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PICList Thread
'PICLIST Digest - 22 Sep 1996 to 23 Sep 1996'
1996\09\24@012056 by Luiz Marques

Patrick wrote:
> Date:    Tue, 24 Sep 1996 08:43:08 +1000
> From:    Patrick Hanson <spam_OUTP.HansonTakeThisOuTspamUWS.EDU.AU>
> Subject: Connecting multiple rs232 lines together
> I am hoping somebody out there in PIC land may be able to help me.
> I am currently working on a simple home automation system using multiple
> 16C84's (max of 10) fitted with rs232 trancievers as slaves and a PC as
> the master controller. I am not sure how to go about connecting all the TXD
> lines on the slaves to the PC without causing possible conflicts. It is
> intend

Gerard Chew wrote:
>  What I'm looking for is like, ok, you connect one pin of the
> Stamp to the Tx, one to the Rx and then ...etc?  What about some sample  code
> to tie the whole thing together?

I sent the following message a few days ago to Mr. Gerard

My 75156 parts just arrived to me but I did not stick them to my prototipe
because I have been busy with overdue projects for my work. However I'll try a
sketch to you (in a robot style!!!?).

Pin 8:  to +5 Vdc
Pin 5:  to GND
Pins 6/7: to RS-485 link (twisted pair) (field wiring)
Pin 1:  Rx Data, to a input port pin of STAMP
Pin 4:  Tx Data, to a output port pin of STAMP
Pin 3:  TX Enable, to another output port pin of STAMP
Pin 2:  Rx Enable, to another output port pin of STAMP

This requires four pins of STAMPs  (too much)
A more economical approach should be tie pin 3 and 2 toghether to a output of
STAMP. This signal means: 0 -> STAMP receiving  or  1 -> STAMP transminting.

If you want to reduce again the pin counts (in STAMP), tie pins 1 and 4
toghether to a STAMP pin. This STAMP port shall be bi-directional, in another
words, this pin will be output when STAMP is transmiting and will be input when
STAMP is receiving. It's easely implemented with tri-state capabilities of

Two pins looks like:

------                             ---------------
     |output pin (flow ctrl) 2,3 |               |6          RS-485 Link
     |---------------------------|               |------------ DATA
STAMP |                           |    75156      |
     |in/out pin (data)      1,4 |               |7
     |---------------------------|               |------------ /DATA
     |                           |               |
------                             ---------------

Sorry I don't write one line of source code yet! But in slave STAMP you must be
in receive mode *listening* to master queries (pins 2,3 -> 0 and pins 1,4 ->
input to STAMP).

I foud National Semi DS75176B at $1 in 500 qty

When I hookup my prototipe I will tell you if it worked out.

I suppose this arrangement work without problems but I'm not sure

Also, I'm interested in a one master many slaves simple protocol. Any pointer?


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