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

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

    I am looking at doing a project in a pretty small building with about 35 devices on a daisy chain network. The existing system is using canbus on a run of crappy 18AWG solid unshielded wire (probably ~800ft) and there are some communication issues. In my experience BACnet MS/TP usually does ok even on 18AWG solid, although we would typically like to see 22-24AWG low cap shielded of course. But I've never done one with this many devices on the 18AWG solid.

    Does anyone have any experience deciding what sort of baud rate you'd select based on the wiring? We generally do 76.8k, but in this case I'm thinking it would be safer to go with 19.2k or so. I am also planning to split the run in 2 as lucky the main control panel is in the middle so that's fairly easy to do.

    Any advice would be appreciated, Thanks.

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    I have had experience with a system that the shield was not landed on the controllers. Landing the shield helped a lot. If the building is that small why not just run new cable? If the existing cable wasn't run correctly you probably don't want to use it anyway. If you are not replacing a running system you can pull the new in with the old if it was installed correctly.

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    Quote Originally Posted by zzip7 View Post
    I am looking at doing a project in a pretty small building with about 35 devices on a daisy chain network. The existing system is using canbus on a run of crappy 18AWG solid unshielded wire (probably ~800ft) and there are some communication issues. In my experience BACnet MS/TP usually does ok even on 18AWG solid, although we would typically like to see 22-24AWG low cap shielded of course. But I've never done one with this many devices on the 18AWG solid.

    Does anyone have any experience deciding what sort of baud rate you'd select based on the wiring? We generally do 76.8k, but in this case I'm thinking it would be safer to go with 19.2k or so. I am also planning to split the run in 2 as lucky the main control panel is in the middle so that's fairly easy to do.

    Any advice would be appreciated, Thanks.
    I have done many 18/2 not cap wiring jobs over the years. But the customer has to know the limitations before hand. Use 9600 or 19.2K and keep network traffic at a minimum and you'll be ok for SIMPLE systems.
    Controls, the cause of... and solution to... all your HVAC problems.

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    Quote Originally Posted by Paul Benkovitz View Post
    I have had experience with a system that the shield was not landed on the controllers. Landing the shield helped a lot. If the building is that small why not just run new cable? If the existing cable wasn't run correctly you probably don't want to use it anyway. If you are not replacing a running system you can pull the new in with the old if it was installed correctly.
    We looked at the option of re-running, but it's a wood frame building with a lot of drywall and some of the network runs go through the slab and up into the walls and are stapled to the studs. The existing system works, but the canbus drops about once a week and requires a power-cycle to get the system going again. It seems like it doesn't recover by itself as soon as there is a network error.

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    Quote Originally Posted by zzip7 View Post
    I am looking at doing a project in a pretty small building with about 35 devices on a daisy chain network. The existing system is using canbus on a run of crappy 18AWG solid unshielded wire (probably ~800ft) and there are some communication issues. In my experience BACnet MS/TP usually does ok even on 18AWG solid, although we would typically like to see 22-24AWG low cap shielded of course. But I've never done one with this many devices on the 18AWG solid.

    Does anyone have any experience deciding what sort of baud rate you'd select based on the wiring? We generally do 76.8k, but in this case I'm thinking it would be safer to go with 19.2k or so. I am also planning to split the run in 2 as lucky the main control panel is in the middle so that's fairly easy to do.

    Any advice would be appreciated, Thanks.
    Good day zzip7,

    Bacnet MS/TP is electrically an RS485 (EIA-485) bus and it does not care what gauge of wire is used. The key parameters are capacitance, inductance, and the characteristic impedance (Zo). Using non RS485 cable can certainly be used and at times can be fine (except for being less electrical noise tolerant), but it entirely depends upon how much electrical noise is present where the cable is run and also how well the equipment's Bacnet MS/TP (RS485) is designed. If you have the right equipment you could make even uber crappy cable work, by tuning the cable using resistors, caps, and inductors. As an example I had to retro-fit some new gear (RS485 based) using the original 40+ old cable. Replacement of the cable was not an option given the size of the building and the cable runs. However, because I was in control of the new gear (my design) I could embed the necessary tuning elements. The result is that I am running this old cable at 460Kbaud with zero issues and I could actually push this even faster, but it is not needed (response time is already instant). Also, the cable runs are 300 to 500 feet. The network transmits and receives over 185000 packets a day with zero losses... and so it is indeed possible.

    For your application, the slower the baud rate the better, as this will allow for any transmission line effects (reflections, etc) to subside before the next packet and thus minimize packet data corruptions, retries, offlines, etc. Also I would not start off with any termination impedances (resistors, caps, etc) unless you have verified the signal integrity with a time domain reflectometer (TDR) or an Oscilloscope. If you have access to a TDR or a scope you can include the termination impedances and observe the signal integrity and decide if it is better or worse. Simply start with the lowest baud rate you need and check the corresponding packet data to see if there are any issues. If all is fine, then you can throttle up if needed.

