Inputs requiring external resistor/jumper? - Page 2
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  1. #14
    Join Date
    Apr 2008
    Location
    Madison, WI
    Posts
    323
    Quote Originally Posted by sysint View Post
    JustJohnson- Ask yourself which method is more susceptible to inaccuracy over distance? (0-10 or 4-20)

    --- now somebody here will tell you I'm just sales... but ignore that guy for now.
    It's true. But it's TECHNICAL sales.

  2. #15
    Join Date
    Nov 2006
    Location
    New Hampshire
    Posts
    255
    now maybe I'm in for a lesson here - but aren't the dipswitch/jumpers on some controllers just adding/subtracting the resistor into the electrical circuit before the A/D converter just like I add an external resistor? All controllers actually measure voltage - even when using a resistance temp sensor - they send out a reference voltage and the returning voltage calculates to a resistance which equals a temperature. I've not had accuracy problems with mA devices with an external resistor at the controller. Now a device that generates a voltage signal at the device over long runs (>200+ feet) I agree with you you get resistance added because of the wire run and your voltage signal is skewed. Perhaps I'm missing something but that's the way I've always done it and I don't have problems with it.

  3. #16
    BACnet is offline Removed by Admin- No Return
    Join Date
    Aug 2009
    Posts
    2,459
    JJ- You are 100% correct.

    Everyone on this board knows that a current signal won't degrade since it's a closed loop system.

    Your assumptions about resistors are right on the money- You can add them yourself or switch/jumper them in if the manufacturer spent the extra 15 cents to make the input selectable. Either way there is no difference electrically.

    Of course you want to make sure you're using a resistor that's +- 1% or less- Those cheap 5% resistors are never worth it.

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