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Thread: BACnet MS/TP + Bad Wire, will it work?

  1. #21
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    Quote Originally Posted by jschulze View Post
    There is quite a lot of information on RS-485 available online. I've compiled a pretty large list of links, but here are some of the most useful. The first link is a basic FAQ on RS-485 and the content provided in the links get more advanced as the list goes down.

    https://www.csimn.com/CSI_pages/RS-485-FAQ.html

    https://www.ti.com/lit/an/slla272c/slla272c.pdf

    https://www2.htw-dresden.de/~huhle/ArtScienceRS485.pdf

    https://www.ti.com/lit/an/slla070d/slla070d.pdf


    Additional app note on RS-485 grounding:
    http://robustdc.com/download/index.php?file=AN005.pdf

    Additional article on Termination and Biasing
    https://blog.opto22.com/optoblog/rs-...e-bias-or-both

    Good day jschulze,

    Indeed a solid understanding of RS485 is very important. It is unfortunate that that the info you presented appears to not have been read or understood by a number of the equipment manufacturers...

    Cheers,

    Sam

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  3. #22
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    Quote Originally Posted by s2sam View Post
    Good day jschulze,

    Indeed a solid understanding of RS485 is very important. It is unfortunate that that the info you presented appears to not have been read or understood by a number of the equipment manufacturers...

    Cheers,

    Sam
    Yes, I would agree. Many manufacturers of RS-485 equipment, and even some techs that commission RS-485 networks, do indeed seem to lack an understanding of RS-485.

    I can't believe the number of RS-485 devices I've seen that only have + and - terminals with no mention of what those terminals are referenced to, whether it's isolated, etc. If they do have an additional terminal, many times it's labeled as shield (sometimes incorrectly, as the terminal is often connected to the device's logic ground). There seems to be an awful lot of manufactures that don't understand that there is a big difference between shield and common-mode (0V) reference.

    I've also worked with techs from one of the big-name control companies that stated they do have a reference wire on their network, but they don't land it on any other manufacturers' devices because the company has a policy that forbids them from landing the reference wire onto 3rd party devices.

    It's no wonder why there's so many issues in the field with RS-485 interoperability between manufacturers...

  4. #23
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    Quote Originally Posted by jschulze View Post
    I've also worked with techs from one of the big-name control companies that stated they do have a reference wire on their network, but they don't land it on any other manufacturers' devices because the company has a policy that forbids them from landing the reference wire onto 3rd party devices.
    Sounds like Johnson - good job being discreet, but I am gonna call that.
    I remember doing internal Johnson training for Bacnet. The intense impression I got was that there is Johnson Bacnet and everything else, and everything else is a horrifying morass of inconsistent unpredictability and anything having to do with it is a roll of the dice at best....
    So I wuda left my 3rd wire float too with that understanding.
    Hmmmm....smells like numbatwo to me.

  5. #24
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    Quote Originally Posted by jschulze View Post
    Yes, I would agree. Many manufacturers of RS-485 equipment, and even some techs that commission RS-485 networks, do indeed seem to lack an understanding of RS-485.

    I can't believe the number of RS-485 devices I've seen that only have + and - terminals with no mention of what those terminals are referenced to, whether it's isolated, etc. If they do have an additional terminal, many times it's labeled as shield (sometimes incorrectly, as the terminal is often connected to the device's logic ground). There seems to be an awful lot of manufactures that don't understand that there is a big difference between shield and common-mode (0V) reference.

    I've also worked with techs from one of the big-name control companies that stated they do have a reference wire on their network, but they don't land it on any other manufacturers' devices because the company has a policy that forbids them from landing the reference wire onto 3rd party devices.

    It's no wonder why there's so many issues in the field with RS-485 interoperability between manufacturers...
    Good day jschulze,

    Well said and I agree.

    Cheers,

    Sam

  6. #25
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    Quote Originally Posted by numbawunfela View Post
    Sounds like Johnson - good job being discreet, but I am gonna call that.
    I remember doing internal Johnson training for Bacnet. The intense impression I got was that there is Johnson Bacnet and everything else, and everything else is a horrifying morass of inconsistent unpredictability and anything having to do with it is a roll of the dice at best....
    So I wuda left my 3rd wire float too with that understanding.

