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5.6 Three-State Devices

Various three-state buffers. Return. Next. 5.6 Three-State Devices. Three-State Buffers The most basic three-state device is a three-state buffer, often called a three-state driver. Eight sources sharing a three-state party line (p386 Figure 5-54).

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5.6 Three-State Devices

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  1. Various three-state buffers Return Next 5.6 Three-State Devices • Three-State Buffers The most basic three-state device is a three-state buffer, often called a three-state driver. • Eight sources sharing a three-state party line (p386 Figure 5-54) What is the purpose for using the 74x138 ?

  2. faster disable enable Return Back Next 5.6 Three-State Devices • Typical three-state devices are designed so that they go into the Hi-Z state faster than they come out of the Hi-Z state. It is important. The turn-on time(tpZL or tpZH) are both larger than the turn-off time(tpLZ or tpHZ). This means that if the outputs of two three-state devices are connected to the same party line, and we simultaneously disable one and enable the other, the first device will get off the party line before the second one gets on.

  3. Return Back 5.6 Three-State Devices The only really safe way to use three-state devices is to design control logic that guarantees a dead time on the party line during which no one is driving it. (P386 Figure 5-54, P387 Figure 5-55) • Standard SSI and MSI Three-State Buffers • 74x125 and 74x126 three-state buffers (P387 figure 5-56 ) • 74x541 (P388 figure 5-57, 5-58) • 74x245 (P389 figure 5-59) • Bidirectional buses (P390 figure 5-60)

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