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The transmission of digital signals plays an important role in high speed computers, not only because the traveling time on the connecting wire is a significant portion of the total delay, but also because the wire is a distributed parameter coupling network which, as a transmission line, becomes an integral part of the computer circuitry and affects the over-all performance. The classical theories of transmission lines are applied to study the signal waveforms. In the case of low input impedance loads, the radial (or parallel) transmission line scheme should be used. A mathematic relationship can be derived among the ramp slope of signals, the input current to the loads, the length and the impedance of the lines, and the terminations. Then the reflections can be predicted. Guided by this, one chooses the optimum line impedance, the maximum length and the proper termination. For high input impedance loads, the tapped transmission line scheme can be used. The multiple junction reflections are expressed in terms of a series which can be worked out by hand or by machine computation. Likewise, there is a relationship among the impedance of the tapped lines and the feeding line, the spacing of loads and the terminations. There are rules to be followed to decide the transmission scheme.
Date of Publication: Aug. 1963