Some cable intended for data, including the popular Category 5 and Category
6 network cables, use 100?„¦, unshielded twisted pairs. These cables are fine for
1-way or full-duplex RS-485 or RS-422 lines. RS-422 allows just one driver
and often uses 100?„¦ cable with a single 100?„¦ termination. A 2-way, half-duplex
RS-485 line would need two 100?„¦ resistors in parallel, which brings the parallel
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combination to 50?„¦., less than TIA-485-A??™s specified minimum. Most RS-485
drivers can source and sink 60 mA and thus will work with 100?„¦ cable and terminations.
But a cable with 120?„¦ or greater characteristic impedance is a better
choice for most RS-485 lines.
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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 functions,
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.
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