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.
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.
I don't know what could prevent the TX line going low. Is the pull-up
resistor value too small ? it is 10K.
Should I use a RS485 network to do this job ?

Lucian


-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Howard Winter
Sent: 23 august 2004 17:35
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: PIC Network Strange Behaviour

Mike,

On Mon, 23 Aug 2004 15:12:30 +0100, Michael Rigby-Jones
wrote:

>...<

> You can use RS232 in a multidrop configuration by a
diodes in the TX line of
> every slave to prevent them loading each other.  It's
a bit of a bodge, and
> RS485 is certainly far superior.

Could you do the bodge with MAX232s involved?  I'd have
thought they would try to assert the + or - voltage
whatever happens, so which way would you connect the
diode?

(Having typed that I've just remembered that RS232
"idles" at -V, so perhaps you treat this as the base
state and anything trying to send would take it to +V ?
You'd have to have a pull-down on the wire somewhere, I
think?)

Cheers,


Howard Winter
St.Albans, England

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