Chapter 7
above example to 222 ft. The chip is rated for use at up to 115,200 bps, which
is fast enough for many PC and microcontroller applications. If your applica-
tion doesn’t require fast bit rates, using slower drivers can help ensure signal
quality.
In addition to reduced transmission-line effects, slower chips reduce the ema-
nated EMI (electromagnetic interference) and can reduce ringing on short lines
as described below.
This brings us to guideline #1 for RS-485:
Use the slowest drivers possible for the bit rate.
If your cable is a long line, the proper termination will help to ensure that the
receiver sees transmitted data without errors. Some short lines can also make
use of terminations.
To properly terminate a long line, you need one more piece of information: the
line’s characteristic impedance. This is the input impedance of the cable if it
were an infinite, open line. Every transmission line has a characteristic imped-
ance. The value varies with the wires’ diameters, their spacing in relation to
other wires in the cable, and the type of insulation on the wires. The value
doesn’t vary with the wires’ physical length but is constant for any length.Table 7-1: The maximum length of a cable that does not behave as a transmission line
varies with the driver’s maximum bit rate. These examples assume a propagation rate of
125 ps/in.
6
#
%7.8(#
+
% 6 (
% (#
+
% (
% (6
% (
MAX3080 115 2500 667 222
MAX483 250 2000 250 83
MAX3083 500 750 200 67
MAX485 2.5 40 3 1
MAX3490 10,000 25 3 1