    Good luck!

    Cheers,

    Sam

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    Quote Originally Posted by control$ View Post
    I have done many 18/2 not cap wiring jobs over the years. But the customer has to know the limitations before hand. Use 9600 or 19.2K and keep network traffic at a minimum and you'll be ok for SIMPLE systems.
    This is exactly my experience. 9600 is often good on most anything. 19.2 is where I end up often in many scenarios, and 38.4 happens if the planets line up right.
    Of course, the gremlin taking it down now may have other ideas for you. Be up front with the customer that he is asking you to do miracles.... this is not only not normal, but not generally a good idea.
    Hmmmm....smells like numbatwo to me.

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    Quote Originally Posted by s2sam View Post
    If you have the right equipment you could make even uber crappy cable work, by tuning the cable using resistors, caps, and inductors.... The result is that I am running this old cable at 460Kbaud with zero issues and I could actually push this even faster,
    I LUV this!
    Sam, is any of the equipment you speak of in some kind of package somewhere? 2 or 3 wires sticking out and a dial you can turn to dial it in while watching an Oscope?
    Seems like you could make a killing with something like that.... just sayin.....
    Hmmmm....smells like numbatwo to me.

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    Quote Originally Posted by numbawunfela View Post
    I LUV this!
    Sam, is any of the equipment you speak of in some kind of package somewhere? 2 or 3 wires sticking out and a dial you can turn to dial it in while watching an Oscope?
    Seems like you could make a killing with something like that.... just sayin.....
    Good day numbawunfela,

    Quote Originally Posted by numbawunfela View Post
    I LUV this!
    Indeed, it is pretty slick what can be done with RS485. There are some that like to bash RS485 for being old, etc... but the reality is that it can work extremely well if one understands the technology and its limits. Depending one's RS485 design you can get 20 Mb/s out of it if one really needed. Like most technologies, once selects the best one for the job and so I use RS485 where it makes sense to do so. For this customer, they saved well over $50K in re-cabling costs, no down-town, no building or area disruption, and I could upgrade the building in stages... A huge win-win I would say.


    Quote Originally Posted by numbawunfela View Post
    Sam, is any of the equipment you speak of in some kind of package somewhere? 2 or 3 wires sticking out and a dial you can turn to dial it in while watching an Oscope?
    Sadly, no. Each piece of equipment provides data and feedback to the user that they then use to determine the next steps. Adding to this the user needs to be needs to adjust or setup the equipment in a manner that allows them to see the information that they a re looking for...so some level of training and experience with the equipment is necessary. Then there is the huge diversity in cabling, site wiring, and also the connected field devices that a user can experience.

    Quote Originally Posted by numbawunfela View Post
    Seems like you could make a killing with something like that.... just sayin.....
    It is hard to say what the market is for such a device, but I have been working on some new test gear that will be quite beneficial to those that do not have access or know how to use such specialty test equipment. However, it will be a while before the device is in the Wild, but once I get closer to releasing it I will let you know.

    Cheers,

    Sam

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    Can you suggest a starting point to learn about this wizardry?

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    Quote Originally Posted by zzip7 View Post
    The existing system works, but the canbus drops about once a week
    I'm always amused at the varying definitions of "works". Either it has a period after it or it doesn't work.

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    Too much data polling on a non cap wire does funny things.
    Controls, the cause of... and solution to... all your HVAC problems.

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    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.

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    Quote Originally Posted by s2sam View Post
    Indeed, it is pretty slick what can be done with RS485.
    It is hard to say what the market is for such a device, but I have been working on some new test gear that will be quite beneficial to those that do not have access or know how to use such specialty test equipment. However, it will be a while before the device is in the Wild, but once I get closer to releasing it I will let you know.
    I am gonna put it on my calender to bug you in a year or so. Hehe.
    I was at a site in midtown Manhattan and had just got done taking measure of static pressures, amp draws, and fan laws to resize a pulley for an exhaust fan for a kitchen to get it working at full capacity. I was leaving to go on vacation, and so I was passing it along to the boss. So I told him they needed a new pulley, grainger part # blah blah, and it needs to be turned 2.5 turns open from the fully closed position when it gets installed.
    'How could you possibly know that?' He asked, dumbfounded. He had been in the HVAC trade for 30 years and had not even a vague concept of the things I was talking about. So I gave him a thumbnail sketch.
    Good to know the fundamentals. Then possibilities open up.
    I was on a site where someone was foolishly running 22 drives via bacnet on 3 trunks in a chiller plant. There had been all sorts of gymnastics to get it working. One day with an oscilloscope and some 20 cent digikey parts and everything is online solid. The person who designed it is unemployed now, but the drives talk.
    Hmmmm....smells like numbatwo to me.

