Designing RS-485 Links and NetworksLight in a vacuum travels 300 million meters/s (186,000 miles/s or 12 in./ns).
An electrical signal in copper wire travels at around 2/3 to 3/4 this speed:
between 200 million meters/s (124,000 miles/s or 8 in./ns) and 225 million
meters/s (140,000 miles/s or 9 in./ns). Other terms for propagation rate are
propagation velocity and transmission velocity.
These are 1-way delays for cables of different lengths:When the cable is short and the rise time is slow, the delays are of no conse-
quence. But as explained below, with long cables carrying signals with fast tran-
sitions, the delays can be long enough to result in reflections that affect the logic
levels at a receiver.
A cable’s propagation delay, or electrical length, equals the inverse of the propa-
gation rate and is expressed as time per unit length. The propagation delay of
light in a vacuum is 85 ps/in. The propagation delay of a signal at 2/3 this speed
is 125 ps/in. Another way to find the 1-way delay is to multiply propagation
delay by cable length.
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The TSB-89-A supplement to TIA-485-A provides the general guideline that a
line is long if the signals’ rise time is less than twice the 1-way cable delay. To
put it another way, the line is long if the cable’s 1-way delay is greater than half
the rise time. The dividing line is somewhat arbitrary. Other sources use 1/4 or
1/6 of the rise time as guidelines. RS-232 lines are always short lines because of
their limited cable length and slew rate.
This function below returns true if a line behaves as a transmission line or false
if the line behaves as a lumped system as determined by the propagation rate,
the wires’ physical length, and the driver’s rise time. The variables are the prop-
agation rate in ps/in (propagationRate), cable length in ft (cableLength), and
driver rise time in ns (driverRiseTime).Length (ft) 2 10 100 1000 4000
One-way delay (μs @8in./ns) 0.003 0.015 0.15 1.5 15