Confusion about serial data voltage levels?

Discussion in 'Embedded Systems and Microcontrollers' started by MikeML, Jul 7, 2015.

  1. MikeML

    Thread Starter AAC Fanatic!

    Oct 2, 2009
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    I have a "black box" which outputs a poorly-documented Serial data stream. I think the data stream may be coming directly from a PIC UART pin with no intervening Max232 driver chip between the PIC pin and the external connector pin. I cannot open the box. I think the data is 9600baud N81, but I am not sure of that.

    If so, is this data stream "inverted" with respect to the signals expected by the serial port input to an old DELL laptop I intend to use to capture the stream to be able to see the data?

    What is the simplest way of interfacing the data to the laptop?

    A single NPN transistor inverter with a pull-up to a 9V battery?
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    Coming out of the PIC the UART will "idle" at a "high or "marking" level. Traditionally RS-232 drivers and receivers each had an inversion so that at the output of the receiver you would again have a data line which there was at "high" or "marking" level when "idle". The problem with the Dell laptop is that it may or may not be happy with levels that were in the range [0V,+9V] since 0 VDC is actually in "no man's land" with respect to signal levels. Try it to see if it works, otherwise you need to fabricate a TTL level to RS-232 level converter.

    BTW --
    a mark level is any voltage in the range [-25V, -3V]
    a space level is any voltage in the range [3V, 25V]
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    In the past I have made a RS232 interface inside a larger backshell using SMT MAX232.
    Another I made using a couple of 1" PVC Pipe caps cemented together that had a strip board circuit inside and inline with the cord, also used on a DELL LT.
    It sounds as though you may need to convert to RS232 if that is the standard used.
    Most of the UART'S in the laptops need to see the -ve.
    Max.
     
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