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    Quote Originally Posted by MaxBurn View Post
    I'm always amused at the varying definitions of "works". Either it has a period after it or it doesn't work.
    Fair. In my world, I would not accept a system that drops regularly, and I would definitely not expect my customer to accept it.

    I guess in this case my thought is that it means the wire isn't completely useless (shorted, broken, etc).

    Sent from my SM-G950W using Tapatalk

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    Quote Originally Posted by fjr2007 View Post
    Can you suggest a starting point to learn about this wizardry?
    Good day fjr2007,

    It really depends upon your current knowledge case. At a minimum one needs to have a solid grasp of electricity, physics... basically electromagnetic understanding. Once here then a solid application of transmission line theory is what is needed. Remember that these areas of knowledge are not specific to RS485/Bacnet/etc but provide the foundation for understanding how the communication signal behaves on wires.

    The majority of people/techs view and consider wires based upon their low frequency or DC properties like resistance. However, as the frequency increases other properties of the wire like Capacitance and inductance affect how the transmitter "sees" the wire and so it is these properties that affect how the communication signal is affected as that signal propagates down the wire. When I mention this to most sparky's they are in disbelief until I do a simple test on some wire. Here I have a spool of some basic communication wire and leave the far end unconnected. To the Sparky the wire has no continuity or has infinite resistance (impedance)... which is 100% true for low frequency or DC signals. However, once I place my TDR on the wire I can tell the sparky the cable length (within +/- 3 feet or so) without altering the cable. Also, I can provide wire length distances where devices are connected and also where an aggressive wire bend was done... or if the cable is water soaked, etc... all this without terminating the cable. Once they see this info they are truly amazed at this "sorcery". Forever after they do not look at all wires as simple resistive elements, but now are far more respectful of the wire when used in communication applications.. and explains why there is specific wire for RS485, Ethernet (e.g. Cat cable), LON, etc. With that said using the "correct" wire for comms does not always guarantee success, but what id does do is remove one variable in what could be causing or contributing to site communication issues.

    Cheers,

    Sam

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    Quote Originally Posted by numbawunfela View Post
    I am gonna put it on my calender to bug you in a year or so. Hehe. <snip>
    Good day numbawunfela,

    I am always the optimist... however, the realist in me realizes that no matter how much I plan or set aside for these projects, they seem to get bumped because of paying customers ... To try and offset this a bit, I try and convince the customer that they "need" one of the devices I am working on and if not, then I try and use the same core technology that I would be using for the internal product. This works well, but still stretches out the development schedule. If only I could get 36 hours of time in a day and then maybe I could catch up.

    Cheers,

    Sam

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    Quote Originally Posted by s2sam View Post
    Good day fjr2007,

    It really depends upon your current knowledge case. At a minimum one needs to have a solid grasp of electricity, physics... basically electromagnetic understanding. Once here then a solid application of transmission line theory is what is needed. Remember that these areas of knowledge are not specific to RS485/Bacnet/etc but provide the foundation for understanding how the communication signal behaves on wires.

    The majority of people/techs view and consider wires based upon their low frequency or DC properties like resistance. However, as the frequency increases other properties of the wire like Capacitance and inductance affect how the transmitter "sees" the wire and so it is these properties that affect how the communication signal is affected as that signal propagates down the wire. When I mention this to most sparky's they are in disbelief until I do a simple test on some wire. Here I have a spool of some basic communication wire and leave the far end unconnected. To the Sparky the wire has no continuity or has infinite resistance (impedance)... which is 100% true for low frequency or DC signals. However, once I place my TDR on the wire I can tell the sparky the cable length (within +/- 3 feet or so) without altering the cable. Also, I can provide wire length distances where devices are connected and also where an aggressive wire bend was done... or if the cable is water soaked, etc... all this without terminating the cable. Once they see this info they are truly amazed at this "sorcery". Forever after they do not look at all wires as simple resistive elements, but now are far more respectful of the wire when used in communication applications.. and explains why there is specific wire for RS485, Ethernet (e.g. Cat cable), LON, etc. With that said using the "correct" wire for comms does not always guarantee success, but what id does do is remove one variable in what could be causing or contributing to site communication issues.

    Cheers,

    Sam
    Do you know of a class anywhere that covers this material? Very important. We have scopes that never get used and no TDR, it's been decades sense I've used that equipment myself.

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    Quote Originally Posted by fjr2007 View Post
    Can you suggest a starting point to learn about this wizardry?
    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

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  22. #19
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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.
    Thanks jschulze!
    Hmmmm....smells like numbatwo to me.

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    Quote Originally Posted by MaxBurn View Post
    Do you know of a class anywhere that covers this material? Very important. We have scopes that never get used and no TDR, it's been decades sense I've used that equipment myself.
    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

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