Objectives
At the end of this chapter, you will be able to:
• Design circuits that use open-collector gates and decoders to drive low-voltage and low-current devices.
• Describe the use of open-collector decoders for memory expansion decoding.
• Describe the different types of peripheral drivers and transistor arrays currently available.
• Design high-current lamp-driver circuits.
• Discuss surge currents in lamp-driver circuits.
• Design driver circuits that use transistor arrays.
• Design addressable latch-driver circuits.
• Design solid-state relay circuits for controlling motors, lamps, and other line-voltage operated devices.
® Discuss the use of optical couplers for isolation.
The simplest driver circuits already exist within the SN7400-series of integrated circuits. These devices have output transistors in which the collector lead of the transistor is uncommitted, and is provided as the output of the particular gate, decoder, multiplexer, or other open-collector-type function. A typical open-collector device is represented by the circuit diagram shown in Fig. 1-1.
In this circuit, which happens to represent an inverter function, the output terminal is the uncommitted collector of the output transistor. Notice, though, that the emitter terminal of the transistor is connected to the ground signal. This means that the output of the transistor will be connected to the ground signal when the transistor is conducting. However, the unconnected transistor collector will have no voltage associated with it when the transistor is no longer conducting. Unlike other SN7400-series devices, the open-collector device does not have a logic one output. The other SN7400-series
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