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newbie need some light :) ...

darkodarko Posts: 5
edited 2008-10-26 16:10 in Propeller 1
Hi , I just start on your world [noparse]:)[/noparse]

I have buy an LCD Parallax SERIAL (27977).

i need to use this LCD with the COM1 for C++ application .

But evrywhere i look the wiring is not the same ...

I have 2 x 5V and 2 x RX so i dont know the one i need ...

I have plug the 5v and the ground . and i use the PIN 3 of my DB9 to one RX but when i send some Data i have kind of : 2$^m

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2008-10-26 14:44
    1) You may have damaged your LCD display. RS232 uses voltages as high as +12V and uses negative voltages too (as low as -12V) and these can damage devices that are made for logic voltages (0 to +5V or 0 to +3.3V).

    2) You may need an RS232 to +5V level converter. A MAX232 or equivalent chip will do this for you. If you choose this method, you should follow the manufacturer's examples and sample circuits.

    3) In this simple case, you may be able to use just a 22K resistor between the LCD display's serial input and the DB9 pin 3. This resistor will protect the LCD display and reduce the voltage.

    4) You still have to properly initialize the C++ serial port for the Baud needed by the serial LCD (set by switches - read the documentation).
  • darkodarko Posts: 5
    edited 2008-10-26 14:51
    If i use the Test Possition the LCD work fine ...

    I thought i had 1 RX for TTL and 1 for RS232 ...
  • Mike GreenMike Green Posts: 23,101
    edited 2008-10-26 15:07
    1) The test position of the switches will not tell you if the input circuitry is damaged.
    The only way to test the input circuitry is to try it in a proper circuit.

    2) The two RX connections are the same. They're connected together.
  • darkodarko Posts: 5
    edited 2008-10-26 15:12
    hoooo ...

    So you think is Dead ? ...
  • darkodarko Posts: 5
    edited 2008-10-26 15:14
    Can i send text by Putty from my DB9 to the lcd ??? for test it with an MAX232 ...
  • Mike GreenMike Green Posts: 23,101
    edited 2008-10-26 15:35
    The microprocessor on the display is pretty rugged and it probably is ok, but that all depends on what you connected it to and how much current it can put out at what voltage. Some computers can put out +13V/-13V at currents up to 30mA or so. Most laptops use a lower voltage (on the order of +6V/-6V).

    You could test it with either a MAX232 or the series resistor I mentioned. If you use the series resistor rather than the MAX232, you'll need to invert the comm port signal when you configure it. RS232 uses a negative voltage for 1 and a positive voltage for 0. The display uses +5V for 1 and 0V for 0. The resistor (and the protective diodes internal to the microprocessor on the display) will limit the negative voltage to 0V and the positive voltage to +5V so the logic will be inverted. The MAX232 includes the necessary inverter.
  • StefanL38StefanL38 Posts: 2,292
    edited 2008-10-26 16:03
    hello darko,

    the online shop says

    "This device can be connected to a PC serial port using a MAX232 line driver.
    The circuit isn't supported by Parallax, but it's possible to make this connection with a few dollars of parts."

    @Parallax: I think it would be very good to revise the manual and add a specification of the serial interface
    that it expects TTL-voltage-level

    AND add even a small circuit showing a standard MAX232-cicrcuit how to interface to PC-voltage-level +-12V

    best regards

    Stefan
  • darkodarko Posts: 5
    edited 2008-10-26 16:10
    ho ... ok I actually use it on laptop ... to i hope the LCD is ok ...

    so i think the best is to use a MAX232 [noparse]:)[/noparse]

    Thx for all your reply.
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