> Soon my electronic gate will be ready witch is
> About 120 feet far from my desk and I need to
> Build bidirectional communication with whole
> House there are doors and switches I need to know
> The status of it. My favorite interface is I2C but the problem
> Is that I2C is not good for 120 feet distance I guess my
> Choice is RS485 differential phase but what I do not know is
> By using 4 wires 2 for receive and 2 for transmit can I communicate
> with all 35 sensors ?. So RS485 works like RS232 I do not see any way
> of sharing from same bus I need to make communication like I2C but
> for 120 feet long any help will highly appreciated.


I2C will work over that distance if you are prepared to allow the slaves to
run at a lower clock rate (actually response time per bit).

RS232 will also work happily multidrop on 4 wires (master tx/slave rx on one
pair, master rx/slaves tx on other) IF you isolate each slave transmitter
behind diodes AND only allow one slave to send at a time AND run the system
slow enough. While the standard does not intend it to be uses in this way,
300 baud will work on wet string (almost literally) and even 9600 may be OK
for this type of app if designed properly. You can even multidrop RS232 on 2
wires with diode isolated transmitters as long as the protocol used prevents
collisions or allows collision recovery.

RS485 is made for this sort of application although 35 sensors may exceed
the fanout of many drivers.
If you are designing the system from scratch , in a 4 wire system you can
easily enough add drivers which can have as much driving capability as you
could reasonably desire. Slower speed will allow you to do very
"non-standard" things.




            Russell McMahon

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