MatchCableImpedance

TableOfContents Category:Sasecurity
=== Impedance matching ===
http://groups.google.com/group/comp.arch.embedded/browse_frm/thread/dcb29743ac37b55a/8b54f2e2e7a55f57?lnk=st&q=RS485&rnum=11#8b54f2e2e7a55f57
Dennis,
 I'm in the process of typing up my notes on RS-485. I'm using it
on a project and needed to get to know it better. I see you have been
given a number of references, but I have a few more to add. Go to TI,
National Semi., and Maxim, and B & B Electronics. I don't have the
URL's, but it won't take you long to find them. In addition, Jan
Axelson had a good article in Circuit Cellar ("Designing RS-485
Circuits") that is available on the internet.
 Now to answer some of your questions. Think of differential as
being the digital signal and the inverse of it on the two lines. At
least that is what you start with, depending on biasing. As you may
suspect, the signal will attenuate over distance. As long as there is
more than a 200 mV difference between A (the true or '+' line) and the B
(or negative or '-' line), the receiver will be able to determine the
state. If the difference is less than 200 mV, then the state of the bus
is unknown.
 How does one insure that the bus state is known? You bias it by
tying the A line to 5 volts through a resistor. Typical values range
from 560 to 750 ohms. The B line is tied to ground with the same value
resistor.
 You may have noticed that I specified a range of bias values. The
reason has to do with termination. First, is termination needed? The
answer is that depends on the signaling speed (bps) and the length of
the transmission line. If the line length is much shorter than the
electrical length of the bit window, then termination is not needed.
When you get the documents mentioned you will see calculations to
determine this. The idea is that the wave front, or Trise, needs time
to dissipate before you are into the center of the bit. It is assumed
that 4 or 5 round trips around the transmission path will attenuate
Trise well enough. Remember the purpose of termination is to match the
cable Zo to the load. 24 AWG wire has a typical Zo of about 120 ohms,
so if termination is needed, there would be a 120 ohm, 1/2 watt resistor
at each end.
 The problem with termination is that it makes the system less
flexible if you plan to be adding and removing nodes at will. So you
may want to do a trade-off where the speeds are slow enough that
termination is not needed. The other problem with termination is that
it is a load on the driver, and increases the current required.
 The number of drivers is not limited. Of course you can't have
more than one driver on at a time because that will play havoc with the
data integrity. The number of receivers is what is limited. A receiver
can have 1, 1/2, 1/4, or 1/8 unit load (check with the receiver
specifications). A unit load is about 12K ohms. So,if all of your
receivers are one unit load, the spec. says that you can have up to 32
of them. More if you use receivers of a fractional unit load.
 Comparing chips is something I have to let up to you because they
all have pros and cons.
 I'm out of time, but I hope this helps somewhat. Let me know if
you have more questions.
Category:Rs485