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  1. #1
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    BACnet Network Wiring

    I am curious about terminating resistors in a BACnet network. I have been told several conflicting ways to use terminating resistors and am hoping someone can clear up how, when, and what to use.

    I am currently under the impression you use a terminating resistor at the end of any network. I typically use the EOD resistor built into the equipment. Not quite sure if I should be using an external resistor, if it needs to be on both sides of the network, and how to chose what size to use.

  2. #2
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    If the equipment has a select-able terminating resistor, then you can use that. If the equipment does not, you have to add your own. You have to have each end terminated with a resistor.

  3. #3
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    On multi-vendor networks it is imperative to understand how each vendor does stuff. Some have built in terminating resistors, others don't - but you must find out and disable all but the end of line ones. I prefer to disable all the built-in stuff, and use physical resistors.
    For the record, we have (3) Fluke Oscilloscopes at my office; I don't use them regularly, but I always do if I am having comm issues, or if I am starting up a multi-vendor trunk.

    Be aware that some VFD's have 'dynamic' end of line resistors. If you are using one of their eol resistors, and you power off the drive - the resistor effectively disappears. Now you only have a resistor on the other end of the trunk...

  4. #4
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    Tech what system are you working with? I've been reading recently a lot of Bacnet wiring manuals for different manufacturers and it seems everyone says something different about something.
    Quote Originally Posted by MatrixTransform View Post
    very soon it is you that will be pwned

  5. #5
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    My most current system has components from several different manufacturers including ACI, Honeywell, Carrier, Grundfos, and Lochinvar.

  6. #6
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    Generally speaking terminating resistors are placed at both 'ends' of the entire trunk. In my experience with working on Delta Controls, terminating resistors are left off unless there are comm problems that can't be remedied by any other means... then I add some terminating resistors to see if things get better.

    Other systems I have worked with like Automated Logic pretty much require the use of their 'sponge bobs' (terminating resistors).

  7. #7
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    Quote Originally Posted by TheControlsFreak View Post
    Generally speaking terminating resistors are placed at both 'ends' of the entire trunk. In my experience with working on Delta Controls, terminating resistors are left off unless there are comm problems that can't be remedied by any other means... then I add some terminating resistors to see if things get better.

    Other systems I have worked with like Automated Logic pretty much require the use of their 'sponge bobs' (terminating resistors).
    Bingo !!!!

    The man wins a prize.

    If you don't know, and if it's a mixed bag of stuff ... leave the terminating resistors off. Unless you have a problem. Then try them.

    Otherwise, if it is all one maker's kind of stuff, follow their rules.

    Although ... fact is I've done that and the maker demanded terminating resistors in ALL cases, and the result was ... they were WRONG. Short runs and/or few controllers ... you got better results without the terminating resistors. LONG run, and/or LOTS of devices ... better with terminators engaged.

    Just my experience with these things. I leave all terminating resistors and bias off ... unless I have problems.

    The very reason it is important to know how the comm run goes. So yah know where EOL is, etc.
    A site where I stash some stuff that might be interesting to some folks.
    http://cid-0554c074ec47c396.office.l...e.aspx/.Public

  8. #8
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    Quote Originally Posted by osiyo View Post
    Bingo !!!!

    The man wins a prize.

    If you don't know, and if it's a mixed bag of stuff ... leave the terminating resistors off. Unless you have a problem. Then try them.

    Otherwise, if it is all one maker's kind of stuff, follow their rules.

    Although ... fact is I've done that and the maker demanded terminating resistors in ALL cases, and the result was ... they were WRONG. Short runs and/or few controllers ... you got better results without the terminating resistors. LONG run, and/or LOTS of devices ... better with terminators engaged.

    Just my experience with these things. I leave all terminating resistors and bias off ... unless I have problems.

    The very reason it is important to know how the comm run goes. So yah know where EOL is, etc.
    The RS-485 spec requires termination. The transceivers on the boards are all of differing brands/tolerances, and of course, noise levels vary, but I have never seen an RS-485 trunk work WORSE with termination. It is part of the design for a reason, without them, a high impedance network is far to prone to noise, even a differential one like RS-485. The terminating resisters, in essence, provide a path for current flow, meaning the signal's current flow down the wire is higher, so that the level of current induced by radio interference is less significant. Terminators therefore raise signal-to-noise levels. This may only be a problem with high noise environments - but I fail to see how anyone would make a conscious decision to forgo their use.

  9. #9
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    Quote Originally Posted by osiyo View Post
    Bingo !!!!

    The man wins a prize.

    If you don't know, and if it's a mixed bag of stuff ... leave the terminating resistors off. Unless you have a problem. Then try them.

    Otherwise, if it is all one maker's kind of stuff, follow their rules.

    Although ... fact is I've done that and the maker demanded terminating resistors in ALL cases, and the result was ... they were WRONG. Short runs and/or few controllers ... you got better results without the terminating resistors. LONG run, and/or LOTS of devices ... better with terminators engaged.

    Just my experience with these things. I leave all terminating resistors and bias off ... unless I have problems.

    The very reason it is important to know how the comm run goes. So yah know where EOL is, etc.
    A lot of people don't understanding the different purposes of terminating and biasing of the network, they are two separate things serving separate functions.

    Kevin
    "Profit is not the legitimate purpose of business. The legitimate purpose of business is to provide a product or service that people need and do it so well that it's profitable."

    James Rouse

  10. #10
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    Thanks for all the information guys. I definitely learned a lot.
    Last question. How do I size EOL resistors?

  11. #11
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    Quote Originally Posted by HuNGRYTeCH View Post
    Thanks for all the information guys. I definitely learned a lot.
    Last question. How do I size EOL resistors?
    Good day HuNGRYTeCH,

    Ideally and in Theory the EOL should be the same as the Characteristic Impedance (Zo) of your communication wiring. This value is usually anywhere from 100 to 200 or so ohms. 120 ohms is considered standard, as a lot of communications have a Zo of 120 ohms. If you do not know or cannot find the Zo spec of the wire, then either ask the equipment vendor or just try the 120 ohm standard and observe your communication signals, stats, etc...

    Cheers,

    Sam

  12. #12
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    Thanks Sam, the wire I use is 120 ohms. I wasn't sure if different equipment required different resistors. 120 sounds good to me though.

  13. #13
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    Quote Originally Posted by HuNGRYTeCH View Post
    Thanks Sam, the wire I use is 120 ohms. I wasn't sure if different equipment required different resistors. 120 sounds good to me though.
    Good day HuNGRYTeCH,

    In theory it should not matter, as CraziFuzzy mentioned... however... Some equipment vendors embed or have a self terminating resistor permanently installed... and so by adding another termination resistor in parallel to this type of equipment can also cause issues. Hardwiring a terminating resistor (i.e. not jumpered, etc), etc is frowned upon but does happen from time to time with vendors. Ideally contact the equipment vendor to see if they have embedded a termination resistor and if so find out its value. Then use this value and your target of 120 ohms to determine what your termination resistor should be (i.e parallel combination of your external termination resistor with equipment embedded termination resistor should equal to Zo or in your case 120 ohms) ... In most cases this should not be the case and so using the Characteristic Impedance (Zo) of the wire is where one should start.

    Cheers,

    Sam

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