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Pull-up resistors

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tim2542(at)sbcglobal.net
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Posted: Tue Jan 04, 2011 11:35 pm Post subject: Pull-up resistors

I'm connecting a GNS430w to a pair of GRT MFD's. GRT calls for 10k     ohm pull up resistors to be used on 4 of the interconnects between     the MFD's and the Garmin, these resistors are fed with 12VDC and

      "T" 'd into the harness.        http://www.grtavionics.com/File/Wiring%20Diagrams/WD-1007-01.pdf      

     All this equipment provides for multiple feed paths, and I have fed     all 3 from the E-Bus and my Aux bus. I have read that the Garmin     will be damaged if these pull ups are not used. So to prevent this     in the case of power from the AUX only I built a little pull up     network using on a DB25 for a "Home" (Thanks Bob) and fed it from     both bus's (E & AUX). Each bus has a 1N4004 diode installed     prior to feeding the 4 resistors to prevent the bus's from back     feeding. Still with me? 

     OK so I check it out when done with my Ohmmeter and discover that     instead of 10K ohms I have 326K ohms. All but 10K of it of course is     from the Diodes. I knew the Diodes had a forward voltage drop, but     didn't realize it was this much. 

     What to do? 

     Thanks, Tim Andres

     

                 

      [quote][b]

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nuckolls.bob(at)aeroelect
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Posted: Wed Jan 05, 2011 5:54 am Post subject: Pull-up resistors

At 02:26 AM 1/5/2011, you wrote:

 I'm connecting a GNS430w to a pair of GRT MFD's. GRT calls for 10k ohm pull up resistors to be used on 4 of the interconnects between the MFD's and the Garmin, these resistors are fed with 12VDC and

  "T" 'd into the harness.  http://www.grtavionics.com/File/Wiring%20Diagrams/WD-1007-01.pdf  

 All this equipment provides for multiple feed paths, and I have fed all 3 from the E-Bus and my Aux bus. I have read that the Garmin will be damaged if these pull ups are not used.



    I kind of doubt this. Some function may not

    behave as intended without a resistor but

    I don't believe there is risk of damage to

    any particular piece of equipment. These

    are signal/control wires with VERY low currents

    and no potential for destructive fault

    currents.



  So to prevent this in the case of power from the AUX only I built a little pull up network using on a DB25 for a "Home" (Thanks Bob) and fed it from both bus's (E & AUX). Each bus has a 1N4004 diode installed prior to feeding the 4 resistors to prevent the bus's from back feeding. Still with me? 

 OK so I check it out when done with my Ohmmeter and discover that instead of 10K ohms I have 326K ohms. All but 10K of it of course is from the Diodes. I knew the Diodes had a forward voltage drop, but didn't realize it was this much. 



    You cannot "measure" the resistance of a diode

    with an ohmmeter. Wire per the diagram. No

    diodes necessary.



    Bob . . .

     [quote][b]

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