RS485 is just a physical layer, as the RS232.

It only rules about how the electrical signal would exist on the wires,
nothing else, nothing related to addressing or something else.

RS232 is an unbalanced system, it uses Ground and a wire to carry the
signal. Any EMI (Electro Magnetic Interference) that "touch" the signal
wire (or ground) would be incorporated to the signal itself, and can
cause communication problems.

RS485 is a balanced system, there is no ground involved, except for the
cable shielding, but there is two wires for the signal (RS232 uses only
one).  Each wire transport a 180¡ rotate signal.  It means that if an
EMI touch the wires, it would generate a voltage with the same polarity
on both wires.  The RS485 receiver chip simple looks for differential
signals, not signal with the same polarity (0¡), so it is automatically
eliminated, and the valid signal is transfered with much more
reliability than RS232. Again, the signal itself has no relation to
ground, only related to the other wire, inverted signal polarity.

How you would use the RS485 is another story.  You can implement several
different protocols, using addressing, multi-drop or something else you
can invent by yourself. All of this can be done either in RS232 or
RS485, doesn't matter so much.

RS485 as you can see, is much more indicated for noisy and connections
with distances bigger than 2 to 3 meters.  It requires a two conductors
shielded cable.

Wagner

Jay Mielke wrote:
> 
> Hello fellow bitheads!!!
> 
> Has anyone out there used the 16C74a with an RS-485 2-wire network?  If so,
> how is it any different than using RS232 (except for levels, of course).  Am
> I required to use addressing?  Would I decode the addressing in software or
> hardware?
> 
> Thanx in advance,
> 
> Jay Mielke
> 
> jay@ticon.net