The higher power consumption is a result of the line??™s lower series resistance. In
Figure 7-6, circuits A and B show how adding a 120?„¦ terminating resistor
decreases the parallel input impedance from 12k to 119?„¦. Assuming 30?„¦ output
impedance for each driver, the current on the line increases from 0.4 to 28
mA.
The higher current also reduces the noise margin. The driver??™s output impedance
absorbs a larger proportion of the output voltage, reducing the differential
voltage at the receivers. If the output impedance of each driver is 30?„¦, one third
of the voltage drops across the drivers??™ output impedances, leaving only 3.3V
across the termination. The received differential signal is still 3.1V greater than
the receiver??™s input threshold, however. Adding a second termination resistor
exaggerates both of these effects.
One way to conserve power is to disable a driver when it isn??™t transmitting. If
the communications path is often idle, disabling drivers that aren??™t transmitting
Designing RS-485 Links and Networks
121
data cuts power consumption dramatically. A circuit that uses spare RS-232 signals
as a power source for an RS-232-to-RS-485 converter can??™t use a resistive
parallel termination because it draws too much current.
In Figure 7-6, circuits C and D show how adding a termination defeats the
fail-safe circuits included in RS-485 receivers.
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