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Problem interfacing Arduino with an rs422 GPS receiver.

Discussion in 'Microcontrollers, Programming and IoT' started by HellasTechn, Jul 14, 2019.

  1. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Hello !

    Im back here with yet an other mysterious problem. A couple of years ago i started building a project that involves an arduino mega and a GPS receiver that has RS422 output at 4800 bps baud rate. Unfortunately the arduino use serial communication TTL level. I have builded a pcb where A and B lines from the GPS are connected on a sn65hvd82 converter chip that in turn feeds the arduino serial TTL input. there is nothing special about my design and no other external components other than a 0.1uf decoupling capacitor between the converters VCC and GND.
    I programmed the arduino to echo back to my computer all the data that receives from the GPS.
    After running my code i noticed that the arduino is having hard time receiving the data. while has no problem at all receiving exactly the same data from an other arduino (no converter or actual GPS involved). I took the Oscope and checked the output from the GPS lines A and B with respect to ground and they looked kind of curved. They where not square as i would expect. The output of the converter looked the same.
    I understand the output of the GPS to be kind of distorted due to the cable etc. but should the sn65hvd82 converter not produce a perfectly square signal ?
    Maybe someone had a similar problem ???

    P.S. I forgot to mention that i did not use terminator resistor between the A and B lines.
     
    Last edited: Jul 14, 2019
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    The information is in the difference between lines A and B, so check the differential voltage. You may use the 'subtract' math function of an oscilloscope when measuring both A and B versus GND.
    That sounds like a malfunction. A possible cause is too much capacitive load on the line.

    Show us a circuit diagram for better understanding, please.
     
  3. HellasTechn

    HellasTechn

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    Apr 14, 2013
    I will make a diagram yes. I do not think that it is the effect of capacitive load because the GPS output looks exactly the same even with nothing connected to it. Unless if you are referring to the Converter output. I will also try to post Oscope pictures. Thanks!
     
  4. HellasTechn

    HellasTechn

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    Apr 14, 2013
    I am still investigating this issue and so far i found no proof that it is a hardware problem. I re scoped the input and output signals with better resolution and they appear to be normal.

    I am also investigating software issues that might be involved.
    This post might be "false alarm"

    Ill write back as soon as i sort it out.
     
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