Multiplexer (MUX) Symbol
Definition: The Multiplexer (MUX) symbol represents a digital or analog combinational logic circuit that selects one of multiple input signals (I0, I1, I2) and routes it to a single output (Out) based on a binary select address applied to the Sel input, functioning as a many-to-one electronic switch, as defined in IEC 60617-12 and ANSI/IEEE 91-1984.
Also known as: MUX, data selector, multiplexer switch, analog multiplexer, selector circuit.
What the Multiplexer (MUX) symbol means
The Multiplexer (MUX) symbol in a circuit or logic diagram indicates that the block selects one input from several available inputs and passes it to the output. The select (Sel) input — a binary address — determines which input is connected to Out. For n select lines, the MUX can switch between 2ⁿ inputs; a 3-input MUX (I0, I1, I2) as shown here uses select addressing to choose among them.
MUX symbols appear in digital logic, analog signal routing, bus switching, and data compression applications. In a schematic, the MUX symbol communicates that multiple signal sources share a single output path, with the active path determined by the select logic. This saves wiring and output pin count in complex systems.
How to identify the Multiplexer (MUX) symbol
The Multiplexer (MUX) symbol is drawn as a trapezoid (wider on the input side, narrowing toward the output) or as a rectangle labelled 'MUX'. Input pins I0, I1, and I2 enter from the left side; the single output pin Out exits from the right; the select input Sel enters from the bottom. The IEC 60617-12 representation uses a rectangle with the qualifying symbol 'MUX' inside. The ANSI/IEEE 91-1984 distinctive shape is a trapezoid with the narrow end pointing toward the output.
Function in a circuit
A multiplexer connects one of its N data inputs to its single output based on the binary select code applied to the Sel input(s). For a 3-input MUX: when Sel selects channel 0, Out = I0; when Sel selects channel 1, Out = I1; when Sel selects channel 2, Out = I2. In digital applications, the MUX handles logic-level signals; in analog applications, the select switches an analog transmission gate. MUXes are used to reduce the number of wires in buses, implement logic functions (any Boolean function of N variables can be realised with a 2ⁿ-input MUX), and route sensor signals to a single ADC input.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-12 defines the multiplexer as a rectangular block with the qualifying symbol 'MUX' inside. Input signals enter from the left; the output exits right; select inputs enter from the bottom. IEC uses dependency notation (labelled with 'G' for AND or 'V' for OR gating) to describe the selection logic. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 91-1984 (IEEE Standard for Logic Symbols) uses a trapezoid shape with the wide end at the inputs and the narrow end pointing toward the output for distinctive-shape MUX symbols. The label 'MUX' or 'DATA SELECTOR' may appear inside. ANSI/IEEE also permits the rectangular IEC-style block with MUX label. |
| Key difference | IEC 60617-12 uses a rectangle with 'MUX' qualifying symbol; ANSI/IEEE 91-1984 uses a distinctive trapezoid shape (wider at inputs, narrower at output). Both carry identical pin labels (I0, I1... for inputs; Out for output; Sel for select). The trapezoid vs. rectangle is the sole visual difference. |
Terminals / pins
| Pin | Name |
|---|---|
| i0 | I0 |
| i1 | I1 |
| i2 | I2 |
| out | Out |
| sel | Sel |
Typical values
Select input width: log₂(N) bits for an N-input MUX; propagation delay: 3–15 ns (74HC series CMOS); channel resistance (analog MUX): 50–300 Ω; on-state leakage current: <10 nA; supply voltage: 2–6 V (CMOS); common IC: 74HC151 (8:1), 74HC153 (dual 4:1), CD4051 (8-channel analog).
