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???
03/10/08 06:52
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 		 #152052 - Almost certainly not!
Responding to: ???'s previous message
Alan Dorman said:
The RS232 port is being used to communicate with a small thermal printer at 9600 baud.

Does the printer ever transmit back to the CPU?

Would it work if I tied these same TX and RX lines...

You can drive multiple inputs from one output - so the MCU's Tx could drive the Rx for both the LCD and printer - but you cannot just short two outputs together to drive a single input!

Would the printer's RS232 see a higher baud rate and ignore it and would the LCD see a lower baud rate and ignore it?

Almost certainly not!
They would almost certainly see the wrong baud rate as junk - just as if you set hypoterminal to the wrong baud rate.

Any suggestions?

You need more pins!
Either to control a multiplexer, or as a soft-UART, or an I2C-connected UART, or other such options that have been discussed before!

I would hate to put in a mechanical switch to select between the printer and the LCD display.




List of 32 messages in thread
TopicAuthorDate
One RS232 port and two devices            01/01/70 00:00      
   Almost certainly not!            01/01/70 00:00      
      neither one talks back            01/01/70 00:00      
         Soft-UART ?            01/01/70 00:00      
         Simplex            01/01/70 00:00      
            responses            01/01/70 00:00      
               The beaty of the 8051            01/01/70 00:00      
                  why all this about bit-banging            01/01/70 00:00      
               Bit banging UART TX.            01/01/70 00:00      
                  Easy to get wrong.            01/01/70 00:00      
                     Point taken, but you can calculate delay.            01/01/70 00:00      
                        Depends on compiler/etc...            01/01/70 00:00      
                           Don't use HLL!            01/01/70 00:00      
                              Cycle counting.            01/01/70 00:00      
                                 Fundamental flaw            01/01/70 00:00      
                              Not an Absolute....            01/01/70 00:00      
                                 RE: Never is a strong word            01/01/70 00:00      
                                    ... and            01/01/70 00:00      
                              assembly verses HLL            01/01/70 00:00      
                           Use a Timer IRQ to set the soft baudrate            01/01/70 00:00      
                              Surprise.            01/01/70 00:00      
                                 High risk            01/01/70 00:00      
                                    Straying into fantasy.            01/01/70 00:00      
                                       Not so fantastic?            01/01/70 00:00      
                  Resistance is too high            01/01/70 00:00      
                  Another reason why this is not so easy...            01/01/70 00:00      
               Timescales            01/01/70 00:00      
      message formatting            01/01/70 00:00      
         Try it now            01/01/70 00:00      
            It's OK now, thanks.            01/01/70 00:00      
   Two RS232 devices            01/01/70 00:00      
      the LCD display            01/01/70 00:00      

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