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Thread: MSTP Bus wiring

  1. #1
    Join Date
    May 2006
    Posts
    668

    MSTP Bus wiring

    Hello,

    We are looking to retofit a honeywell system to a BACnet system. The current honeywell system is using a 2 wire for the bus communication (Microsac and W1616 controllers).

    Can we keep the existing 2 wire bus for the MSTP communication run?

    Thanks for the pro and con's

  2. #2
    I would suggest you find out the specs on the existing cable. BACnet specifies 30 pico Farads per linear foot for the cable at 9600 Baud Rate. We typically run at 76.8k and use 12.5 pF per linear foot cable. Different controls manufactures specify different cable requirements, some dont list requirments at all. I would suggest you look at your controllers you plan to install and see what they recommend for a wiring specification as well as any equipment with factory mounted BACnet controls and meet or exceed the requirements specified. The shorter the cable run and the fewer controllers as well as the lower the baud rate you can get away with less desirable cable. An option is to break the communcations bus into smaller segments to re-use the existing cable.

    From what I have seen in the field, most of the existing cable on sites is 50pF or higher (assuming an install 15 years old). One might want to also inpsect the cable to verify it was installed properly and not pulled to tight, installed next to or on top of high RF equipment like light fixtures ect.


    In the end you will need to make your decision on if it can be re-used. I am sure there are others on this site that can provide some guidance with their field experience.

  3. #3
    Join Date
    Feb 2005
    Location
    Edmonton, AB Canada
    Posts
    605
    Quote Originally Posted by integrator View Post
    Hello,

    We are looking to retofit a honeywell system to a BACnet system. The current honeywell system is using a 2 wire for the bus communication (Microsac and W1616 controllers).

    Can we keep the existing 2 wire bus for the MSTP communication run?

    Thanks for the pro and con's
    Good day Integrator,

    My suggestion would be to remove the 2-wire cable and install a suitable (i.e. low capacitance communicatoin cable...one that has a Characteristic impedance within its spec and be twisted) RS485 cable (1 twisted pair with an additional, thrid, wire for the reference lead). I say this, as if you install the proper cable you can then be assured that the cable will not cause you grief today or in the future should you wish to operate at higher baud rates. Secondly, the third wire (the reference) is quite important for long term reliability, etc. I have posted about the above information before ad nauseum and so I will not repeat it here.

    If you chose to re-use the existing and possible older 2-wire you run the risk of reduced long term reliability (do differential ground/references), etc. Secondly, the existing wire characteristics may have limitations on its data rate and noise immunity and may overall affect the communication data integrity (i.e. the cable may cause weird communication issues, random offlines, etc).

    Cheers,

    Sam

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