Diode D2
Designing RS-485 Links and Networks
137
blocks this substrate current and allows the active driver to co-exist with disabled
drivers.
The protection is guaranteed only when the common-mode voltages are within
the chip??™s specified limits. RS-422 interface chips don??™t have the protection
diodes. For this reason, RS-422 allows only one driver per line.
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3
One way to ensure that a ground path exists between nodes is to include a
ground wire in the cable (Figure 7-14). TIA-485-A recommends connecting a
1/2-Watt, 100?„¦ resistor in series between each node??™s signal ground and the
ground wire. The resistors protect the components by limiting current in the
ground wire if the ground voltages vary.
%
RS-485 cables can be much longer than RS-232 cables. Over long distances,
the nodes??™ grounds may vary by many volts. Chapter 5 introduced galvanic isolation
as a way of making a circuit immune to ground noise in other circuits.
As with RS-232, if the nodes along an RS-485 line have a common earth
ground and a ground wire, ground currents from all sources will choose the
Figure 7-13: Schottky diodes in RS-485 drivers block large substrate currents between
an active driver and disabled drivers.
Chapter 7
138
path of least resistance. If the power supplies of all nodes use the same electrical
system and their ground wires connect at an earth ground, the ground connection
might be quiet.
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