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  1. #1
    Join Date
    Apr 2008
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    Confused MODBUS RS-485 cable???

    I am working on a project which requires MODBUS integration to Automatic Transfer Switches. I can't recall the ATS manufacturer but we were told that they recommend we use one of the following RS-485 cables:

    Belden 89729
    Belden 82729
    Alpha 58902

    I'm not sure about the last two yet but the cost for 1000' roll of Belden 89729 is over $3K The Belden 89729 specs are shown below. Is there something special about this cable which may be critical for this vendor? I didn't believe there was anything special about MODBUS RS-485 cable. I'm looking for a MUCH cheaper replacement. Any help would be greatly appreciated. Thanks.

    24 AWG stranded (7x32) TC conductors, plenum, foam FEP insulation, twisted pairs, individually shielded
    w/Beldfoil® (100% coverage), 24 AWG stranded TC drain wire, overall fluorocopolymer jacket.
    Physical Characteristics (Overall)
    # Pairs AWG Stranding Conductor Material Dia. (mm)
    2 24 7x32 TC - Tinned Copper 0.6096
    Total Number of Conductors: 4
    Insulation Material:
    Insulation Material Wall Thickness (mm)
    FFEP - Foam Fluorinated Ethylene Propylene 0.483
    Inner Shield
    Inner Shield Material:
    Inner Shield Trade Name Type Inner Shield Material Coverage (%)
    Beldfoil® Tape Aluminum Foil-Polyester Tape 100
    Inner Shield Drain Wire AWG:
    Inner Shield Drain Wire Stranding: 7x32
    Inner Shield Drain Wire Conductor Material: TC - Tinned Copper
    Outer Jacket
    Outer Jacket Material:
    Outer Jacket Material Nom. Wall Thickness (mm)
    PVDF - Fluorocopolymer 0.432
    Overall Cable
    Overall Nominal Diameter: 6.629 mm
    Pair Color Code Chart:
    Number Color
    1 White/Blue & Blue/White
    2 White/Orange & Orange/White
    Mechanical Characteristics (Overall)
    Operating Temperature Range: -70°C To +150°C
    Bulk Cable Weight: 40.181 Kg/Km
    Max. Recommended Pulling Tension: 146.791 N
    Min. Bend Radius/Minor Axis: 69.850 mm

  2. #2
    Join Date
    Feb 2005
    Edmonton, AB Canada
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    Good day,

    Ideally RS485 should have the following specs:

    twisted-pair with an additional wire for Reference which does not need to be twisted
    Characteristic Impedance (Zo): 100 to 200 ohms
    Mutual capacitance <= 12.5 pF/Foot
    Capacitance to Shield <= 60 pF/Foot.
    Propagation velocity >= 60%

    If the wire you select does not provide a spec for its Characteristic impedance then it is not a communication cable.

    Will RS485 work with cables that do not meet the above specs? Sometimes... but will be site (background electrical noise), installation (daisy chained, stubs, cable length, etc), and equipment (how many devices, data rate, etc) specific.



  3. #3
    Join Date
    Mar 2005
    Pacific Time Zone
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    I would make calls to different wire manufactures/vendors and tell them you need bids for that type of cable or equivalent.
    We normally run a 2 conductor cable, likely very similar to what your spec is calling for, when I first used it the cable made me think of coax cable. I know the stuff isn't cheap, but I'll try to get more details later if I can.
    Doing a quick search - 4 conductor, but nowhere near the same FEP.
    "How it can be considered "Open" is beyond me. Calling it "voyeur-ed" would be more accurate." pka LeroyMac, SkyIsBlue, fka Freddy-B, Mongo, IndyBlue
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  4. #4
    Join Date
    Jun 2004
    Eugene, Oregon
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    Siemens uses anixter cable.

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