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Here's a diagram of the configuration I have. In fact, I want the
circuit to function without the MAX232 connected, but it doesn't. That's
what I'm trying to figure out: why does now it only functions with the
MAX232 connected to the line.

Lucian


-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of D. Jay Newman
Sent: 23 august 2004 18:06
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: PIC Network Strange Behaviour

> I do not want to use RS485 or RS232, only the USART from the PICs. The
> TX line is pulled high on the master side with a pull-up resistor, and
> the slaves have a diode connected with the cathode to the TX pin of
the
> micro.

Strange. I've never used a diode for this, but then I've only used
ansychronous serial (which is what most people mean when they say
RS232 -- which is purely an electrical standard) either point to point
or in a strictly master-slave protocol. I did use pull-ups though when
I was using TTL level ansychronous serial.

> It has worked previously, but now it only works with a MAX232 circuit
> connected to the line. The slave can receive data, it cannot transmit,
> so I guessed it is the TX line that it cannot pull low. The problem is
> not at the slave level, as I tried with another slave and I have the
> same result, while it worked before.

As you describe it, the MAX232 is actually connected to both the PICs.
This could load the network too much for the PICs to communicate between
themselves.

> I don't know what could prevent the TX line going low. Is the pull-up
> resistor value too small ? it is 10K.

I would suggest *not* using the same UART to do both RS232 and TTL level
asych serial.

> Should I use a RS485 network to do this job ?

If your two PICs are right next to each other (or within a few feet),
then TTL-level (what comes out of the PIC) is fine.

If your two PICs are either in an electrically noisy environment or if
they are separated by some distance, I would use RS485. I am moving to
RS485 for my internal robotics communications because it solves much of
my noise problems.
--
D. Jay Newman           ! DCX - it takes off and lands base down,
jay@sprucegrove.com     !       as God and Robert Heinlein intended.
http://enerd.ws/robots/ !

--
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