Chapter 7
combination to 50Ω., less than TIA-485-A’s specified minimum. Most RS-485
drivers can source and sink 60 mA and thus will work with 100Ω cable and ter-
minations. But a cable with 120Ω or greater characteristic impedance is a better
choice for most RS-485 lines. $
Metal shielding is effective at blocking noise due to capacitive, electromagnetic,
and high-frequency magnetic coupling. The shielding is typically grounded at
one end only. If the line has a single power source, the shield ground is at this
node. Many RS-485 lines don’t require shielded cable.
Unlike RS-232, the RS-485 standard doesn’t specify a connector, signal func-
tions, or pin assignments. Many links use RJ-type modular connectors
(described in Chapter 5). On any connector, keep the two signal wires (A and
B) next to each other.
The two differential lines for each signal should be in the same twisted pair.
Also be careful not to transpose the wires: all of the drivers’ and receivers’ A pins
should connect to one wire, and all of the B pins, to the other.. !!#
An RS-485 line can extend for thousands of feet. The differential drivers cause
equal and opposite return currents that essentially cancel each other, and thus it
may seem that RS-485 has no need for a ground connection at all. With few
exceptions, however, the entire line should share a ground connection. Chapter
5 introduced the topics of power supplies, grounding, and isolation. This chap-
ter looks at grounding and isolation as it relates to RS-485 lines, which may
extend much farther than RS-232.0
.
The currents in RS-485 balanced lines are nearly but not exactly equal. The
currents differ slightly due to imbalances between the components and noise
that isn’t exactly equal in both wires. The current in the ground wire may be
very small but isn’t zero. If there is no ground connection, the energy in the
return current has to dissipate somehow, possibly as radiated energy that shows
up as EMI.