Designing RS-485 Links and Networksremains charged and the receiver’s internal pull-up and pull-down resistors hold
input A > B.
In Figure 7-8B, a capacitor charges to half the differential voltage. Figure 7-8C
replaces the capacitor with a resistor equal to 1/4 the line’s characteristic imped-
ance. The terminations in B and C aren’t recommended when the largest source
of noise is from magnetic fields.*
1
When more than two computers share a communications path, how the com-
puters connect to the cable can also affect signal quality. Figure 7-9 shows sev-
eral network topologies, or wiring configurations. RS-485 drivers and receivers
are designed for use in a bus, or linear, topology, where the network cable
begins at one node and connects in sequence to each of the other nodes. This
topology enables using a termination at each end of the bus and brings us to
guideline #3 for RS-485:
Wire the nodes in a bus topology.
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 through-
out 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 arrange-
ment 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 run-
ning 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 con-
nects 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-