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8502Re: [nslu2-general] Question about adding a read only serial port

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  • Rob Lockhart
    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?
      >
      >
      >


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