Designing RS-485 Links and Networksblocks this substrate current and allows the active driver to co-exist with dis-
abled 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.#
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, 100Ω 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 iso-
lation 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 theFigure 7-13: Schottky diodes in RS-485 drivers block large substrate currents between
an active driver and disabled drivers.