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Thread: rs232 problems

  1. #1
    Constant Bitrate
    Join Date
    Nov 2004
    South West

    rs232 problems

    hi there, hope somebody can help? i am trying to connect a touchscreen controler card to my laptop. the disstrictions say the card can get its power either via 2 terminals @ 5v or through the rs232 port, the cable that came with it has a 5 pin connector to plug to the board and a 9 pin d plug on the other end. they are marked as data in, data out, rs232 ground, +ve and +v3. before attaching to the board i tested with a volt meter but the power between both the power lines and the ground reads -7.7 volts! do i need to change something on the laptop or is this not an rs232 port (its marked I/O and my modem used to plug in hear and work fine) any help would be great, thanks.

  2. #2
    Maximum Bitrate starfox's Avatar
    Join Date
    May 2002
    Your touchscreen controller manual says you can do that? I didn't think there was enough current available on a serial port to drive anything other than maybe a low end PIC... My preference would be to find a more reliable 5V line and connect that up.

    Serial ports are weird, iirc the specification is really loose and you're expected to see anything between 5V to 20V at the receiver end, and to improve signal quality a "0" is a positive voltage beyond 5V and a "1" is a negative voltage beyond -5V. If you're going to use these, i'd suggest putting a zener diode across them. Does your manual say what you're supposed to plug the positive voltage lead into?

  3. #3
    Constant Bitrate
    Join Date
    Nov 2004
    South West
    the amp draw should only be small as it is just 1 board about 2inch square that connects to a glass plate that sits over the tft. it came with a lead 5pin plug connects to board and 9 pin female to pc. 2 of the wires are data in/out 1 is ground and 2 are marked positive power. it does not say the voltage but power can be supplied on another connector which is marked 5v. wanted to power through the rs232 line to rate on having to find a 12v to 5v converter (any ideas for one of these by the way?)

  4. #4
    Raw Wave
    Join Date
    Jun 2003
    You would normally switch the RTS, DTR and TD lines to +V...the apps should do this.

    Depending on the chipset used on the laptop, you may not well get the required specification to power the device.

    Have a read here :

    Have you tried it on a desktop? Laptop normally have a weaker port.

  5. #5
    Low Bitrate
    Join Date
    Nov 2003
    Radio Shack has a 7805 chip - a 5v regulator. Very easy to use and will get that 12v down to 5v regulated. I'm with Starfox - don't use the serial power unless you have to.

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