Texas Instruments SN74LS399D

Overview

SN74LS399D from Texas Instruments is a quadruple 2-input multiplexer with storage, implementing four independent 2:1 data selectors each with latch functionality. It operates at TTL logic levels, supports 0°C to 70°C ambient temperature, features 16-pin PDIP packaging, and is used in digital control logic for data routing and register staging in legacy industrial controllers.

For engineers reviewing the SN74LS399D datasheet, SN74LS399D pinout, SN74LS399D application, or SN74LS399D equivalent, key selection criteria include its dual-function multiplexing + latching capability, guaranteed propagation delay under 25 ns, TTL-compatible input thresholds, and availability in through-hole PDIP-16 for prototyping and repair of vintage digital systems.

Technical Context

The SN74LS399D integrates four identical 2-input multiplexer sections, each with an independent select (S) input and a common latch-enable (G) input that controls storage of selected outputs. Each section routes one of two inputs (A or B) to its output (Y) based on S, then holds that value when G is asserted.

All logic functions conform to standard TTL electrical specifications: V_CC = 5 V ±5%, I_OL = 8 mA minimum sink capability, and guaranteed noise margins of 0.4 V (low) and 0.4 V (high) across temperature. No internal pull-ups or open-collector outputs are present.

Key Specifications

Pinout & Package

SN74LS399D is supplied in a 16-pin plastic dual in-line package (PDIP), with 0.3-inch body width and standard through-hole mounting footprint.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar multiplexer-with-storage applications.

Compared with SN74LS157N and SN74LS253N, the SN74LS399D uniquely combines multiplexing and latching in a single package-reducing component count and board space where synchronized data capture is required without adding discrete latch ICs.

Availability

SN74LS399D is available at Aetrix Electronics and suitable for industrial control logic, digital test equipment, and legacy computer peripheral designs requiring stable component supply and long-term obsolescence mitigation.

Supply support for SN74LS399D includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments is a U.S.-based semiconductor company founded in 1930, specializing in analog and embedded processing technologies with broad industrial, automotive, and consumer product portfolios.

The SN74LS399D belongs to TI's legacy 74LS logic family, designed specifically for robust, low-power digital control and data routing in commercial-grade systems from the 1970s–1990s.

FAQ

What is the primary function of the SN74LS399D?

The SN74LS399D is a quadruple 2-input multiplexer with integrated latch functionality. Each of its four channels independently selects between two data inputs (A or B) based on a select signal (S), and stores the resulting output when the global latch-enable (G) input is asserted. This enables synchronized data routing and hold operations in digital control systems without external latching components.

Does the SN74LS399D support 3.3 V operation?

No, the SN74LS399D is strictly a 5 V TTL device. Its absolute maximum supply voltage is 5.5 V, and it requires V_CC = 5 V ±5% to meet specified timing and logic thresholds. Operation at 3.3 V will result in undefined outputs, excessive propagation delay, and potential failure to recognize valid logic levels.

Is the SN74LS399D pin-compatible with the SN74LS157?

No, the SN74LS399D is not pin-compatible with the SN74LS157. While both are quad 2:1 multiplexers in 16-pin PDIP packages, the SN74LS399D uses pin 16 for both V_CC and the active-low latch-enable (G) signal, whereas the SN74LS157 dedicates separate pins for V_CC and enable. Their pinouts differ significantly and require PCB layout changes for substitution.

What is the purpose of the latch-enable (G) input on the SN74LS399D?

The latch-enable (G) input on the SN74LS399D is an active-low global control that freezes the output state of all four multiplexer channels simultaneously. When G is low, the current Y outputs are held regardless of changes to S or data inputs. This allows precise timing alignment-such as capturing sensor readings at a system clock edge-without requiring individual channel latches.

Can the SN74LS399D drive LED indicators directly?

No, the SN74LS399D cannot reliably drive standard LEDs directly. Its output sink capability is rated for 8 mA maximum, which may be insufficient for visible LED brightness, and it lacks current-limiting design. Driving LEDs requires external series resistors and careful current calculation; for indicator applications, dedicated LED-driver ICs or buffer stages are recommended instead.