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Question about adding a read only serial port

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  • kajsde
    Hi, I am referring to adding the serial port page http://www.nslu2-linux.org/wiki/HowTo/AddASerialPort In particular the section - Read only connection
    Message 1 of 3 , Oct 10, 2011
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      Hi,

      I am referring to adding the serial port page
      http://www.nslu2-linux.org/wiki/HowTo/AddASerialPort

      In particular the section - "Read only connection without level conversion". I have a doubt regarding the connections that are mentioned here. Can someone shed some light on what exactly is meant by
      the three connections mentioned below :

      1) What I did was connecting the 5V VCC provided by USB to the VCC pin of the converter
      2) both GNDs (The one of the NSLU2 and the one of USB) to the GND pin and the NSLU2s
      3) TXD pin directly to the converters RXD pin

      As far as I understand - there are 4 pins on the NSLU2 (3.3V, Rx,Tx,GND) and there are two sides of the USB-RS232 converter. The USB side has four pins (5v, +Data, -Data, Gnd) and the serial port side has 9 pins. Can someone tell me exactly what are the connections here - or am i missing something?
    • Rob Lockhart
      Sorry for the delay, I don t often check my messages here. I think you are missing something critical here. The NSLU2 uses a TTL serial interface which means
      Message 2 of 3 , Oct 29, 2011
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        Sorry for the delay, I don't often check my messages here.

        I think you are missing something critical here. The NSLU2 uses a TTL
        serial interface which means that the signal levels are either 0V or 3.3V.
        RS232 uses positive/negative voltage levels which are supposed to be
        +/-12V. Some devices only output about 6-9V though but so long as it is -6
        to +6, that's enough to be used. This conversion from USB (0V/5V) to +/-6V
        is done with internal charge pumps which serve as a level converter. So
        let's summarize:

        USB-to-RS232 converter
        USB connections: GND, DATA+, DATA-, +5VDC
        RS232 connections: Tx = pin 2, Rx = pin 3, GND = pin 5.

        Note that RS232 always provides either high or low power for Tx, and I
        believe with no characters transmitted a voltmeter should show a negative
        voltage.

        You cannot connect the RS232 output of a USB-to-RS232 adapter directly to
        the NSLU2 TTL serial port, as that may damage the board trying to send
        negative voltages into the TTL serial port and overvoltage (>3.3V) to the
        TTL serial port.

        If the device shown in the USB-RS232 adapter are no longer present, you'll
        have to use a third device which converts RS232 to TTL. Please see this
        URL:
        http://www.futurlec.com/Mini_RS232_TTL_3V.shtml

        That converts the RS232 signaling to TTL, which is what the NSLU2 requires.
        Or if you prefer to keep your USB-RS232 adapter in-tact, you can use this:
        http://www.diybin.com/products/DFRobot-RS232-TTL-Converter.html

        or you can try your luck on Ebay:
        http://www.ebay.com/sch/i.html?_nkw=RS232+TTL+Converter

        I hope that helps!
        -Rob


        On Mon, Oct 10, 2011 at 04:51, kajsde <amitkupadhyaya@...> wrote:

        > Hi,
        >
        > I am referring to adding the serial port page
        > http://www.nslu2-linux.org/wiki/HowTo/AddASerialPort
        >
        > In particular the section - "Read only connection without level
        > conversion". I have a doubt regarding the connections that are mentioned
        > here. Can someone shed some light on what exactly is meant by
        > the three connections mentioned below :
        >
        > 1) What I did was connecting the 5V VCC provided by USB to the VCC pin of
        > the converter
        > 2) both GNDs (The one of the NSLU2 and the one of USB) to the GND pin and
        > the NSLU2s
        > 3) TXD pin directly to the converters RXD pin
        >
        > As far as I understand - there are 4 pins on the NSLU2 (3.3V, Rx,Tx,GND)
        > and there are two sides of the USB-RS232 converter. The USB side has four
        > pins (5v, +Data, -Data, Gnd) and the serial port side has 9 pins. Can
        > someone tell me exactly what are the connections here - or am i missing
        > something?
        >
        >
        >


        [Non-text portions of this message have been removed]
      • Rob Lockhart
        In regards to the specifics of the read-only without level conversion, this will need a voltage level converter to work. If you don t, you may damage the
        Message 3 of 3 , Oct 29, 2011
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          In regards to the specifics of the read-only without level conversion, this
          will need a voltage level converter to work. If you don't, you may damage
          the buffer or IO cell pad by continually trying to have the pad source
          voltage to compensate for the below-ground TTL input at steady state.

          I was thinking that it may be possible to use a series diode and a 3k
          pull-down to ground to keep noise away, along with a voltage divider for
          12V to 3.3V level conversion. That may work.

          Here's another idea not yet on the Wiki but I'm pretty sure that it works:
          http://www.botkin.org/dale/rs232_interface.htm

          Here's another idea that should work but I don't have a way to test it.
          Please excuse my crude MS-Paint drawing but it should suffice. The diode
          is to simply block the below-ground voltage levels (which is where the
          5-10k resistor pull-down helps). The voltage divider gets 4.0V to the
          diode input, and diodes usually have a 0.7V drop, which then would get 3.3V
          to the TTL input.

          Regards,
          -Rob


          [image: image.png]

          On Mon, Oct 10, 2011 at 04:51, kajsde <amitkupadhyaya@...> wrote:

          > Hi,
          >
          > I am referring to adding the serial port page
          > http://www.nslu2-linux.org/wiki/HowTo/AddASerialPort
          >
          > In particular the section - "Read only connection without level
          > conversion". I have a doubt regarding the connections that are mentioned
          > here. Can someone shed some light on what exactly is meant by
          > the three connections mentioned below :
          >
          > 1) What I did was connecting the 5V VCC provided by USB to the VCC pin of
          > the converter
          > 2) both GNDs (The one of the NSLU2 and the one of USB) to the GND pin and
          > the NSLU2s
          > 3) TXD pin directly to the converters RXD pin
          >
          > As far as I understand - there are 4 pins on the NSLU2 (3.3V, Rx,Tx,GND)
          > and there are two sides of the USB-RS232 converter. The USB side has four
          > pins (5v, +Data, -Data, Gnd) and the serial port side has 9 pins. Can
          > someone tell me exactly what are the connections here - or am i missing
          > something?
          >
          >
          >


          [Non-text portions of this message have been removed]
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