74238 (Dual 4-Line to 16-Line Decoder/Demultiplexer)

Description

The IC 74238 is a Dual 4-Line to 16-Line Decoder/Demultiplexer. It is part of the 7400 series of TTL (Transistor-Transistor Logic) ICs. The 74238 is primarily used for selecting one out of multiple lines based on the binary code at its input, making it a useful component in digital systems requiring data routing or control.

IC 74238 Description:

The 74238 contains two independent 4-line to 16-line decoders/demultiplexers, each capable of selecting one of 16 possible output lines, based on a 4-bit binary input code. This IC is commonly used for data selection, address decoding, or demultiplexing applications in digital circuits.

Key Features:

• Dual 4-Line to 16-Line Decoder/Demultiplexer: The IC has two 4-to-16 line decoders, meaning it can decode a 4-bit input and select one of 16 output lines.
• Active Low Outputs: The 74238 features active low outputs, meaning that when a specific output line is selected, it goes low (0) while all others remain high (1).
• Three-State Outputs: The IC provides three-state outputs that can be disabled (high impedance), making it ideal for bus systems where multiple devices share a common data path.
• TTL Logic: As a TTL device, the 74238 is designed to work with TTL logic levels (e.g., 0V for low and 5V for high).
• Output Enable Pin: The IC has enable pins that control whether the decoder is active or not. When the enable pin is low, the IC will select the appropriate output; when the enable pin is high, all outputs are in a high-impedance state.

Pin Configuration (16-pin package):

Functional Description:

1. 4-Line to 16-Line Decoder:
   • The 74238 decodes a 4-bit input (A0, A1, A2, A3) into 16 output lines. The output is active low, meaning the selected line goes low (0), while all the other output lines remain high (1).
   • The 4-bit input code determines which of the 16 output lines is selected. For example:
     • If the input code is 0000, the output line 0 will go low.
     • If the input code is 0001, output line 1 will go low, and so on.
2. Output Enable Pin:
   • The enable pin is used to control whether the decoder is active or not. When the enable pin is low, the decoder is active, and the selected output line will be low while others stay high.
   • When the enable pin is high, all output lines are in a high-impedance state (disconnected), making the outputs appear as if they are not connected to the circuit.
3. Three-State Outputs:
   • The three-state outputs allow the IC to be used in multiplexed or bus systems. When the decoder is disabled (enable pin high), all outputs go into a high-impedance state, effectively disconnecting them from the circuit and allowing other devices to control the bus or data path.
4. Active Low Outputs:
   • The outputs are active low, meaning that when a particular output line is selected, it will be pulled low (0), while all other outputs remain high (1).

Applications:

1. Data Multiplexing/Demultiplexing:
   • The 74238 can be used in demultiplexing applications where data or control signals are routed to one of several output lines. A 4-bit input selects the desired output line.
2. Address Decoding:
   • It is often used in address decoding applications, where a specific address is used to enable one of many devices in a system, such as in memory chips, I/O ports, or peripheral devices.
3. Memory Bank Selection:
   • In computer systems, the 74238 can be used to select specific memory banks or chips based on the address lines.
4. Digital Signal Routing:
   • The IC can route a signal to one of multiple outputs based on the input address, making it useful in digital systems where a signal needs to be distributed to different parts of a circuit.
5. Bus Systems:
   • The three-state outputs allow the IC to be used in bus systems, where multiple devices share a common set of output lines, and only one device drives the line at a time.
6. Control Systems:
   • The 74238 is often used in control systems to activate different outputs based on control signals, allowing for more flexible control of multiple devices.

Electrical Characteristics:

• Supply Voltage (Vcc): Typically 5V (operates from 4.75V to 5.25V).
• Input Voltage (Vih/Vil): The input voltage levels follow typical TTL logic levels. Vih is typically 2V for a high input, and Vil is typically 0.8V for a low input.
• Output Voltage (Voh/Vol): The output will be high (1) for all lines except the selected one, which will be low (0).
• Power Consumption: The IC has relatively low power consumption typical for TTL logic ICs.