Key Selection Information
| Selection Parameter | Confirmed Information |
|---|---|
| Function | 3-line to 8-line decoder/demultiplexer |
| Logic Family | LS-TTL |
| Supply | 5 V-class TTL operation |
| Outputs | 8 active-low decoded outputs |
| Enable | Three enable inputs for cascading/strobes |
| Use Case | Legacy TTL address/chip-select decode |
| Package | SOIC (D), 16 pins |
| Packing | Large tape and reel |
Product Overview
SN74LS138DR is the LS-TTL version of the classic 138 one-of-eight decoder. It is relevant when a legacy 5 V board, timing model or input/output loading has already been qualified around LS-TTL behavior.
Although SN74HC138 performs the same Boolean decode function, it uses CMOS electrical characteristics and a wider supply range. Replacing LS with HC should therefore verify TTL input thresholds, output loading, edge/timing and supply-current assumptions rather than relying only on pin/function similarity.
The DR order code is the SOIC large-reel production format, retaining a familiar leaded footprint for serviceable legacy logic assemblies.
For related devices, see HKEQGOO’s Signal Switches, Multiplexers, Decoders category.
Technical Specifications
| Parameter | Verified Information |
|---|---|
| Manufacturer | Texas Instruments |
| Exact MPN | SN74LS138DR |
| Function | 3-line to 8-line decoder/demultiplexer |
| Logic Family | LS-TTL |
| Supply | 5 V-class TTL operation |
| Outputs | 8 active-low decoded outputs |
| Enable | Three enable inputs for cascading/strobes |
| Use Case | Legacy TTL address/chip-select decode |
| Package | SOIC (D), 16 pins |
| Packing | Large tape and reel |
| Manufacturer Status | Active / Production |
| Part Marking | LS138 family marking per TI addendum |
Functional Highlights
Legacy electrical compatibility can be the real reason to keep LS138
Mature systems may depend on LS-TTL input thresholds/output loading or qualified timing. A newer CMOS part is not automatically a risk-free substitute.
Three enable pins make 138 devices useful as address-strobe gates
The decoder can be disabled globally or cascaded into larger address maps without adding separate AND/OR gates.
Design Considerations
- Treat HC/LS substitutions as an electrical-interface change even if the truth table and pinout match.
- Check fan-out and VOL/VOH requirements of the actual legacy loads.
- Preserve active-low chip-select timing during power-up/reset.
Product Status and Exact Order Code
Manufacturer status: Active / Production
Texas Instruments lists SN74LS138DR as Active / Production exact SN74LS138DR SOIC-16 large-reel order code per current TI addendum.
Related Engineering Resources
Quality & Verification Support
Third-party inspection or testing can be arranged according to customer requirements. Testing fees are borne by the buyer. Additional verification requirements can be discussed before shipment. After an order is placed, physical product, label, packaging, and Date Code photos may be provided where appropriate.
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