The fail-safe circuits ensure that
the receiver sees a defined logic level when the inputs are open. Without a termination,
the internal pull up and pull down in many RS-485 receivers hold
Figure 7-6: (A) and (B) show that a parallel termination increases power consumption
and decreases the noise margin. (C) and (D) show how the termination defeats a
receiver??™s internal open-circuit fail-safe circuits.
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input A more positive than input B. Adding a termination lowers the open-circuit
differential voltage to just a few millivolts. This chapter shows how to add
circuits that replace the internal fail-safe circuits on terminated lines.
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Another type of termination used in some systems is the series, or back, termination.
Instead of a parallel resistor across the lines at the end of the cable, the
termination resistor is at the driver, in series with the line (Figure 7-7). The termination
plus the driver??™s output impedance equal the line??™s characteristic
impedance.
When the output switches, half of the voltage drops across the output impedance
and termination. The initial current is half as large as the final current, and
the receiver sees a voltage half as large as the final voltage. The receiver??™s high
impedance causes most of the voltage to reflect back to the driver. The driver
and termination, which together equal the characteristic impedance, absorb the
entire reflection.
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