Part A
74HC139D
Toshiba Semiconductor and Storage
- Lifecycle
- Active
- Stock
- 4290 PCS
- Package
- SOIC-16
- Series
- Logic - Signal Switches, Multiplexers, Decoders
Replacement verdict
Not a drop-in replacement
The 74HC138D and 74HC139D are not functionally interchangeable despite sharing the same 74HC CMOS logic family and 16-pin SOIC package. The 74HC138D is a single 3-to-8 line decoder/demultiplexer with three enable inputs, while the 74HC139D contains two independent 2-to-4 line decoders/demultiplexers with only one enable input each. This fundamental architectural difference in decoder topology, channel count, and enable logic prevents direct substitution in existing designs without circuit redesign.
Different decoder topology
Different channel count
Different enable input count
Different address line count
Key differences
Rows are prioritized by design impact. Highlighted values require attention during substitution.
Key electrical and mechanical differences between the two devices
| Parameter | 74HC138D | 74HC139D | Why it matters |
|---|---|---|---|
| Function | 3-to-8 line decoder/demultiplexer | Dual 2-to-4 line decoder/demultiplexer | Defines the core logic function; these parts are not functionally interchangeable because one decodes 3 inputs to 8 outputs while the other contains two independent 2-to-4 decoders. |
| Number of Decoders | 1 | 2 | Determines how many independent decoding channels are available in a single package, affecting circuit partitioning and board space. |
| Number of Outputs | 8 | 8 (4 per decoder) | Total output count affects how many peripheral devices or address lines can be selected. |
| Address Inputs per Decoder | 3 (A0, A1, A2) | 2 (A0, A1 per decoder) | The number of select inputs determines the maximum number of unique output states that can be decoded. |
| Enable Inputs | 3 (two active-low, one active-high) | 2 (one active-low enable per decoder) | Enable inputs allow cascading and gating of decoders; different enable configurations affect how easily the device integrates into a larger address decoding scheme. |
| Output Type | Active-low | Active-low | Output polarity must match the active level expected by the driven device or bus. |
| Supply Voltage Range | 2 V to 6 V | 2 V to 6 V | Defines compatibility with 3.3 V and 5 V logic systems and ensures the part operates within the target system's power rails. |
| Logic Family | HC (High-speed CMOS) | HC (High-speed CMOS) | Logic family determines input thresholds, drive strength, and compatibility with other logic devices in the system. |
| Package | SO16 (SOIC-16) | SO16 (SOIC-16) | Package footprint and pin count must match the PCB layout for a drop-in replacement. |
| Operating Temperature Range | -40 °C to +125 °C | -40 °C to +125 °C | Ensures reliable operation across industrial and automotive temperature environments. |
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Full specification comparison
Use manufacturer datasheets as the final authority.
| Specification | 74HC139D | 74HC138D |
|---|---|---|
| Series | 74HC | 74HC |
| Part Status | Obsolete | Active |
| Type | Decoder/Demultiplexer | Decoder/Demultiplexer |
| Circuit | 1 x 2:4 | 1 x 3:8 |
| Independent Circuits | 2 | 1 |
| Current - Output High Low | 5.2mA, 5.2mA | 5.2mA, 5.2mA |
| Voltage Supply Source | Single Supply | Single Supply |
| Voltage - Supply | 2V ~ 6V | 2V ~ 6V |
| Operating Temperature | -40°C ~ 85°C | -40°C ~ 125°C |
| Mounting Type | Surface Mount | Surface Mount |
Frequently asked questions
What is the main functional difference between the 74HC138D and the 74HC139D?
The 74HC138D is a 3-to-8 line decoder/demultiplexer with one active-LOW enable input and two active-LOW enable inputs, producing eight mutually exclusive active-LOW outputs. The 74HC139D contains two independent 2-to-4 line decoders/demultiplexers, each with its own active-LOW enable input and four active-LOW outputs.
Are the 74HC138D and 74HC139D pin-compatible?
No. The 74HC138D is typically supplied in a 16-pin package with a single 3-to-8 decoder function, while the 74HC139D is also typically supplied in a 16-pin package but contains two 2-to-4 decoders. Their pin assignments and internal signal routing differ, so they are not direct pin-for-pin replacements.
Can the 74HC138D be used to replace the 74HC139D in an existing design?
Not directly. The 74HC138D provides one 3-to-8 decoding function, whereas the 74HC139D provides two independent 2-to-4 decoding functions. Replacing one with the other requires redesign of the address decoding logic, enable signal routing, and output connections, and must be validated against the manufacturer datasheet.
How many outputs does each device provide?
The 74HC138D provides eight active-LOW outputs (Y0 to Y7). The 74HC139D provides two sets of four active-LOW outputs, for a total of eight outputs, but they are organized as two independent 2-to-4 decoders rather than one 3-to-8 decoder.
What are the enable input configurations for the 74HC138D and 74HC139D?
The 74HC138D has three enable inputs: one active-LOW (E1 or G2A) and two active-LOW (E2 and E3 or G2B and G1), depending on the manufacturer datasheet naming. The 74HC139D has two independent active-LOW enable inputs, one for each 2-to-4 decoder section.
Do both devices operate at the same supply voltage and logic levels?
Yes. Both the 74HC138D and 74HC139D are 74HC-series CMOS devices and are specified for a typical supply voltage range of 2.0 V to 6.0 V, with standard CMOS input thresholds and rail-to-rail output swing, as stated in manufacturer datasheets.
Which device is better for applications requiring two separate 2-to-4 decoders?
The 74HC139D is the appropriate choice because it integrates two independent 2-to-4 decoders in one package. Using a 74HC138D for the same function would require additional logic or a different decoding scheme and would not provide two independent enable-controlled 2-to-4 decoders.
Are the 74HC138D and 74HC139D available in the same package type?
Both are commonly available in SO16 and other 16-pin packages, but package availability depends on the specific manufacturer and ordering part number. Designers must verify the exact package suffix and mechanical dimensions from the manufacturer datasheet before layout or substitution.