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???
09/23/09 17:59
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Msg Score: +1
 +1 Good Answer/Helpful		 #169081 - It is all in the subpressed details
Responding to: ???'s previous message
Kiran V. Sutar said:
Hi Per Westermark,
Per Westermark said:
And what do you think happens with the signals, when you connect two transmitters together?

Per Westermark said:
Parallel the 2 TX lines of the 8051's together and also the RX lines of the 8051's together.
Now treat them as 1 port to the PC with appropriate buffering etc.

I think Tony Gelonese had meant; was to short the TX pins of both micros to each other and connect that pin to TransmitterINPUT pin of MAX232....I think this is workable.

Per Westermark said:
Don't you think the PC will have a hard time hearing any messages sent from one of the two 8051 chips?
This will not be a problem if you have protocol such as Modbus (where PC is the Master), or even simple protocol that you use for RS485 multidrop networks.

Correct. And my original post did contain the following text:
Some serial designs are using tri-state and some are using open-collector drive, to allow multiple masters on a wire. RS232 is designed for one master in each direction.


It is a very special case that 8051 chips have weak internal pull-up. But that is not a general case. Some chips have options to configure push-pull operation. And only the datasheet will tell if it will automagically turn on push-pull of an I/O pin, when that pin is used as UART transmit.

This is a forum where people have very varying knowledge levels. And very varying processors. And the intention to solve very varying problems.

It is absolutely vital to not just post a one-liner claiming to be a general solution, when in fact there are a number of "gotchas" hidden in the solution. Besides the requirement that the processor only makes use of a weak pull-up - or have other ways of deactivating the transmit pin - a reader must also be informed that such a solution will limit the possible protocol options.

List of 71 messages in thread
TopicAuthorDate
Communication between two 8051            01/01/70 00:00      
   I'm going to recommend            01/01/70 00:00      
      I'm going to recommend            01/01/70 00:00      
   I'd recommend ...            01/01/70 00:00      
      irrelevant, however            01/01/70 00:00      
      chip            01/01/70 00:00      
         multiple chips ....            01/01/70 00:00      
            Perhaps you don't need two UARTs            01/01/70 00:00      
   Side note, multiple COM ports            01/01/70 00:00      
      FTDI Multi-UART to USB            01/01/70 00:00      
   Clarify?            01/01/70 00:00      
   Parallel 8051 TXs and RXs            01/01/70 00:00      
      Two transmitters            01/01/70 00:00      
         Which lesson to give?            01/01/70 00:00      
            and ..            01/01/70 00:00      
            Like good little children            01/01/70 00:00      
            Jimmy Neutron/Phineas & Ferb            01/01/70 00:00      
            Tony Gelonese's method should work...            01/01/70 00:00      
               no, and yes            01/01/70 00:00      
         some more thought...Per Westermark            01/01/70 00:00      
            It is all in the subpressed details            01/01/70 00:00      
               SIlabs chips would burn            01/01/70 00:00      
      No, That won't "just work"            01/01/70 00:00      
         I think there's more to this than meets the eye            01/01/70 00:00      
            Not always interrupt response instantly            01/01/70 00:00      
               Since the O/P hasn't been back ...            01/01/70 00:00      
                  serial communication            01/01/70 00:00      
                     you are aware?            01/01/70 00:00      
                        You are also aware?            01/01/70 00:00      
                     Synchronize transfers            01/01/70 00:00      
                        Connecting two TX outputs            01/01/70 00:00      
                     Unnecessary complication!            01/01/70 00:00      
                        Different name            01/01/70 00:00      
                        Walter and Walter            01/01/70 00:00      
                           Name            01/01/70 00:00      
                              what are you ashamed/afraid/... of            01/01/70 00:00      
                              Walter, Please eschew obfuscation and equivocation            01/01/70 00:00      
                     That Phytec Board has RS485 and I2C and SPI            01/01/70 00:00      
                        summarizing            01/01/70 00:00      
                           to a nail everything looks like a hammer            01/01/70 00:00      
                        actually ... he hasn't said he needs that ...            01/01/70 00:00      
                           Often important information arrives quite late            01/01/70 00:00      
                           Actually, he did!            01/01/70 00:00      
                              It's not complete            01/01/70 00:00      
                                 he's so stingy with information            01/01/70 00:00      
   Some more Thought....            01/01/70 00:00      
      This assumes perfect synchronization            01/01/70 00:00      
      the longest reply ever            01/01/70 00:00      
         Schematic is Incomplete            01/01/70 00:00      
      Missing boundary limits            01/01/70 00:00      
         Use p3.0 and p3.1            01/01/70 00:00      
            huh?            01/01/70 00:00      
            You need to clarify your requirement!            01/01/70 00:00      
            This contradicts an earlier post from you            01/01/70 00:00      
               agree with most, but???            01/01/70 00:00      
                  clock-for-clock send            01/01/70 00:00      
                     Still not clear!            01/01/70 00:00      
                        I think you lost the thread            01/01/70 00:00      
                           If they're to remain in precise synchronization ...            01/01/70 00:00      
                              Ridiculous            01/01/70 00:00      
                                 Come on, Walter!            01/01/70 00:00      
                                 It's routinely done ...            01/01/70 00:00      
                                    Don't relive your past again and again            01/01/70 00:00      
                                       MCU is a component, not a computer system.            01/01/70 00:00      
                                          Tell me more about femtosecond precision            01/01/70 00:00      
                                             not even then            01/01/70 00:00      
                                                Watch out            01/01/70 00:00      
                                                   Some suggestions for low-jitter processor activation            01/01/70 00:00      
                                                That's exactly what it is            01/01/70 00:00      
                  The two slaves must be in the same state            01/01/70 00:00      
            Is this yet another problem?            01/01/70 00:00      

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