Deltayqpuwp (Hôte)
| | Links and Networks UNDEFINED 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. 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. 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 121 Chapter 7 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. Series Terminations 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. The voltage thus doubles and brings the voltage and current to their final values after just one reflection. This type of termination can be useful in full-duplex lines between a single driver and receiver. prom-electric.ru |
