Most RS-485 chips include a fail-safe feature that holds input A more positive
than input B when no signal is applied to the receiver. The fail-safe works fine
on lines that don??™t use terminating resistors.
But as Figure 7-6 showed, the fail safe is defeated on lines with terminations.
Figure 7-10 shows a solution that adds two 620?„¦ resistors: one from input A to
+5V and one from input B to ground. The terminating resistor at the end with
the biasing resistors is increased to 130?„¦. This configuration holds terminal A
about 500 mV more positive than terminal B when no drivers are enabled. The
TTL outputs of the receivers are logic highs. When a driver is enabled, a logic
low at the driver??™s input brings line B more positive than line A and brings the
receivers??™ outputs low.
Chapter 7
128
Biasing the entire network requires one pair of resistors. Networks that use a
primary/secondary protocol typically place the biasing resistors at the primary
node. If a node becomes disconnected from the network or a network wire
opens, the internal fail-safe circuits hold the inputs at logic 1.
The external fail-safe components have a small effect on the current. When a
driver is enabled and line A is more positive than line B, the output current is in
the same direction as the bias current, so the two currents add. When the driver
brings line B more positive than line A, the bias current is in the opposite direction
and subtracts from the signal current.
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