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Language:
Expertise: Beginner
May 2, 2000

Meanings of RS232 Signals

Question:
Can you explain the interaction of the RS232 signals (RTS, CTS, DTR, etc.). I need to write a software utility to transmit and receive characters between two computers, but I am unsure about the order of raising/lowering signals and checking for them. I also don't quite understand them when I do.

How do I know when I have sent a character, and when do I know there is a character for me?

Answer:
RS232 is the standard for communication between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE). I've outlined what each of the signals you've mentioned do, as simply and briefly as I could:

  • Request to send (RTS): Think of this one as raising your hand in class. You want to speak but you want to be recognized before you go ahead and transmit your information.
  • Clear to send (CTS): I think of this signal as saying "Over" on a walkie-talkie to indicate that you're done speaking and are now listening for the person's reply on the other end.
  • Data terminal ready (DTR): This signal indicates to the device that the DTE is on.
The simplest order for these signals would be DTR to RTS to (send data when received CTS) then CTS.
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