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PICList Thread
'Multi-drop serial bus'
1995\04\24@122910 by Paul Greenwood

> Hello,
>         There was a thread on the list a while back about implementing a
> multi-drop serial bus for PIC communications (RS422, I believe).  Can
> anyone help me find it?  I would like to know the resolution, ie, did
> they find a way to do it?  Is there existing code to implement this bus?
> Any help appreciated.
> Thanks,
> Martin Kirk
> Arizona State University
> (602) 263-9270


       I don't remember the discussion but let me explain something that works
quite well for me at low data rates (1200 baud).  I haven't hooked it to a
scope to see the waveforms but I THINK the design is sound.  What I do is tie
the Rx line going to the computer to RTS through a 2.2K resistor.  I write my
programs to pull RTS low (-12V).  I then connect the Rx line through an
opto-isolated transistor to +12V.  I can then switch the line from -12V to
+12V by energizing the optoisolator.  Also, I can connect many of these
together this way.  Then, I just connect the Tx pin of the computer to all
my PICs through a large resistor (I think I used 1Mohms so there was very
little loading).  Now, you have to work on your protocol because collisions
can definately take place.  Also, a limitation is that you cannot have one
PIC talk to another - just to the computer.

       If anyone has any comments on this, I'd like to hear them.  I've been
using this method for a long time - with no problems at all.


           -- Paul Greenwood --  (

"We don't care.  We don't have to.  We're the Phone Company."

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