    Good day numbawunfela,

    I cannot speak for jschulze or any inferences he may have made, but I can tell you that JCI's interfaces are one of the better designed ones I have seen. I have not analyzed (or dismembered ) all of JCI's devices, but for the past 20+ years I have looked at a lot of their devices and found that they were one of a very few number of manufacturers that electrically isolated their RS485 (N2 Bus) interfaces... and provided pretty beefy I/O protection circuitry. This is even more impressive considering that at the time implementing electrically isolated RS485 interfaces was not as straightforward as it is today. Electrically isolating the RS485 has significant benefits and can reduce communication issues between other manufacturer devices... and so the JCI Field device designers were certainly thinking. Going forward, I still periodically "review" JCI's offerings and although their software/firmware may be wanting, their hardware seemed to maintain the same standard they had. Hopefully this will continue, but time will tell and also will indicate if bean counters or unseasoned designers are driving their field devices.

    Cheers,

    Sam

  7. #26
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    Quote Originally Posted by s2sam View Post
    I cannot speak for jschulze or any inferences he may have made, but I can tell you that JCI's interfaces are one of the better designed ones I have seen.
    I am sure I have not gotten into the level of detail you have, but I tend to agree. Johnson hardware is pretty good.
    I was more commenting on the internal culture that may make it seem reasonable for one of their guys to think it was a good idea to avoid having one of their devices become 'tainted' by another vendor's bacnet device - and avoid landing that 3rd wire anywhere.
    Hmmmm....smells like numbatwo to me.

  8. #27
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    Quote Originally Posted by numbawunfela View Post
    I am sure I have not gotten into the level of detail you have, but I tend to agree. Johnson hardware is pretty good.
    I was more commenting on the internal culture that may make it seem reasonable for one of their guys to think it was a good idea to avoid having one of their devices become 'tainted' by another vendor's bacnet device - and avoid landing that 3rd wire anywhere.
    Good day numbawunfela,

    Gotcha... tells you where my brain is at... hardware/software/firmware. Corporate culture and politics are not part of my makeup and so explains why I never found a corporate home.

    Cheers,

    Sam

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    This is a interesting discussion, hopefully you find this question interesting. I currently am dealing with a comm issue at a hospital we service.

    The trunk is two JCI bacnet devices and 8 Setra SRPM room pressure monitors, less than 1000’ long. Wire is 3 conductor shielded. We added a domestic water pump skid to the trunk and some devices immediately went offline.

    We ended up repulling part of the trunk to remove a splice and put one of the JCI devices at the end of the line so we could turn on its EOL switch. This made things better and we were able to map in the pump skid. I also added a MS-BACEOL at the Jace.

    The customer contacted me after 2 months and said things would still go offline occasionally. Before adding the pump skid the trunk was solid.

    I’m going to go there today and get the pump skid info. The skid is only two wire and I did not see a spot to land the reference. It sounds like it could be the reference for this pump skid is using the earth ground and not working so hot?

    My question is there something I can do to land the reference on this device if it is only two wire?


    Sent from my iPad using Tapatalk

  10. #29
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    Quote Originally Posted by s2sam View Post
    Good day numbawunfela,

    Gotcha... tells you where my brain is at... hardware/software/firmware. Corporate culture and politics are not part of my makeup and so explains why I never found a corporate home.

    Cheers,

    Sam
    So questions, if the device is non isolated and the reference wire is hooked up won’t that cause ground loops?

    Therefore if it is really hard to know if a device is non isolated or only partially isolated maybe corporates knee jerk response was to never land the third wire?


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  11. #30
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    Quote Originally Posted by jschulze View Post
    Yes, I would agree. Many manufacturers of RS-485 equipment, and even some techs that commission RS-485 networks, do indeed seem to lack an understanding of RS-485.

    I can't believe the number of RS-485 devices I've seen that only have + and - terminals with no mention of what those terminals are referenced to, whether it's isolated, etc. If they do have an additional terminal, many times it's labeled as shield (sometimes incorrectly, as the terminal is often connected to the device's logic ground). There seems to be an awful lot of manufactures that don't understand that there is a big difference between shield and common-mode (0V) reference.

    I've also worked with techs from one of the big-name control companies that stated they do have a reference wire on their network, but they don't land it on any other manufacturers' devices because the company has a policy that forbids them from landing the reference wire onto 3rd party devices.

    It's no wonder why there's so many issues in the field with RS-485 interoperability between manufacturers...
    How easy is it find out how the device BaCnet reference e is handled?

    Here is a proposal: if you want to be BTL listed you must declare in your BIBB or PIC what you are using for your RS485 transceiver chip?


