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Serial Communcation Conversion / Noise Coupling

  1. May 3, 2013 #1
    I have a two way radio that I'm trying to control remotely through 120 ft of 25 conductor cable at 22 gauge. The radio uses serial communication between the actual transmitter and control head unit. After I wired the pin's up accordingly, there is to much noise pickup through the cable and it doesn't function properly. I'm trying to come up with a solution on how to fix and these are the solutions I have found. I'm wondering if someone can provide me with additional help or which might be the best way to go.

    1. Place ferrite beads around the cable about every 10 ft or so and try to remove the noise.

    2. Try and design a AC coupled circuit that passes the pules and removes any AC hum or interference. (If this is the way to go, can someone give me a good starting point on how to get this going?)

    3. Possibly convert the serial communication to some other form of transmission which noise has minimal effect.

    4. Try to design a wireless control for the serial data. ( I would also need some help on this)


    Thank you for the input..

    - Andrew
     
  2. jcsd
  3. May 6, 2013 #2
    On a really long cable run usually the problem is with the wire capacitance itself. What this does is distort the sharp edges of a digital signal to make it look more and more sinusoidal (rounded/sawtooth instead of square). At some point the receiving circuit can no longer recognize the signal.

    If this is the problem (and I would bet it is) then 1 and 2 really can't help you because it isn't outside interference that is your problem but a distortion of your original signal.

    You can try to get low capacitance wire that should help.


    3 and 4 could both work. Ethernet could work, I think it is good up to 100 meters or more. Wireless works over that range as well.
     
  4. May 7, 2013 #3

    berkeman

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    To communicate that far, you need to use a twisted pair for each communication channel, and it needs to be terminated at the receiving end.

    Can you say more about what-all you are doing with the cable? Why so many conductors?
     
  5. May 7, 2013 #4
    Thank you for the replies. Berkeman, I'm using this cable to remotely control the two way radio remotely. If you remove the faceplate, there is 25 pin total connector that runs to the actual transmitter unit. I'm not sure what each conductor does, but every one of them is wired between the control face plate and the actual radio. When I tried this, the radio came on and worked fine but I can hear slight noise coming through the audio along with the (2) 7 segment displays that aren't functioning properly. My next idea was to try and get 100 ft of actual DB-25 cable and see if it will work that way.
     
  6. May 7, 2013 #5

    berkeman

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    Hmm. That sounds like it may be difficult to do. There might be a way to make converter boxes at each end, with good quality serial communication on a set of twisted pair cables run between the converter boxes. Can you find the pinout and signal description, or is it likely a proprietary interface? What band is the radio for?
     
  7. May 7, 2013 #6
    It is a Motorola Radius 16 ch 40 watt radio. It covers the 70cm ham band. I do have a pin out that shows what each pin is used for. Does twisted pair cable actually perform better in terms of capacitance buildup throughout longer distances?
     
  8. May 7, 2013 #7

    berkeman

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    Capacitance can be an issue for much longer distances, or for higher frequencies (like the Ethernet that you alluded to in an earlier post). But for your distance, attenuation should not be an issue if the twisted pair transmission line (TPTL) is properly driven and terminated.

    What are some of the signals? Can you post a scan of the pinout diagram and pin descriptions?
     
  9. May 7, 2013 #8

    Attached Files:

  10. May 7, 2013 #9
    Since I have the multi channel radio, it looks like I'm trying to correct the communication in pins 12 and 13. All the other functions seem to work right. I can key the transmitter and see the tx light and receive light come on but the display and channel select isn't functioning.
     
  11. May 7, 2013 #10

    berkeman

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    It looks like you should drive the Display Clock & Display Data (and maybe the Enable signal too) as individual twisted pairs with terminations at the far end. You can use RS-485 differential drivers and receiver ICs to do the single-ended to differential conversion at both ends. Use Cat-5 cable to give you multiple twisted pairs in one sheath. You might also want to do the same with the SCI+ signal, if that is doing downlink TX comm in a RS-232 fashion.

    I'll try to look closer later tonight. Is there a more detailed pin description somewhere in the docs?
     
  12. May 7, 2013 #11
    That's about all I could find for the information. I'm trying to understand a little bit more on the possible scenario you gave me. Using my RS-232 DB 25 communication output, convert that to RS-485 and transmit those signals over CAT-5 twisted pair cable, then on the receive end convert the RS-485 signals back to RS-232? Am I understanding that correct?
     
  13. May 7, 2013 #12

    berkeman

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    Since the display has clock and data lines, that is not RS-232 communication. And for the SCI+ line, if that is RS-232 communication, it should work over that distance, I believe. The problem comes with unterminated digital lines -- there will be too much ringing to allow reasonable comm.

    Do you have an oscilloscope to be able to look at some of the lines if we need to?
     
  14. May 7, 2013 #13
    I do have an older tektronix oscilloscope. It's not one of the newer, nicer digital displays that has all the measurement features. I'll try to get that hooked up and get some images and data on what exactly I'm looking at.
     
  15. May 9, 2013 #14
    UPDATE: I have one of the 2 channel units that do not rely on the clock signal and data signal.. I'm going to hook one of those up and see if it works on that unit.
     
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