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  1. #1
    Join Date
    Apr 2003
    Posts
    108

    NiagaraAX-Bacnet units dropping out

    I have some Mcquay units with bacnet cards. 2 separate buildings with 2 rooftop units on each jace. Sometimes it happens a few times a day and sometimes it will go days without happening. The units will drop out and just about when they ramp down, they will start coming back up. Im thinking i am losing comms for second and once it comes back it starts the units back up. When i check the change of state history extensions on the occupancy point, i see a fault around these times. It is happening on both jaces at different buildings.Is there any settings that i can change to prevent this from happening? There are only two units daisy chained on this bus so there should not be any issues with the comm line.

  2. #2
    Join Date
    Jul 2008
    Posts
    1,475
    Quote Originally Posted by droptchevy View Post
    ... There are only two units daisy chained on this bus so there should not be any issues with the comm line.
    Well, that simply isnt true. The number of devices on the wire is not what will be screwing up yr comms.

    MSTP is notoriously crap . check your physical and make absolutely sure of wire type, terminations etc etc.
    1 + 1 = 3 ( *** for very large values of 1)

    ...everybody wants a box of chocolates and long stemmed rose

  3. #3
    Join Date
    Apr 2007
    Location
    San Diego, CA
    Posts
    1,332
    Quote Originally Posted by droptchevy View Post
    The units will drop out and just about when they ramp down, they will start coming back up...
    There are only two units daisy chained on this bus so there should not be any issues with the comm line.
    If you only have three nodes on the network, assuming JACE_X is assigned MAC 1, AHU_Y = MAC 2, and AHU_Z= MAC 3, then the max Master setting would be set to 3. Termination should match the wire impedance. Hopefully you're using low capacitance wiring. Is this a 2 or 3 wire RS-485 setup? Are you using the shield as a shield or the signal common? Does the comm wiring run anywhere near the VFDs or anything high voltage in their control panel?

  4. #4
    Join Date
    Dec 2011
    Location
    Winnipeg,Manitoba
    Posts
    33
    The thing about wiring in the VFD's is its so hard to keep the shielded cable away from the high voltage, what would be the best minimum distance in inches that the cable would not be affected ?

  5. #5
    Join Date
    Mar 2005
    Location
    Pacific Time Zone
    Posts
    4,208
    Depends on the VFD. Some are more noisy than others and then you have the issue of just how 'good' the ground to the VFD actually is. I've tried to make it standard to just run shielded wire to/from VFD's even for LON, which normally doesn't need a shield.
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  6. #6
    Join Date
    Dec 2011
    Location
    Winnipeg,Manitoba
    Posts
    33
    Its a standard that communications to a VFD should be a shielded cable.

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