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  1. #1
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    OEM bacnet terminations

    I see Carrier calls for a BT485. Are there any OEM bacnet terminations that are different than a 120 ohm resistor?

  2. #2
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    ALC also uses spongebobs. I have seen EOL termination devices for Reliable Controls (for use with other manufacturers devices if a RC controller is not at the end of a segment), Delta Controls and Trane. In a pinch a 120 Ohm resistors should work. Good rule of thumb is to have matched EOLs on a segment.

    kontrol out
    "Good" - Jocko
    "Open is as open does." - Forrest Gump
    "Can't we all just get a Lon?" - Garry Jack
    "BACnet: integration or interrogation?" - The Janitor
    "Interoperability? You can't handle interoperability!" - Nathan R. Jessup
    “What’s that? Aaa… open protocols? Don’t talk about…. open protocols? Are you kidding me? Open protocols? I just hope we can hardwire an interface!” - Jim Mora Watch it here!

  3. #3
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    For your standard BACnet MSTP network using run of the mill RS-485 cabling, you want to match the termination with the characteristic impedance of the wire. In most cases, that's 120 Ohm. I've seen some sites try to use Belden 9463 (Blue Hose cable) with RS-485 devices. It'll work, but the characteristic impedance is 100 Ohm for that type of wiring so it's important to match it up.

    When I used to work for Auto-Matrix, their PUP controllers used 249 Ohm terminating resistors. This was based on the serial drivers that the hardware engineers selected for use with a common platform that supported both PUP and BACnet MSTP. I personally think it was overkill since both protocols are not FSK-based and 120 Ohms would have worked perfectly fine.

  4. #4
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    120 ohm should work in most cases.
    if you want to get real fancy look at this:
    https://www.ccontrols.com/pdf/ds/DS-485TER00-AA0.pdf

  5. #5
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    Carrier does sell them, part number is TERM485 (qty 26). They are a 120ohm resistors. We like them better as they are in hard plastic similar to heat sink.
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  6. #6
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    I have ordered 300 120 Ohm resistors from eBay for a fraction of the cost of buying them from an OEM control company. Use them on generic IEA-485 busses. Also I have seen EOL resistors bring a network down, if they do this and the network functions without them leave them off. If you have an oscope view the network characteristics either them on and off and make an educated decision.

    kontrol out
    "Good" - Jocko
    "Open is as open does." - Forrest Gump
    "Can't we all just get a Lon?" - Garry Jack
    "BACnet: integration or interrogation?" - The Janitor
    "Interoperability? You can't handle interoperability!" - Nathan R. Jessup
    “What’s that? Aaa… open protocols? Don’t talk about…. open protocols? Are you kidding me? Open protocols? I just hope we can hardwire an interface!” - Jim Mora Watch it here!

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