Part A
74HC138D
Toshiba Halbleiter und Speicher
- Lifecycle
- Active
- Stock
- 2131 Stückzahl
- Package
- SOIC-16
- Series
- Logic.Signal Schalter, Multiplexer, Decoder
Replacement verdict
Not a drop-in replacement
The 74HC126D and 74HC138D are not functionally interchangeable. The 74HC126D provides four independent non-inverting buffer/line driver channels with 3-state outputs, while the 74HC138D is a 3-to-8 line decoder/demultiplexer with active-low outputs. Their logic functions, channel configurations, output behavior, and pin assignments differ fundamentally, making direct substitution impossible without redesigning the surrounding circuit.
Different logic function
Different channel count
Different output type
Different pinout
Parts at a glance
Part B
74HC126D
Toshiba Halbleiter und Speicher
- Lifecycle
- Active
- Stock
- 7257 Stückzahl
- Package
- SOIC-14
- Series
- Logic c Puffer, Treiber, Empfänger, Transceiver
Key differences
Rows are prioritized by design impact. Highlighted values require attention during substitution.
Key electrical and mechanical differences between the two devices
| Parameter | 74HC126D | 74HC138D | Why it matters |
|---|---|---|---|
| Logic Function | Quad Buffer/Line Driver with 3-State Outputs | 3-to-8 Line Decoder/Demultiplexer | The core logic function determines whether the devices can be substituted at all; these are fundamentally different functions and are not functionally interchangeable. |
| Number of Channels / Outputs | 4 independent buffers | 8 decoded outputs | Channel count affects how many signals can be handled and how the device maps into a circuit's signal path. |
| Output Type | 3-State (tri-state) non-inverting | Active-LOW decoded outputs (non-tri-state) | Output type determines bus-sharing capability and whether outputs can be tied together; tri-state allows high-impedance bus connection while standard outputs do not. |
| Control Inputs | Output Enable (OE) per buffer | Three enable inputs (two active-LOW, one active-HIGH) | Control inputs define how the device is gated and cascaded; different enable schemes change the logic design and wiring requirements. |
| Number of Inputs | 4 data inputs plus 4 enable inputs | 3 address inputs plus 3 enable inputs | Input count and arrangement determine the logic selection capability and pin usage in a design. |
| Supply Voltage Range | 2.0 V to 6.0 V | 2.0 V to 6.0 V | The supply voltage range must match the system rail to ensure reliable logic levels and safe operation. |
| Logic Family | 74HC (High-Speed CMOS) | 74HC (High-Speed CMOS) | Same logic family ensures compatible input thresholds, output drive, and noise margins when mixing devices. |
| Package | SOIC-14 (D suffix) | SOIC-16 (D suffix) | Package footprint and pin count must match the PCB layout; different pin counts prevent direct drop-in replacement. |
| Propagation Delay (typical) | Approximately 11 ns at 4.5 V | Approximately 15 ns at 4.5 V | Propagation delay affects timing margins and maximum operating frequency in synchronous or timing-critical designs. |
| Pin Count | 14 pins | 16 pins | Pin count must match the PCB footprint; a mismatch makes physical substitution impossible without a board redesign. |
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Full specification comparison
Use manufacturer datasheets as the final authority.
| Specification | 74HC138D | 74HC126D |
|---|---|---|
| Series | 74HC | 74HC |
| Part Status | Active | Active |
| Type | Decoder/Demultiplexer | - |
| Circuit | 1 x 3:8 | - |
| Independent Circuits | 1 | - |
| Current - Output High Low | 5.2mA, 5.2mA | 7.8mA, 7.8mA |
| Voltage Supply Source | Single Supply | - |
| Voltage - Supply | 2V ~ 6V | 2V ~ 6V |
| Operating Temperature | -40°C ~ 125°C | -40°C ~ 125°C (TA) |
| Mounting Type | Surface Mount | Surface Mount |
| Logic Type | - | Buffer, Non-Inverting |
| Number of Elements | - | 4 |
| Number of Bits per Element | - | 1 |
| Output Type | - | 3-State |
Frequently asked questions
What are the primary functional differences between the 74HC126D and 74HC138D?
The 74HC126D is a quad non-inverting buffer with 3-state outputs, while the 74HC138D is a 3-to-8 line decoder/demultiplexer with active-low outputs. The 74HC126D provides four independent buffer channels with output enable pins, whereas the 74HC138D decodes three binary select inputs into eight mutually exclusive active-low outputs.
Can the 74HC126D and 74HC138D be used interchangeably in a design?
No. The 74HC126D and 74HC138D have different logic functions, pin configurations, and output types. The 74HC126D is a quad 3-state buffer, while the 74HC138D is a 3-to-8 decoder/demultiplexer. They are not pin-compatible and cannot be directly substituted without redesigning the circuit.
What are the supply voltage and logic family specifications for both devices?
Both the 74HC126D and 74HC138D are members of the 74HC high-speed CMOS logic family. They typically operate over a supply voltage range of 2.0 V to 6.0 V, with standard 74HC input and output voltage thresholds. Both are available in SOIC-16 packages as indicated by the D suffix.
How do the output drive characteristics of the 74HC126D and 74HC138D compare?
Both devices are 74HC CMOS logic parts and typically provide symmetrical output drive with standard 74HC DC output current ratings, such as ±4 mA at 5 V VCC. The 74HC126D has 3-state outputs that can be placed in a high-impedance state, while the 74HC138D has standard active-low push-pull outputs without a high-impedance state.
What is the pin count and package type for the 74HC126D and 74HC138D?
Both the 74HC126D and 74HC138D are typically offered in a 16-pin SOIC package, denoted by the D suffix in the part number. The 74HC126D uses the 16 pins for four buffer channels with shared enable controls, while the 74HC138D uses the 16 pins for three select inputs, three enable inputs, and eight decoded outputs.
Which device is suitable for expanding microcontroller I/O or address decoding?
The 74HC138D is specifically designed for address decoding and demultiplexing, making it suitable for expanding microcontroller I/O or selecting peripheral devices via three select lines. The 74HC126D is used for buffering and bus isolation with 3-state control, not for decoding multiple outputs from a binary input.
Do the 74HC126D and 74HC138D have the same enable or control logic?
No. The 74HC126D has output enable pins that control the high-impedance state of each buffer, with active-high enables on the standard 74HC126. The 74HC138D has three enable inputs (typically two active-low and one active-high) that must all be valid to activate the selected output, and its outputs are mutually exclusive active-low signals.
Are the 74HC126D and 74HC138D compatible with 74LS or 74HCT logic levels?
The 74HC126D and 74HC138D are 74HC CMOS devices with CMOS input thresholds, so they are not directly compatible with 74LS TTL logic levels at the same supply voltage without level shifting. For TTL-compatible input thresholds, 74HCT versions such as 74HCT126D or 74HCT138D would be required instead.