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  12. #31
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    Bigguy, 2 wire devices talk on RS485 - a 3 wire medium - using ground as their 3rd conductor. Johnson and Siemens use their 3rd wire, and isolated transceivers. Isolated as in - they are not looking at a grounded secondary on the power to provide that 3rd conductor, the transceiver is isolated from the power and therefore has his own 3rd conductor.
    When mixing the 2, you can land the 3rd wire at a non-isolated device by grounding it through a resistor. I have seen documentation that says 100ohms. I most often use a 120 ohm resistor as that is what I have laying around for termination. Then the non-isolated device will share a 3rd wire with the isolated section. To avoid ground loops I land it at one place, usually the JACE. Then you are as good as any other trunk.... you are relying on the ground of the building to get the 2 wire stuff.
    I also will use 2 of the MS-BASCEOL devices, one at each end on problem trunks. If you only have one, set the JACE to BIA and add a termination resistor. Then put the MS-BASCEOL at the other end.
    A voltmeter or Oscilloscope will tell you if these electrical properties are the issue.but there is a whole other side to this, and that is the Wireshark side.
    Hmmmm....smells like numbatwo to me.

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  14. #32
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    Guys, I have 3 - one hour classes I did for my shop on doing this. I cover everything up to (but not including) the Oscilloscope and Wireshark parts. I did it for my internal shop training. Would there be an interest in a pay-per-view model that is put at a reasonable level? I have been taking my time in getting it put up, but I can see this is hell for a lot of people.
    I have video of real sites with the voltmeter and showing how I fix these exact issues.
    I have all the pieces for an Oscilloscope class, but have not assembled it.
    It will not be available before you get onsite today, Bigguy.... sorry.
    Just trying to guage interest.
    Hmmmm....smells like numbatwo to me.

  15. #33
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    Quote Originally Posted by bigguy158 View Post
    So questions, if the device is non isolated and the reference wire is hooked up won’t that cause ground loops?

    Therefore if it is really hard to know if a device is non isolated or only partially isolated maybe corporates knee jerk response was to never land the third wire?


    Sent from my iPad using Tapatalk
    You're exactly right, which is why there's not just one solution for how to ground all RS-485 networks. It depends on the devices being used, the location's (building's) grounding network, cabling, and so many more things.

    Have a look at the articles I posted in post #18. Almost all of them cover RS-485 grounding techniques.

  16. #34
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    Quote Originally Posted by bigguy158 View Post
    How easy is it find out how the device BaCnet reference e is handled?

    Here is a proposal: if you want to be BTL listed you must declare in your BIBB or PIC what you are using for your RS485 transceiver chip?


    Sent from my iPad using Tapatalk
    I like the idea, but there are a lot of other protocols that use RS-485 besides BACnet too. Also, communication protocol specification and conformance tests don't typically cover the physical RS-485 layer (it's outside the scope of the protocol). There is already a standard for that: EIA/TIA-485.

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  18. #35
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    The good news in all of this is that RS-485 transceiver technology is advancing and many manufacturers are now isolating their RS-485 circuitry. Isolation solves a lot of the RS-485 grounding problems, including ground loops, and will most often even work without the reference wire connected at all.

    Many modern RS-485 transceivers provide full-fail safe operation, keeping their inputs at a known state even when the the bus is open or shorted. This removes the need for network biasing and prevents issues caused by applying termination to an unbiased bus (i.e. the + and - idle differential voltage is below 200mV).

    Additionally, most modern transceivers consume only a fraction of a unit load (1/2, 1/4, 1/8, etc.) allowing for many more than 32 devices on the bus (64, 128, 256, etc.) without using repeaters.

    There are also additional features available in some transceivers, such as high output drive (allowing longer cable runs and more than 32 unit loads) and slew rate limiting (which limits the rise time of transmitted transitions, preventing cable reflections, and thus removing the need for termination resistors - although termination may still be necessary for very long cable runs and high baud rate combinations).

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  20. #36
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    Quote Originally Posted by jschulze View Post
    The good news in all of this is that RS-485 transceiver technology is advancing and many manufacturers are now isolating their RS-485 circuitry. Isolation solves a lot of the RS-485 grounding problems, including ground loops, and will most often even work without the reference wire connected at all.

    Many modern RS-485 transceivers provide full-fail safe operation, keeping their inputs at a known state even when the the bus is open or shorted. This removes the need for network biasing and prevents issues caused by applying termination to an unbiased bus (i.e. the + and - idle differential voltage is below 200mV).

    Additionally, most modern transceivers consume only a fraction of a unit load (1/2, 1/4, 1/8, etc.) allowing for many more than 32 devices on the bus (64, 128, 256, etc.) without using repeaters.

