A stub is the wires that connect a node to the network cable. Stubs should be as
short as possible. Many sources recommend limiting stub length so the stub??™s
1-way delay is 1/4 to 1/2 of the signals??™ rise time.
Sometimes connecting the nodes along a bus isn??™t convenient. Wiring throughout
a house lends itself to using cables that branch from one or more central
locations, in a star, or hub-and-spoke, topology. An advantage to this arrangement
is that if a connection opens at a node, communications among the others
can continue normally.
For a setup like this, there are several options:
??? Use slow drivers to increase the rise time and allow longer stubs. With the
MAX3080??™s minimum rise time of 667 ns, a stub of 1/3 the rise time is 150
ft.
??? Wire the nodes as a bus, even if this means each node has a pair of wires running
to it then doubling back before going on to the next node. The line is
twice as long but performance isn??™t compromised.
??? Add a repeater circuit to regenerate the RS-485 signals where a stub connects
to the main bus. The regenerated signals begin a new RS-485 line.
This chapter has more about repeater circuits.
Another topology used by some networks is the ring, where each node receives
from a one node only and transmits to a single, different node. Each connec-
Chapter 7
126
Figure 7-9: Network nodes can connect using any of several wiring topologies.
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