Connect with us

questionable RS-232

Discussion in 'General Electronics Discussion' started by foTONICS, Aug 5, 2014.

  1. foTONICS

    foTONICS

    332
    9
    Sep 30, 2011
    I was probing an RS-232 connection at work and came across this waveform. At first it looked okay but as I zoomed in I noticed there were no nice clean transitions, is it normally this rounded or is this waveform being distorted by something?
     

    Attached Files:

    • 1.jpg
      1.jpg
      File size:
      140.6 KB
      Views:
      159
    • 2.jpg
      2.jpg
      File size:
      175 KB
      Views:
      161
    • 3.jpg
      3.jpg
      File size:
      173 KB
      Views:
      127
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,266
    Nov 28, 2011
    It looks like there's some serious low-pass filtering occurring somewhere! Excessive cable capacitance maybe? Slew-rate-limited driver trying to transmit a high data rate?
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,178
    2,690
    Jan 21, 2010
    Does it work? RS232 is pretty resilient. The voltage swings required for the receiver are generally far smaller than provided by the drivers. This is partially to overcome issues with driving long cables (I expect this is a fairly long cable).

    Having said that, this might not be real RS232 as the signal does not seem to be be swinging from a positive to a negative value. It looks to be swinging from 0 to some higher voltage (around 10V?)
     
  4. gorgon

    gorgon

    603
    23
    Jun 6, 2011
    For me it looks like this is a normal RS232 +/- 12V signal, with some cable RC included.

    From the pictures I would assume you are running 115200 baud, and that is speedy if you have som cable length involved.
    RS232 is defined with max cable length vs max speed, due to the design of the interface.

    After the standard there is a deadband between +/-3V, and your signal is well above that, as far as I can tell.
    I suppose there are no problems wih this communication?
     
  5. foTONICS

    foTONICS

    332
    9
    Sep 30, 2011
    no I don't think there is problems. I don't use this signal, I just happened to be probing it with a scope and saw how rounded everything was. just goes to show how not everything is textbook examples
     
  6. gorgon

    gorgon

    603
    23
    Jun 6, 2011
    The reason for the rounding is a combination of capacitivity in the cable, a small resistance in the cable, and last but not least the 300 ohm serial resistance in the RS232 driver output. Together this will make a lowpass filtering effect on the signal, and the max frequency of the transmission is highly depending on the length of the cable.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-