AVR-Chat

Russell Shaw wrote:
> Tom Becker wrote:
> 
>> >> ... Unless you are making a unidirectional
>>
>>>>connection, you need at LEAST 3 wires for a proper RS232-C link.  
>>>>Ground, TX and RX.
>>>
>>>That's what i said before, unless the line is half-duplex.
>>
>>Do you mean unless the line is simplex?
> 
> 
> Yes, for simplex which is one-way only, and half-duplex.
> 
> Half duplex is when the ends alternate between being a transmitter
> or receiver, using the same wire. You need tri-stateable transmitters
> for that, and a software protocol. It's usually less hassle just to
> use 3 wires and full duplex for 1:1 connections. For a multidrop
> half-duplex network, the tristateable method can be used, but you
> need to make your own higher current tristateable rs232 transmitters.
> I've done that, but it's not very economical for component count and
> RS485 would be better. I've also tried the dominant pull-down method
> like i2c and canbus, but then that's hardly rs232 anymore.

If you use a max3322, it is tristateable and has 1Meg input impedance,
so you can do multidrop rs232 with standard drive-level transmitters.

http://www.ferret.com.au/articles/67/0c012a67.asp