SN74ALS251D

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Logic - Signal Switches, Multiplexers and Decoders are widely used in many applications such as switching, multiplexing and decoding signals. One such device is the SN74ALS251D. It is an 8-input multiplexer (MUX) which can select one of eight digital signals (A0 - A7) to be passed via the output (Y) while the others are kept at a low level. The selection is based on the three address lines (A, B, C), thus allowing the user to select any one of the eight input lines to be passed through the output.

The SN74ALS251D is a sophisticated, dual-rail input multiplexer that accepts two sets of input signals - one set at the upper (A-side) pins and the other at the lower (B-side) pins. Both sets have 8 inputs, designated as A0 to A7 and B0 to B7, respectively. All the input pins are compatible with 5V logic inputs. The multiplexer has a single output (Y) that reflects the state of the A or B line that is selected by the three address lines (A, B, C). This device serves the purpose of routing any of the inputs, depending on the logic levels of the address lines, to the output. As such, it serves as an ideal solution for applications requiring multiplexing or switching.

The SN74ALS251D also offers protection to connected ICs in data bus applications by providing a set of bus-hold circuitry for the data bus lines. The bus-hold circuits includes a weak Schmitt-Trigger comparator with hysteresis, which holds the input pins steady in the event of the failure of the driving source and the proper operation of the associated IC. This ensures that the data lines are kept at a valid logic level. The chip also has an enable pin that allows the multiplexer to become enabled or disabled from the outside.

The operating principle of SN74ALS251D multiplexer is simple. The three selection inputs (A, B, and C) are used to select which one of the input lines goes through the output (Y). When A and B are low, and C is high, the logic level at A0 is passed to Y and so on. When all three selection inputs are low, the output remains in a high-impedance state. The output can transmit either a high or low signal, depending on the state of the selected input lines. A typical block diagram of the SN74ALS251D is shown below.

The SN74ALS251D is a stand-alone logic device and does not require external components for its operation. It is a low power, low supply-voltage multiplexer with a fast propagation delay time and wide input common mode range. It also offers both low-voltage and low-power design characteristics that make it suitable for many different applications. It is available in 7.5V and 5.0V varieties.

In an automotive context, for example, the SN74ALS251D multiplexer can be used to select from multiple signal sources such as engine sensors or switches. In industrial applications it can be used to provide signal selection and routing among multiple devices. In communication systems, it can facilitate signal routing between various data processing units and modems. The multiplexer also finds useful application in instrumentation systems, computer peripheral designs, and other electronic devices.

Both the functioning principle and practical applications of the SN74ALS251D multiplexer are evidence of its unique capability to route any of the input signals to the output in a very efficient way. Furthermore, it is cheap and requires minimal external components, making it a popular choice for various signal routing applications.

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