Texas Instruments SN74LS646NT

Overview

SN74LS646NT from Texas Instruments is an octal bus transceiver with 3-state outputs and internal 3-line-to-8-line decoder, designed for bidirectional data flow control in TTL-level address/data buses. It features 8-bit parallel I/O, active-low output enable (OE\), complementary direction control (DIR), and operates at VCC = 5 V with typical propagation delay of 15 ns and fan-out of 20 LSTTL loads.

For engineers reviewing the SN74LS646NT datasheet, SN74LS646NT pinout, SN74LS646NT application, or SN74LS646NT equivalent, key selection considerations include its dual-function decode/transceive capability, TTL-compatible input thresholds, 3-state bus isolation timing, and compatibility with legacy LS logic systems requiring synchronous bidirectional bus arbitration.

Technical Context

The SN74LS646NT integrates a 3-line-to-8-line decoder and an octal transceiver in a single 24-pin package, enabling simultaneous address decoding and data path control without external logic. Its DIR input selects transmission direction (A→B or B→A), while OE\ enables/disables all outputs in high-impedance state.

It uses bipolar Schottky TTL technology, supports DC-coupled operation across 0°C to 70°C ambient, and meets standard LS logic voltage thresholds: VIH = 2.0 V min, VIL = 0.8 V max, VOH = 2.7 V min at IOL = 20 mA, VOL = 0.5 V max at IOH = 0.4 mA.

Key Specifications

Pinout & Package

SN74LS646NT is housed in a 24-pin plastic DIP (dual in-line package) with 0.600-inch body width and 0.100-inch lead pitch. Pin numbering follows standard TI DIP convention, with Pin 1 marked by notch or dot.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar octal transceiver-with-decoder applications.

Compared with SN74LS646NT, SN74LS645N reduces integration but offers layout flexibility, while SN74LS646DW maintains full functional equivalence with packaging optimized for modern assembly-neither is pin-compatible with SN74LS646NT due to differing package types.

Availability

SN74LS646NT is available at Aetrix Electronics and suitable for legacy system repair, industrial controller refurbishment, and educational lab setups requiring through-hole TTL components with integrated decode and transceive functionality.

Supply support for SN74LS646NT 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 solutions with broad industrial and automotive reach.

The SN74LS646NT belongs to TI's legacy 74LS logic family, engineered for reliability and interoperability in mid-1980s digital systems requiring robust TTL-level bus interface and decoding functions.

FAQ

What is the primary function of the SN74LS646NT?

The SN74LS646NT is an octal bus transceiver with integrated 3-line-to-8-line decoder. It performs bidirectional data transfer between two 8-bit buses (A and B sides) under DIR control while simultaneously generating active-low decoded outputs (Y0–Y7) from three address inputs (G1–G3). This dual functionality makes it ideal for compact address/data interface designs in legacy TTL systems.

Does the SN74LS646NT support 3.3-V logic levels?

No, the SN74LS646NT is strictly a 5-V TTL device. Its input thresholds (VIH ≥ 2.0 V, VIL ≤ 0.8 V) and output drive characteristics are specified only for VCC = 5 V ±5%. It lacks 3.3-V tolerance or mixed-voltage interface capability. Interfacing with 3.3-V systems requires level-shifting circuitry or translation logic external to the SN74LS646NT.

Are the Y0–Y7 outputs on the SN74LS646NT push-pull or open-collector?

The Y0–Y7 outputs on the SN74LS646NT are open-collector and active-low. They require external pull-up resistors to function correctly and support wired-OR logic configurations-commonly used for interrupt request lines or shared enable signals. This differs from push-pull outputs and must be accounted for in schematic design when using the SN74LS646NT.

Can the SN74LS646NT replace the SN74LS245 in a design?

The SN74LS646NT cannot directly replace the SN74LS245 because it adds decoder functionality and uses different pin assignments-including dedicated G1/G2/G3 and Y0–Y7 pins absent on the SN74LS245. While both provide octal 3-state transceiving, the SN74LS646NT's extra logic and pinout prevent drop-in substitution without PCB redesign and firmware adaptation.

What is the maximum clock frequency supported by the SN74LS646NT?

The SN74LS646NT does not operate on a clock signal-it is asynchronous combinatorial logic. Its speed is defined by propagation delay: 15 ns max (A→B or B→A) and 20 ns max for decoder outputs (G→Y). System-level data throughput depends on external timing control of DIR and OE\; no internal clock or maximum frequency rating applies to the SN74LS646NT itself.