Texas Instruments SN74LS444N

Overview

SN74LS444N from Texas Instruments is a quadruple tridirectional bus transceiver in a 24-pin PDIP package, supporting bidirectional data flow with three-state outputs, 2.0 V typical input threshold, and 15 mA output drive capability. It enables isolated bus communication in legacy TTL-based industrial control backplanes and memory expansion interfaces.

For engineers reviewing the SN74LS444N datasheet, SN74LS444N pinout, SN74LS444N application, or SN74LS444N equivalent, key selection factors include tridirectional control logic (DIR/CLK), 3-state enable timing, DC electrical compatibility with LS-TTL families, and thermal derating in through-hole PCB layouts.

Technical Context

The SN74LS444N implements four independent tridirectional data paths, each controlled by separate DIR (direction) and CLK (clocked enable) inputs. Its internal architecture uses dual-edge-triggered latches and complementary output drivers to support simultaneous read/write arbitration on shared buses without external glue logic.

It operates strictly within the standard LS-TTL voltage and timing envelope: V_CC = 4.75–5.25 V, t_pd = 20 ns max (A→B or B→A), and I_OH/I_OL compliant with TTL fan-out requirements. No internal pull-ups, level-shifting, or CMOS-compatible thresholds are present.

Key Specifications

Pinout & Package

SN74LS444N is housed in a 24-pin plastic dual in-line package (PDIP) with 0.600-inch body width and standard through-hole pitch. Pin 1 is marked by a notch or dot; pin numbering follows counter-clockwise convention from top view.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar tridirectional bus transceiver applications.

Compared with SN74LS444N, SN74LS245N lacks per-channel tridirectionality and requires additional logic for bidirectional operation, while SN74ALS444N offers identical functionality with superior speed and power efficiency-making it the optimal upgrade path for performance-sensitive retro-design refreshes.

Availability

SN74LS444N is available at Aetrix Electronics and suitable for industrial backplane bus interfaces, legacy memory expansion subsystems, and EPROM programmer adapters requiring stable component supply and long-term obsolescence management.

Supply support for SN74LS444N 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 innovator founded in 1930, specializing in analog, embedded processing, and logic technologies with broad industrial and automotive reach.

The SN74LS444N belongs to TI's legacy 74LS logic family, designed specifically for robust, noise-tolerant digital interconnect in cost-sensitive, high-reliability industrial control and instrumentation systems of the 1980s.

FAQ

What is the maximum clock frequency supported by SN74LS444N?

The SN74LS444N does not contain an internal clock oscillator. Its CLK input is an asynchronous enable signal used to control three-state output timing. System-level maximum operating frequency depends on propagation delay (20 ns max) and bus loading; practical limit is ~25 MHz for point-to-point connections. The SN74LS444N must be synchronized externally via system clock edges applied to its CLK pins.

Does SN74LS444N support hot-swap or live insertion?

No, SN74LS444N is not hot-swap rated. Its inputs lack current-limiting protection and its outputs do not implement slew-rate control. Insertion into a live 5-V bus risks latch-up or damage due to supply sequencing violations. Always power down the system before installing or removing the SN74LS444N.

Can SN74LS444N interface directly with CMOS logic?

No, SN74LS444N cannot interface directly with standard CMOS (e.g., 74HC or CD4000) without level translation. Its V_IH = 2.0 V and I_OH = –0.4 mA are insufficient to reliably drive CMOS inputs, which typically require ≥3.5 V and >1 mA. A 74LS245 or discrete resistor-transistor translator is required for mixed-logic systems.

What is the purpose of having three V_CC pins on SN74LS444N?

The three V_CC pins (pins 8, 16, 24) provide distributed power delivery to different functional blocks inside the SN74LS444N die. This reduces localized voltage droop and supply noise coupling between the four independent transceiver channels, improving noise immunity and timing consistency-especially during simultaneous switching events across multiple data paths.

Is SN74LS444N RoHS compliant?

No, SN74LS444N is not RoHS compliant. As a legacy PDIP device manufactured prior to 2006, it contains leaded solder and leaded die attach materials. Texas Instruments discontinued RoHS-compliant versions of this part; modern alternatives like SN74ALS444N remain leaded unless explicitly ordered as "green" variants (e.g., SN74ALS444N3).