SN74LS138DR

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Logic-signal switches, multiplexers, decoders are the key components of digital systems used to route and condition signals. Among these switching devices, the SN74LS138DR is used extensively in digital systems due to its high performance, small room requirement and low power consumption. Its application field and working principle is introduced in this paper.

The SN74LS138DR is a triple three-input decoder/demultiplexer. It provides eight mutually independent outputs and is designed to accept two-level data such as two-rail design of digital logic or transistor-transistor logic (TTL). It is fabricated from advanced low-power Schottky TTL (LSTTL).

The SN74LS138DR features active-low outputs, which indicate that a logic low level output is obtained when the three binary input lines are at the proper combination, whereas the remaining seven outputs are active-high. The three binary inputs A0, A1, A2 and the active-low output enable, E, function as an additional input for the device. In addition, the device features an active-low output inhibit (E) and an active-high output enable (OE) to permit the system designer to disable selected outputs without disturbing the logic state of other outputs.

The primary application areas of this device include address decoding, data selection, control information and programmable logic. As shown in the figure below, within a digital system, the SN74LS138DR is connected in a special manner called remote sensing. This eliminates the usual pair of inverters that provide noise margins to decode TLL logic systems. Remote sensing is a technique of extracting data out at the remote point without the necessity of transistor-transistor logic level repeating buffers. This arrangement simplies the logic design and reduces power consumption.

Here is a detailed description of SN74LS138DR working principle. When one of the inputs of the device is activated, the corresponding output is activated at the same time. When all the three binary input lines are in a proper combination, the appropriate output line is enabled. When one of the inputs of the device is activated, the other seven outputs are turned off. When all the three input lines are at zero, the output will remain off making it suitable for address lines, data lines and control lines.

The SN74LS138DR is a powerful device since all eight outputs can be used simultaneously depending the combinations of input lines. The device is capable of handling two-rail design, thus eliminating the use of Schmitt trigger buffers and inverters. The device is extensively used in decoding applications and due to its low power consumption and small room requirement, it is widely used in laptop and pocket size computers as well as in digital systems.

In summary, the SN74LS138DR is a powerful decoder/demultiplexer device widely used in digital systems. Its application field includes address decoding, data selection, control information and programmable logic. It features remote sensing technique which eliminates the necessity of a pair of inverters, simplifying the logic circuit design. Its low power consumption and small room requirement makes it perfect for laptop and pocket size computers.

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