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MC74172B Datenblatt(PDF) 11 Page - ON Semiconductor |
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MC74172B Datenblatt(HTML) 11 Page - ON Semiconductor |
11 / 14 page MC75172B MC75174B http://onsemi.com 11 Termination Resistors Transmission line theory states that, in order to preserve the shape and integrity of a waveform traveling along a cable, the cable must be terminated in an impedance equal to its characteristic impedance. In a system such as that depicted in Figure 18, in which data can travel in both directions, both physical ends of the cable must be terminated. Stubs, leading to each receiver and driver, should be as short as possible. Leaving off the terminations will generally result in reflections which can have amplitudes of several volts above VCC or below ground. These overshoots and undershoots can disrupt the driver and/or receiver operation, create false data, and in some cases damage components on the bus. Figure 18. Typical EIA−485 System NOTES: 1. Terminating resistors RT must be located at the physical ends of the cable. 2. Stubs should be as short as possible. 3. Circuit ground of all drivers and receivers must be connected via a dedicated wire within the cable. Do not rely on chassis ground or power line ground. #8 TTL R TTL TTL #3 TTL R En #2 TTL TTL En TTL R TTL #1 R TTL RT #1 En D #3 D #5 D TTL En #6 R TTL R En #5 R #6 D #7 RT TTL TTL R En TTL #4 D 5 “off” drivers (@ 0.06 U.L. each), +8 receivers (@ 1.0 U.L. each) = 8.3 Unit Loads RT = 120 Ω at each end of the cable. #4 D #2 120 Ω Twisted Pair |
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