Chapter 7
RS-485 can connect anywhere from 2 to over 200 computers. The bit rate can
be as slow as 300 bps or less on up to 10 Mbps. Transceivers that support even
higher rates are available for applications that don’t need to comply with
TIA-485-A. Cables can be very short or thousands of feet long.
Over short distances at slow bit rates, the component and cable choices are less
critical, though even here the right choices can save power and reduce noise.
Over long distances and at fast bit rates, selecting the proper cables, drivers,
receivers, and related components is critical.
7
The theory behind how digital signals behave in long-distance lines requires
thinking about the voltages on the line in a different way. On a long line, you
can’t assume that a voltage transfers instantly and perfectly from driver to
receiver.
An RS-485 cable can behave as an electrically long or short line. The terms long
and short don’t refer to the cable’s physical length but instead to the amount of
time required for a signal to propagate along a wire to the receiver. The amount
of time varies with the physical length of the wires, the frequencies in the sig-
nals carried by the wires, and how fast the signals travel in the wires.
When the wires are physically short and the frequencies are low, the time
required for signals to propagate down the wires has little effect on signal qual-
ity. The circuit is considered a lumped system, and the wires form a short line.
In many respects, you can think of short lines as perfect, zero-impedance con-
ductors. When an output switches state, you can assume that the input at the
other end of the cable instantly sees an identical voltage.
When the wires are physically long and the frequencies are high, the time
required for a signal to propagate down the wires is significant. This type of cir-
cuit is considered a distributed system, and the wires form a long line. Another
name for a long line is transmission line. On a long line, terminating compo-
nents can help ensure that the receiver sees a clean signal by reducing reflected
voltages on the line.
Understanding how long and short lines behave requires understanding the
effects of two parameters: rise time and cable delay.