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Problem with recently purchased MAX232IN

frolic
Prodigy 90 points

After a week working with my MCU RS232 interface, I was always able to get the terminal to respond to an ASCII string, but the MAX232I it would not allow data to be received by the MCU. I placed a scope and observed that the low level was 2.5 volts. I tried 2 other chips from the batch I bought from either Mouser or Digikey.  Same result! I found in my collection a MAXIM RS232 and placed it in my circuit and to my surprise, the low level shifted to 1 volt and the MCU was able to receive the data. According to TI data sheet this low level should be at MAX 0.8 volts. What is going on here. Do I have a ground loop problem?

  • 0
    TI__Guru**** 164726 points

    Frolic,

    Are you using the same setup as figure 4 in the data sheet? The left signal are logic level and the right side is the RS232 port signals.
    The +/-8.5V label is for reference only. The device generate these voltages automatically.

    Which pin has the wrong voltage?

    Regards,
    Ron Michallick

  • 0 in reply to Ron Michallick
    Prodigy 90 points

    Thank you for your rapid response. Yes, I  did use that setup. That was not my problem. I  I found the problem this morning. It appears that I had a fine (and I mean fine) solder bridge between the RX and TX lines on my micro controller pins. Not a short but effectively  a pull up resistor of a few K. Don't asked how I found it, but I did. All works according to specs.

    Also with regards,

    John Paul Froehlich (Frolic)