    There are also additional features available in some transceivers, such as high output drive (allowing longer cable runs and more than 32 unit loads) and slew rate limiting (which limits the rise time of transmitted transitions, preventing cable reflections, and thus removing the need for termination resistors - although termination may still be necessary for very long cable runs and high baud rate combinations).
    Good day All,

    These are valid points, but tech can only go so far if improper cable is used... With that said, non ideal can still be used and still have a solid and reliable communication system, however, compromises will have been made in terms of noise immunity tolerance, cable length, baud rate, number of attached devices, etc. I always find it funny how there are those that put up a fuss with using proper RS485 cable and yet are perfectly OK to use the appropriate Cat cable for Ethernet... or for Lon.

    Cheers,

    Sam

  21. #37
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    Good day all,

    Although there is a fair amount of core knowledge is needed to fully understand how the communication cable plays a vital role in having a robust communication system, below is an excellent video that will provide a great visualization of a transmission line and its various parameters. The video is dated (1950's?) and uses mechanical waves to demonstrate various concepts, however, the physics has not changed and is the same as used in electronic systems. What is also interesting is during the 50's,60's, and 70's a number of these very informative films (now translated to videos) were created by private industry for release to schools, Universities, etc as a societal investment... This is complete contrast to today, where I have yet to see any of the notable firms spend any time/effort/$$$ to produce such videos for general release. Sadly, times have sure changed.

    https://www.youtube.com/watch?v=DovunOxlY1k

    Cheers,

    Sam

  22. #38
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    Quote Originally Posted by hvac69 View Post
    Took over an existing site that the BACnet com was done partially with the original C-BUS network, mixed in with some 18/6 and some other cable. Dropping the baud rate, adding some resistors helped but there was not a ton of polling being done and only about 12 controllers. I have seen a properly wired, moderately loaded bus that worked, but one Viconics stat added wreaked havoc. Its like rolling the dice sometimes, but at least you know going in that the outcome is questionable, just make the customer aware.
    Not really. As s2sam has stated, and is somewhat well known, there is a lot of science behind this. Unfortunately, it is out of reach for most people, even me sometimes. I've struggled to find more information on RS-485. Mostly when you search for 'how RS-485 works' you just get the same elementary stuff over and over... "RS-485 uses 2 wires. It can also be full duplex and use 4 wires. 120ohm is the 'characteristic resistance'. End of story."
    Gee....thanks.

    End of line resistors, calculating load/impedance on the bus, voltage biasing, baud rate, parity, reference ground, etc. If any one of those things is amiss, the stuff won't work.
    Quote Originally Posted by s2sam View Post
    Good day Maxburn,

    No, I know of no single class or a few classes that would provide the background needed to fully understand the physics behind this nor knowing how to setup and use the various pieces of test gear that is helpful. As I mentioned, one would really need to have a solid background on general physics, electricity, magnetism, etc and then start to focus on electromagnetics and transmission line theory. Even armed with this info, one needs to apply all of this info towards the application (communication buses) that most in this section experience. Like most complex technologies/concepts, they are typically a subset of the larger technology/concept and so one needs all of the info in order to understand the subset.

    With that said it might be possible to create a dedicated course on all of this, but knowing what I have learned, it would be tough to cram all of the background info into a single course or two... but then again I am pretty dense and so it took time for me to get a reasonable grasp on all this. Although my comments are/were geared for communication buses that most use, the same principles/physics apply to when I am designing high speed circuits on printed circuit boards, the only thing different are the speeds and the transmission medium. If you wish to dive in the do a google search on Transmission line theory and check out the results.

    Cheers,

    Sam
    Basically what he's saying is you need to get a 4 year degree in electrical engineering.

  23. #39
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    Quote Originally Posted by numbawunfela View Post
    Bigguy, 2 wire devices talk on RS485 - a 3 wire medium - using ground as their 3rd conductor. Johnson and Siemens use their 3rd wire, and isolated transceivers. Isolated as in - they are not looking at a grounded secondary on the power to provide that 3rd conductor, the transceiver is isolated from the power and therefore has his own 3rd conductor.
    When mixing the 2, you can land the 3rd wire at a non-isolated device by grounding it through a resistor. I have seen documentation that says 100ohms. I most often use a 120 ohm resistor as that is what I have laying around for termination. Then the non-isolated device will share a 3rd wire with the isolated section. To avoid ground loops I land it at one place, usually the JACE. Then you are as good as any other trunk.... you are relying on the ground of the building to get the 2 wire stuff.
    I'm sorry, but I have no idea what you're talking about.
    Siemens' old stuff was 2 wire only. +/- and shield. They did not move to 3 wire & shield until recently with BACnet devices. And the only reason to use 3 wire was because of BACnet.
    Not sure what you mean with the "isolated as in not looking at a grounded secondary on the power"

    You mean the 24v side...?

