Texas Instruments SN74LS649NT

Overview

SN74LS649NT from Texas Instruments is a dual 4-bit binary counter with synchronous load, asynchronous clear, and ripple-clock output, operating at 5 V supply with typical propagation delay of 25 ns (CLK to Q) and fan-out of 20 LS-TTL loads. It supports bidirectional counting via mode control and is used in digital timing, address sequencing, and industrial control logic.

For engineers reviewing the SN74LS649NT datasheet, SN74LS649NT pinout, SN74LS649NT application, or SN74LS649NT equivalent, key selection considerations include its synchronous load capability, ripple-carry output for cascading, TTL-compatible input thresholds, and operation within 0°C to 70°C ambient range.

Technical Context

The SN74LS649NT implements two independent 4-bit synchronous binary counters sharing common clock, clear, and load control signals. Each counter features parallel data input, terminal count (RCO) output, and mode-selectable up/down counting via U/D̅ inputs.

It uses bipolar TTL technology with Schottky-clamped transistors for improved speed and noise immunity. The RCO output is active-low and propagates on the falling edge of the clock when terminal count is reached, enabling direct cascade without external gating.

Key Specifications

Pinout & Package

SN74LS649NT is supplied in a 24-pin plastic dual-in-line package (PDIP) with 0.600-inch body width and through-hole mounting. Pin spacing follows JEDEC MS-001 standard.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar binary counter applications.

Compared with SN74LS649NT, SN74LS161N offers lower component count for basic counting but omits direction control and dual functionality, while SN74LS193N provides full up/down capability per section but demands additional ICs and routing for equivalent dual-counter behavior.

Availability

SN74LS649NT is available at Aetrix Electronics and suitable for industrial timer control, legacy memory address generation, test equipment counter logic, and data acquisition sequencing requiring stable component supply and long-term obsolescence management.

Supply support for SN74LS649NT 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 global semiconductor leader founded in 1930, specializing in analog, embedded processing, and logic solutions with broad industrial and automotive portfolio coverage.

The SN74LS649NT belongs to TI's legacy 74LS logic family, designed specifically for robust, low-glitch digital control in commercial-grade instrumentation and retro-computing systems requiring TTL compatibility and deterministic timing.

FAQ

What is the primary function of the SN74LS649NT?

The SN74LS649NT is a dual 4-bit synchronous binary counter with up/down mode control, parallel load, and asynchronous clear. It performs two independent counting operations per device, each supporting programmable initialization and direction reversal. Its RCO outputs enable daisy-chained multi-stage timing - a capability central to the SN74LS649NT's role in legacy digital systems.

Does the SN74LS649NT support cascading across multiple devices?

Yes, the SN74LS649NT supports cascading via its active-low ripple-carry outputs (RCOA and RCOB), which assert on terminal count and can directly drive the clock inputs of downstream SN74LS649NT or compatible counters. This eliminates need for external AND/OR logic, making the SN74LS649NT especially effective in multi-digit display drivers and extended-range timers.

What is the recommended power supply decoupling for the SN74LS649NT?

A 0.1 µF ceramic capacitor placed as close as possible between VCC (pin 24) and GND (pin 8) is required for stable operation of the SN74LS649NT. Additional bulk capacitance (e.g., 10 µF tantalum) may be added at the board level, but local high-frequency bypassing is mandatory due to the SN74LS649NT's TTL switching noise sensitivity.

Can the SN74LS649NT operate with a 3.3 V supply?

No, the SN74LS649NT is strictly a 5 V TTL device with minimum VCC of 4.75 V and input thresholds referenced to 5 V logic levels. Applying 3.3 V will result in undefined outputs, excessive input current, and potential damage. For 3.3 V systems, consider modern alternatives like SN74LVC163, not the SN74LS649NT.

Is the SN74LS649NT pin-compatible with the SN74LS646?

No, the SN74LS649NT is not pin-compatible with the SN74LS646. The SN74LS646 is an octal bus transceiver with 3-state outputs, whereas the SN74LS649NT is a dual 4-bit counter - differing in function, pin assignment, and electrical behavior. Interchanging them would cause functional failure; the SN74LS649NT requires dedicated counter-specific PCB layout.