Where the Multiplexer (MUX) symbol is used
- Microcontroller I/O expansion: a MUX routes multiple sensor outputs to a single ADC input, multiplying the number of sensors readable with one ADC
- Digital bus switching: MUXes select between multiple data buses or memory banks to route to a single processor data bus
- Boolean function generation: any N-variable truth table can be implemented with a 2ⁿ-input MUX by applying logic levels to select inputs and data inputs
- Serial communication multiplexing: a MUX routes one of several UART or SPI devices to a microcontroller's single serial port
- Video signal routing: analog MUXes select between camera inputs to route one video signal to a single display output
- Test and measurement: automated test equipment uses MUXes to connect multiple DUT (device under test) nodes to a single measurement instrument
Example
In a data acquisition system schematic, a MUX symbol (I0 = temperature sensor, I1 = pressure sensor, I2 = flow sensor; Out connected to an ADC input; Sel driven by a microcontroller GPIO pin) allows a single-channel ADC to read three sensor voltages sequentially. The microcontroller cycles the Sel address through 00, 01, 10 at 1 ms intervals, reading I0, I1, and I2 in turn through the shared Out-to-ADC path.
Key facts
- The Multiplexer (MUX) symbol represents a combinational circuit that routes one of N input signals (I0, I1, I2...) to a single output (Out) based on the binary select address on the Sel input.
- The three standard terminals visible in this symbol are I0, I1, I2 (data inputs), Out (selected output), and Sel (select address input).
- For N select lines, a MUX can switch between 2ⁿ inputs; the generic MUX symbol with one Sel line switches between two inputs, with two Sel lines between four, and so on.
- IEC 60617-12 uses a rectangle with 'MUX' inside; ANSI/IEEE 91-1984 uses a trapezoid (wider at inputs, narrower at output) as the distinctive shape.
- Any Boolean function of N variables can be implemented using a 2ⁿ-to-1 MUX by programming select inputs as variables and setting data inputs to the corresponding truth table outputs.
- Analog MUX ICs (e.g. CD4051 8-channel, CD4052 dual 4-channel) switch analog signals with channel resistance of 50–300 Ω and are used for sensor routing and signal conditioning.
- The inverse of a MUX is a demultiplexer (DEMUX), which routes one input to one of many outputs; MUX and DEMUX symbols are complementary in bus transmission diagrams.
Frequently asked questions
What does the MUX symbol mean in a circuit diagram?
The MUX (multiplexer) symbol means the block selects one of multiple input signals and routes it to a single output. The select input determines which input is connected to the output. It acts as a digitally-controlled many-to-one switch.
What does the multiplexer symbol look like?
The multiplexer symbol is either a trapezoid (ANSI/IEEE 91-1984, wider at inputs, narrowing toward output) or a rectangle labelled 'MUX' (IEC 60617-12). Inputs (I0, I1, I2...) enter from the left, the output (Out) exits right, and the select input (Sel) enters from the bottom.
What is the difference between a multiplexer and a demultiplexer symbol?
A multiplexer (MUX) symbol shows many inputs converging to one output (many-to-one). A demultiplexer (DEMUX) symbol shows one input diverging to many outputs (one-to-many). Both use a trapezoid shape in ANSI/IEEE 91; the MUX trapezoid narrows toward the output, the DEMUX narrows toward the input.
How many select lines does a 4-input MUX have?
A 4-input (4:1) MUX requires 2 select lines (S1, S0) because 2² = 4. A 2-input MUX needs 1 select line; an 8-input MUX needs 3 select lines; a 16-input MUX needs 4 select lines.
What is the difference between a digital MUX and an analog MUX?
A digital MUX switches logic-level (binary) signals and is built from standard CMOS or TTL gates. An analog MUX uses transmission gates (CMOS switches) to route analog voltages with low on-resistance (50–300 Ω). Analog MUX ICs include the CD4051 (8-channel) and CD4052 (dual 4-channel).
What IEC standard defines the multiplexer symbol?
The multiplexer symbol is defined in IEC 60617-12 (binary logic elements). The qualifying symbol 'MUX' inside a rectangle identifies the multiplexer function. The equivalent ANSI/IEEE standard is ANSI/IEEE 91-1984 (IEEE Standard for Logic Symbols for Digital Logic).
What common ICs use the multiplexer symbol?
Common MUX ICs include the 74HC151 (8-to-1 digital MUX), 74HC153 (dual 4-to-1 digital MUX), 74HC257 (quad 2-to-1 MUX), CD4051 (8-channel analog MUX), and CD4052 (dual 4-channel analog MUX). Each is represented by the MUX block symbol with appropriate input/output pin labels.
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