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TRS3122E: DOUT not going negative

zack polizzi
Prodigy 20 points
Part Number: TRS3122E

I am debugging a circuit with a TRS3122E connected to a microcontroller. The RS232 output voltage ranges from 0V to 5V instead of the -5V to 5V I would expect. Attached is the schematic and scope trace. I would appreciate suggestions on how to debug this issue.

  • 0
    TI__Guru 62910 points
    Hi Zack,

    What do you see on the V- pin? It should normally have a voltage lower than -5 V. If you don't see a negative voltage on this rail, then the issue would most likely be related to the charge pump circuitry. If that is the case, I would check the capacitor connections and make sure that any polarized capacitors are installed with the proper polarity.

    If V- looks OK but still DOUT does not go negative, perhaps the negative voltage is getting clamped by something else -- are there any other components connected to DOUT? Or, are you sure that the logic analyzer/probe you are using supports negative inputs? (If DIN were a constant value, you should be able to double-check the measurements using a voltmeter if you suspect a measurement error is possible.)

    Regards,
    Max
  • 0 in reply to Max Robertson
    Prodigy 20 points
    Thanks, my logic analyzer analog input apparently only handles positive voltage (but reports slightly negative voltages also..). Looks like that was the issue and not the chip or circuit.