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Joseph Hebert
08/06/07 09:22
Read: 327 times
Okemah, Ok
United States

#142817 - Three pins required
Responding to: Ellappan Varadharaj's previous message
Hi Ellappan,

For a standard DB9 serial port, you only need to use three pins. They are

Pin 2 RxD (Receive Data)
Pin 3 TxD (Transmit Data)
Pin 5 GND (System Ground)

It is common for people new to this to believe they need to use all of the other control/handshaking pins and signals, but they are mistaken. Serial communication is via an Asynchronous UART, which means there is no need to worry about sending and receiving data at the same time. Moreover, with the great disparity in operating speeds between even the slowest -controllers and fastest UARTs, overrunning data simply isn't a problem that is realized very often. Okay, you might encounter an application where you need to worry about it, which is to say that I can't guarantee mathematically that you'll never overrun a UART buffer, but it isn't likely.

To connect your 805x to the serial port, using a MAX232 (or equivalent), connect the TxD pin of your -controller to a transmitter input on the MAX232, and connect the corresponding transmitter out pin of the MAX232 to the RxD pin (Pin 2) of the DB9. Then connect the TxD pin (Pin 3) of the DB9 to a receiver input on the MAX232, and connect the corresponding receiver output to the RxD pin of your -controller. Finally, connect the GND pin (Pin 5) of the DB9 to the GND/Common of your -controller circuitry.

Now, having said all of that, I suggest that as soon as you've done this, and have convinced yourself that it works, get a USB/UART Bridge chip (e.g. Silabs CP3102) and learn how to use it instead. You're going to need to soon, if not already.

Good luck,


List of 13 messages in thread
interfacing with serial port of the PC      Ellappan Varadharaj      08/06/07 01:06      
   this is why we have FAQs here      Jan Waclawek      08/06/07 01:22      
   Three pins required      Joseph Hebert      08/06/07 09:22      
      are you sure that site is alive?      Richard Erlacher      08/06/07 12:07      
         well, slightly different      Jan Waclawek      08/06/07 12:08      
            Thanks Jan      Joseph Hebert      08/06/07 12:32      
               oh-er-that's all right it's a pleasure...      Jan Waclawek      08/06/07 13:31      
                  Superman says      Andy Neil      08/06/07 16:59      
                  English      David Smith      08/07/07 13:01      
                     Only French and German?      Joseph Hebert      08/07/07 15:12      
      Transmit and Receive      Andy Neil      08/06/07 17:39      
         I hate this nomenclature      Jan Waclawek      08/07/07 02:16      
      Googled CP3102...      Stanley Lio      08/07/07 06:51      

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