I am trying to connect RS232 Port to Raspberry Pi using PL2303 USB To RS232 Converter

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and PL2303 USB To RS232 TTL Converter

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These Both connect at


I try minicom command to read data from this port

sudo minicom -b 9600 -o -D /dev/ttyUSB0

But it shows nothing on screen.

Then i check voltage difference between GND and TX and it gives about 9 Volts which is not supported by raspberry pi.

Please tell me how to make a circuit for voltage level shifter so it works with raspberry pi via PL2303 USB To RS232 TTL Converter or GPIO pins?

Please also tell me any necessary commands/library to use RS232 serial Port?

  • it is unclear what you are asking. If you are trying to use both it won't work. The bottom device should connect from Pi to USB on computer
    – Milliways
    Apr 16, 2015 at 15:12
  • no-no we are not use both device at a time. the upper device is working on windows 7 and we can read data from it, but it not working on raspberry pi.
    Apr 16, 2015 at 16:15
  • how to use /dev/ttyUSB0 as /dev/ttyACM0 or /dev/ttyAMA0 for serial communication
    Apr 16, 2015 at 16:18

1 Answer 1


You can use a voltage divider which consists of two resistors for each pin. You will be able to find info and instructions if you search for that term. The library you use will depend on the language but this is very standard and all will have one.

EDIT - of course this only works for dropping voltage, you would need a transistor to go the other way.

  • can you give me circuit for rs232 level shifter from 9-12volt to 3.3volt, that works with raspberry pi
    Apr 17, 2015 at 5:29
  • it's probably best to just use a purpose made chip such as cpc.farnell.com/texas-instruments/max232in/… there will be circuit diags. in the technical specs farnell.com/datasheets/1885872.pdf The additional advantage of this is a built-in voltage pump using 5V input (you need to add a capacitor)
    – paddyg
    Apr 17, 2015 at 10:15
  • NB you need to make sure you get a 3.3V compatible version, search around and read the specs, poss MAX3232 is the one you want.
    – paddyg
    Apr 17, 2015 at 10:30
  • In addition to voltage translation, you must invert the signal. The intended-purpose serial level shifters pretty much all do that implicitly. May 16, 2015 at 22:59

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