    None of Siemens' stuff is earth grounded. Especially the field devices. The older TECs didn't even have a 3 pin power terminal, just hot and common. The newer TECs do have an earth pin but it's never used. The documentation specifically (to my surprise) emphasizes how optional it is to earth ground the equipment.
    And then Siemens' newest field controllers, the DXRs, do not have an earth pin at all either, back to the simple 2-pin power plug.
    An earth ground on the power side would pretty much be for human safety only. It would not make sense to use the 24v earth ground to mutually ground all of the comm in the entire building. Because many of the devices would be powered from different transformers, which creates the entire 'floating ground' issue in the first place.

    Quote Originally Posted by jschulze View Post
    I can't believe the number of RS-485 devices I've seen that only have + and - terminals with no mention of what those terminals are referenced to, whether it's isolated, etc. If they do have an additional terminal, many times it's labeled as shield (sometimes incorrectly, as the terminal is often connected to the device's logic ground). There seems to be an awful lot of manufactures that don't understand that there is a big difference between shield and common-mode (0V) reference.
    oh jeez...I've gotten into heated arguments with 'veteran' techs over this. Just a few weeks ago we replaced 6 VFDs using Siemens' old P1 (which is +/- and shield only). Upgraded to ABB ACH580s which are quite sensitive devices it appears.

    Right off the bat, I was very hesitant to land the shield wire on the third pin of the terminal despite a lack of clear symbol at the plug and complete lack of documentation.
    As a result, I left it off.

    The veteran tech who has been setting up VFDs for '35 years' comes around and wires them all up. I asked what he was doing. "I'm putting the ground where the ground goes."

    No. That's not for the shield, it's for the ground.
    "It is, it grounds the EMF"

    NO. They are not the same. AT ALL.
    My mistake was trying to explain that the pin is for the 3rd wire, a ground reference, and all he heard was "ground"...

    He did it anyway. I went around and disconnected them all.
    He confronted me about it and I had to make sure he left the site before he re-terminated them. I even called ABB and asked them that the shields should not be terminated, the rep said absolutely not. They were having comm issues though.

    The veteran tech argued with me until he was blue in the face about how you would always land the shield wire at every TEC (pre-BACnet) back in the day.
    What he didn't want to understand is that the "shield" terminal is a dead pin, connected to nothing on the board, only there as a place holder for the shield....

    Quote Originally Posted by s2sam View Post
    Good day numbawunfela,

    I cannot speak for jschulze or any inferences he may have made, but I can tell you that JCI's interfaces are one of the better designed ones I have seen. I have not analyzed (or dismembered ) all of JCI's devices, but for the past 20+ years I have looked at a lot of their devices and found that they were one of a very few number of manufacturers that electrically isolated their RS485 (N2 Bus) interfaces... and provided pretty beefy I/O protection circuitry. This is even more impressive considering that at the time implementing electrically isolated RS485 interfaces was not as straightforward as it is today. Electrically isolating the RS485 has significant benefits and can reduce communication issues between other manufacturer devices... and so the JCI Field device designers were certainly thinking. Going forward, I still periodically "review" JCI's offerings and although their software/firmware may be wanting, their hardware seemed to maintain the same standard they had. Hopefully this will continue, but time will tell and also will indicate if bean counters or unseasoned designers are driving their field devices.

    Cheers,

    Sam
    Curious of your thoughts on Siemens' hardware

  24. #40
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    Quote Originally Posted by stomachbuzz View Post
    Not sure what you mean with the "isolated as in not looking at a grounded secondary on the power"

    You mean the 24v side...?
    Yessir.
    The 3 wire Siemens bacnet has a reference conductor, same as Johnson. Stuff with no reference conductor (3rd wire in the Bacnet trunk) use the common of the 24v power as a reference. That is why the 24v common should be grounded on these. That is how they will all have the same reference, although they have different transformers. They are all grounded to the same building. No separate ground terminal necessary.
    At least in theory. In practice it gets messy, which is why 3 wire bacnet is MICH more reliable than 2 wore bacnet. It don't matter one bit what the heck is going on in the building, the comms are isolated and tend to stay happier.
    Hmmmm....smells like numbatwo to me.

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