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RS-232/TTL encoder

Discussion in 'Troubleshooting and Repair' started by foTONICS, May 23, 2012.

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  1. foTONICS


    Sep 30, 2011
    Hey all,

    I have a DTE/DCE board which takes a TTL signal from one end and translates it to a RS-232 signal (-15/+15 out of phase signal). These boards are already made, I'm just testing them to make sure they are still functional.

    I have a Tektronix osciliscope monitoring the TTL signal on channel 1 and the RS-232 signal monitored on channel 2.

    Typically I just touch coinciding pins and see if I have the appropriate voltages, but a couple of the boards behave funny. When I apply the TTL input to the input pin the voltage drops drastically (about a volt, maybe less). but when I take it off the input it returns to normal (I also get no RS-232 signal out).

    So I have checked visually, looking for shorted out traces and such but find none.

    I have noticed though that whoever built this board has bad soldering practices. There are quite a number of those blobbed on cold solder joints.

    I guess my question would be could cold solder joints with contaminants inside attenuate a signal like that? My first assumption was that There was a short to ground since I was losing the signal the second it touched the input pin, is that a safe assumption?

    -Thanks guys!
  2. peter1978


    Jun 12, 2012
    no,I don't think so.

    {link removed}
    Last edited by a moderator: Jun 12, 2012
  3. davenn

    davenn Moderator

    Sep 5, 2009
    I almost deleted your whole post, peter, rather than just the possible spam link

    a response like you gave isnt very helpful to the poster, would you care to elaborate as to why you dont think so, so that he can learn something :)

  4. gorgon


    Jun 6, 2011
    Normally you use an output to an input, but this is maybe just a way of talking.

    You should check out the driver chip to see if all voltages are present. Depending on how old the boards are there may be several voltages coming in, or they are made on the board. The driver chip will need a +3V3 or +5V, plus the driver voltages, at least the negative voltage (-5 to -12V) maybe an additional positive voltage (5 to 12V)

    If the board is very old you'll have a set of 1488/1489 driver/ reciever. Those will need +5, +12 and -12V. If more recent a xxx232 like chip, it will generate the voltage with chargepumps and some capacitors. If the charge pump is made with electrolytic capacitors, they may dry out and stop working, with no voltage, or only one present.
    They have 2 charge pumps, one that double the 5V, and one to invert the resulting +10V to -10V.

    The cold solder joint could make the supply voltage erratic, and if you load an output with a non powered chip, you may very well see a drop in signal, and no output.
    It's all depending on the type of driver used.

    TOK ;)
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