The RS232 circuit is for interfacing with the PC. It is not connected
between the two PICs, but on the master side, to the PC. The two PICs
are still connected as before, but now the slave can only talk when the
RS232 circuit is connected.


Lucian


-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Howard Winter
Sent: 23 august 2004 17:00
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: PIC Network Strange Behaviour

Lucian,

On Mon, 23 Aug 2004 15:08:09 +0300, Lucian wrote:

> Hello all,
>
> I have a problem connecting two PICs (16F877A) on two boards. I use a
> communication protocol and until yesterday it worked fine. I connected
a
> MAX232 circuit to one board and after that, the boards won't connect
> without this circuit plugged.

I'm confused - you had it working with the two PICs connected directly,
then you added a MAX232 to one board,
"inserting" it into that serial connection?

If so then it shouldn't work - the PICs connected directly are using 0V
= data "Low" and 5V = data "High") but
a MAX232 is a level-shifter that implements RS232 which is +V / -V
(where V is 5 to 15V, I think) with +V =
data "Low"  and -V = data "High".  If I understand you correctly you
need a matched pair of MAX232s or none at
all.

> One board is the master and the other is the slave. The slave has to
> request for a permission to transmit, but the signal never gets to the
> master. I think that the problem is with the TX line which cannot be
> pulled LOW by the slave. Each slave has a diode connected on the TX
line
> to allow more slaves on the line.

You can't share a serial connection with a MAX232 - it's expecting to
drive the line and not be overriden by
something else (or to put it another way, RS232 is a point-to-point
protocol).  You need RS485 which is
designed for shared serial busses, and there are appropriate driver
chips available.

> I have the feeling that I might have missed some details, but I hope
> someone will understand my problem and answer it. Should I give more
> detail, please ask !

Yes, a diagram of some sort would help - I may have completely
misunderstood you!

Cheers,


Howard Winter
St.Albans, England

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