the RS485 data. Just need to make sure this stays within the common mode spec of the transceivers, and doesn't make any of the protection devices start to conduct.probablty not an issue at the voltage levels involved here.You may want to add protecion against incorrect connection.

author: ajb
author_rank: Super Contributor
timestamp: November 08, 2021, 03:51:56 pm
content: Quote from: elepo on November 07, 2021, 08:30:06 amMost devices provide their own internal isolated power for the isolated transceivers because it's more flexible and simplifies installation. There are lots of existing installations that only have one pair of wires, with or without isolation, so a device that requires external isolated power is harder to integrate into a situation like that. For a new installation, running external isolated power means twice as many connections to deal with, and now you have an additional power supply that needs to be specified based on the number of transceivers anticipated, and the voltage drop needs to be dealt with based on what those devices can tolerate and the distances involved. Plus you now have power inputs on all of those devices that should/must be protected against faults, and the power supply is an additional single point of failure for the entire system. It's all just enough of a pain to make providing local isolated power for the transceivers within each device is easier for most systems.