Texas Instruments SN74LS670D

Overview

SN74LS670D from Texas Instruments is a 4×4 register file IC with three-state outputs, designed for temporary data storage and addressable read/write operations in TTL-based digital systems. It features 16-bit bidirectional data I/O, independent write-enable control per word, and operates at 0°C to 70°C with typical propagation delay of 35 ns (address-to-data) and 25 ns (write-enable-to-write). It is used in microprocessor peripheral interfaces, ALU register stacks, and early minicomputer data-path buffering.

For engineers reviewing the SN74LS670D datasheet, SN74LS670D pinout, SN74LS670D application, or SN74LS670D equivalent, key selection considerations include its 4-word × 4-bit architecture, TTL-compatible input thresholds, three-state output drive capability, and SOIC-16 packaging for board space-constrained legacy logic designs.

Technical Context

The SN74LS670D implements a synchronous, address-decoded register file using bipolar TTL circuitry. Each of its four 4-bit words is selected via a 2-bit address bus (A0, A1), with separate read (R̅/W̅ = HIGH) and write (R̅/W̅ = LOW) modes controlled by a single bidirectional R̅/W̅ line. Write operation requires active-low WE (Write Enable) assertion per addressed word.

Its three-state outputs are disabled when chip select (CS̅) is HIGH, allowing direct bus sharing without external buffers. The device uses standard LS-TTL input thresholds (V_IL = 0.8 V, V_IH = 2.0 V) and drives ±8 mA loads, ensuring compatibility with 74LS family logic levels and timing budgets.

Key Specifications

Pinout & Package

SN74LS670D is housed in a 16-pin SOIC (Small Outline Integrated Circuit) package with 1.27 mm pitch, 7.5 mm body width, and 2.00 mm max height - optimized for automated assembly and higher-density PCB layouts than through-hole alternatives.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar register file applications.

Compared with SN74LS670N and SN74LS670NSR, the SN74LS670D offers identical register file functionality and timing in a tube-packaged SOIC variant - making it ideal for low-to-medium volume SMT builds requiring quick-turn availability and no reel-handling infrastructure.

Availability

SN74LS670D is available at Aetrix Electronics and suitable for microprocessor peripheral interfaces, ALU register stacks, minicomputer data-path buffering, and legacy test equipment control logic requiring stable component supply across long-lifecycle industrial programs.

Supply support for SN74LS670D 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, embedded processing, and logic products with broad industrial and aerospace heritage.

The SN74LS670D belongs to TI's legacy 74LS logic family, engineered for reliable, low-power TTL-compatible digital building blocks in control, instrumentation, and computing systems from the 1970s onward.

FAQ

What is the primary function of the SN74LS670D in a digital system?

The SN74LS670D functions as a 4-word × 4-bit addressable register file with three-state bidirectional I/O. It stores temporary data values that can be individually written or read under address and control-line supervision. Its design enables fast, deterministic access without external memory addressing logic - making the SN74LS670D especially valuable in microprocessor support circuits and discrete ALU implementations where minimal latency and predictable timing are critical.

Does the SN74LS670D support simultaneous read and write operations?

No, the SN74LS670D does not support true simultaneous read and write. Its R̅/W̅ pin determines direction: HIGH enables read (outputs driven), LOW enables write (inputs sampled). While one word can be read and another written in successive cycles using independent WE lines, concurrent access to the same 4-bit I/O bus is prevented by the single-direction control. This behavior is inherent to the SN74LS670D's architecture and confirmed in its functional truth table.

What is the significance of the NC (No Connect) pin on the SN74LS670D?

Pin 16 of the SN74LS670D is designated NC (No Connect) - meaning it is internally unbonded and electrically isolated. It must remain unconnected on the PCB and should not be tied to V_CC, GND, or any signal. This pin exists for mechanical symmetry and package compatibility within TI's SOIC-16 footprint family, not for functionality. Leaving it floating poses no risk, but routing or soldering to it may compromise SN74LS670D reliability or violate package specifications.

Can the SN74LS670D be used in place of static RAM in simple systems?

The SN74LS670D can substitute for small-capacity SRAM (e.g., 16×1 or 16×4) in systems where address decoding is simple and write control is word-granular. Unlike SRAM, it lacks automatic address incrementing or burst modes, but offers faster write-enable gating via four independent WE pins - a feature absent in most 16-bit SRAMs. However, the SN74LS670D requires explicit address generation and lacks chip-enable power-down, so it is best suited for fixed-function register storage rather than general-purpose memory expansion.

How does the SN74LS670D handle bus contention when multiple devices share the same data lines?

The SN74LS670D prevents bus contention using its active-low CS̅ (chip select) input: when CS̅ is HIGH, all I/O pins enter high-impedance (three-state) mode regardless of R̅/W̅ or address state. This allows safe connection to a shared data bus alongside other three-state devices like transceivers or additional register files. Proper timing - ensuring CS̅ is asserted before R̅/W̅ transitions - is essential to avoid glitches, and this behavior is fully specified in the SN74LS670D's DC and switching characteristics tables.