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rs-232 question — Parallax Forums

rs-232 question

henry99henry99 Posts: 67
edited 2008-07-14 14:54 in BASIC Stamp
Do most PC ports and machines that take rs-232 input work fine with 0-3V ttl or 0-5V ttl or do you need some sort of external driver (say a maxim rs-232 chip)?

I assume the best that can happen is that it doesn't work but the worst is that the incoming +/-10V or whatever the rs-232 spec is will fry the inputs of the BS.

Are there any quick and easy solutions to convert 0-3.3V ttl to rs-232 without an external voltage supply?

Thanks for all the help.

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2008-07-14 04:21
    RS232 ports require some kind of compatible driver. The MAX232 works nicely for a 5V system. The MAX3232 is the equivalent for a 3.3V system. If you connect an RS232 port to a 5V or 3.3V device, you'll fry the low voltage device. The Basic Stamp manual shows how to use a series resistor to protect the Stamp pin. Look in the chapter on the SERIN statement. The Stamp's 5V/0V outputs can drive an RS232 input. Look in the chapter on the SEROUT statement for details. Also note that the Stamps use a 3 transistor circuit for programming the Stamp. This translates the RS232 input to 5V/0V TTL levels. The Stamp Manual also has schematics of the Stamp modules that show you the circuit.

    3.3V is just barely high enough to satisfy an RS232 input. You're really better off using a MAX3232 or equivalent to convert 3.3V/0V logic levels to RS232 signal levels. The trick that the Stamp uses can also be used with the Propeller for RS232 inputs, not reliably for outputs. There is a 3 transistor converter for the Propeller as well. It "steals" power from the RS232 receive line (from the PC or whatever's transmit line) to provide an idle voltage for the RS232 transmit line (to the PC or other device).
  • henry99henry99 Posts: 67
    edited 2008-07-14 08:15
    Is there an equivalent to the MAX3232 chips but with built in capacitors?

    Isn't it silly that that the chip requires capacitors but they don't surface mount those things or make them somehow in silicon? Would save me a ton of soldering time on a large number of components...

    Thanks for the help!
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2008-07-14 12:00
    MAX-232 requires external caps.

    MAX-233 is the all-in-one device.
  • Mike GreenMike Green Posts: 23,101
    edited 2008-07-14 14:54
    Similarly, the MAX3232 is the 3.3V device that requires external caps and the MAX3233 is the all-in-one device.

    It's hard to make large capacitors on silicon. The MAX3233 can't deliver as much current as quickly to a long or relatively high capacitance load as the MAX3232 with its external capacitors. Including discrete capacitors in a hybrid package would make the thing much more expensive.

    Post Edited (Mike Green) : 7/14/2008 3:00:10 PM GMT
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