>Hi.
>
>What would be the easiest way to have multiple 16C84's talk to the same
>Max232?
>I don't want to use PortB for this, so open drain configuration as
>described in one of the Parallax Stamp application notes is out of the
>question. At least as far as I can see.
>It is crucial to use only one MAX232.

Why don't you use PORTA.4 (RA4) which is an "Open Collector" (I presume Open
Drain)?  The only extra component required is a pull-up.

ie:

                               Vdd
                                ^
                                |
                                >
                                <  Pull Up Resistor
                                >
                                <
                                |
   RA4 -------------------------+---------------- Max232 Input
   PIC1                         |
                                |
   RA4 -------------------------+
   PIC2

This is a pretty simple circuit and should work for pretty decent speeds (as
long as the lines are short and the capacitance is low).

Now, you still have a pretty significant issue of handshaking (making sure
that the two PICs don't try to communicate at the same time.

I'd be interested in finding out how you would resolve it (or what the other
people on the list would suggest).


Just thinking about it, you could have two lines between the PICs for
Handshaking:

ie

   PIC1                                         PIC2
        |                                      |
 HshkIn |<-------------------------------------| HshkOut
        |                                      |
HshkOut |------------------------------------->| HshkIn
        |                                      |

The Check Code would be:

Loop
  Set HshkOut                   ;  Indicate that you want to Send
  if ( HshkIn is Set )          ;  Is the Other One Sending/Trying to Send
    Reset HshkOut               ;  Stop the Request
    Wait Random Time            ;  Wait a Random Amount of Time
    goto Loop                   ;  Start All Over
  Send Character                ;  Send the Character
  Reset HshkOut                 ;  Finished, Release the Resources

This can be implemented easily in PIC assembler.  The "Random Time" would be
decided by counting an ansynchronous time interval.

What does everybody else think/comments?

myke
>
>
>Would this work?
>                          /--------------------+-----[ R2 ]---- VCC
>                        |/c                    |
>PortA.0 -----[ R1 ]-----|                      |
>  PIC 1                 |\e                    |
>                          ------+-- Max 232 in |
>                                |              |
>                              |/e              |
>      Port A.0 ----[ R3 ]-----|                |
>                              |\c              |
>                                ---------------+
>
>etc.
>
>I am not good at this, so any help appreciated.
>
>yours,
>Ronald
>
>

Today, the commercial sector is advancing computer and communication
technology at a breakneck pace.  In 1992, optical fiber was being installed
within the continental U.S. at rates approaching the speed of sound (if
computed as total miles of fiber divided by the number of seconds in the year).

Aviation Week and Space Technology, October 28, 1996