|
 by
Bob Perrin
Start
• Arm Yourself •
RS-485 101 • Getting
Grounded • Shielding
• Topology •
Termination • Idle-state
Biasing • Transients
• Review Time •
Sources
TOPOLOGY
If the signals on the
network are slow, the bit edges are long, and the
cable runs are short, topology is not an issue.
But, the question of network topology will crop
up from time to time.
As soon as transmission-line
effects begin to show up, there is only one simple
topology for managing them. Figure 5 shows several
network topologies. Only the daisy chain is easy
to manage reflections on.
 |
| Figure 5—Many
common network topologies exist, but the daisy
chain is the most reliable for RS-485 networks. |
This is not to say,
for example, that it’s impossible to implement
a star configuration with RS-485 devices. Keeping
reflections under control in a star topology is
more art than science in a practical network. The
best way to ensure a robust and reliable RS-485
network is to build it around a daisy-chain configuration.
There are several rules
of thumb to follow when predicting if the line is
long enough to be a transmission line. One common
rule states that transmission-line effects will
begin to occur when the signal rise time is less
than 4× the one-way propagation delay of the cable
[7].
Most twisted-pair have
a propagation speed of 66–77% of the speed
of light. Cable manufacturers publish this specification
for their network cables. By knowing the approximate
length of the network cable, the one-way propagation
can be computed by knowing that PropTime
equals CableLength divided by PropSpeed.
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|
THE
ART AND SCIENCE OF RS-485