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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