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

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  • 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 1 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?
      >
      >
      >


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