> 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.

RS-485 is an electrical spec only and leaves a lot to the individual
implementation.  Typically you use the differential lines to carry *one*
half-duplex signal.  All the transmitters on the bus cooperate using a
higher level protocol to decide which one of them is allowed to transmit at
a time.

Another common implementation is to run two differential pairs when there is
a "main controller".  The main controller is always the sender on one pair,
and different slaves respond on the other pair as the main controller asks
them to.  This scheme maps well to hardware UARTs because the send and
receive for each unit on the bus can be hard wired to one of the pairs.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics