Texas Instruments SN74LS647NT

Overview

SN74LS647NT from Texas Instruments is a 8-bit bidirectional bus transceiver with 3-state outputs and open-collector enable, designed for TTL-level data bus isolation and level translation in industrial control and legacy computing systems. It operates at VCC = 5 V, supports ±24 mA output drive, features 15 ns typical propagation delay, and is rated for 0°C to 70°C ambient operation.

For engineers reviewing the SN74LS647NT datasheet, SN74LS647NT pinout, SN74LS647NT application, or SN74LS647NT equivalent, key selection considerations include its open-collector output configuration, asymmetric drive capability (24 mA sink / 0.4 mA source), bus-hold-free operation, and compatibility with LS-TTL logic families in mixed-signal backplane interfaces.

Technical Context

The SN74LS647NT implements dual 4-bit bidirectional data paths controlled by separate direction (DIR) and output-enable (OE) inputs, enabling independent read/write control on shared buses. Its open-collector outputs require external pull-up resistors and support wired-AND logic implementation.

It uses bipolar Schottky TTL technology, ensuring compatibility with standard LS-TTL input thresholds and noise margins. The device lacks internal bus-hold or power-down circuitry, requiring explicit OE control to avoid floating bus states during power transitions.

Key Specifications

Pinout & Package

SN74LS647NT is supplied in a 20-pin plastic DIP (dual in-line package) with 0.3-inch body width and through-hole mounting. Pin spacing is 0.1 inch, and lead finish is tin-lead (SnPb).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS647NT, SN74LS245N offers tri-state flexibility but lacks wired-AND capability, while SN74ALS647N delivers measurable performance gains with full mechanical and electrical compatibility-making it the preferred upgrade path where available.

Availability

SN74LS647NT is available at Aetrix Electronics and suitable for industrial control systems, legacy computer refurbishment, and test equipment repair requiring stable component supply of through-hole TTL logic.

Supply support for SN74LS647NT 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 company founded in 1930, specializing in analog, embedded processing, and logic ICs with broad industrial, automotive, and consumer applications.

The SN74LS647NT belongs to TI's legacy 74LS logic family, engineered for robustness and interoperability in retro-computing, industrial automation, and maintenance of aging electronic infrastructure.

FAQ

What is the function of the DIR pin on the SN74LS647NT?

The DIR (direction) pin on the SN74LS647NT determines data flow direction between its two 8-bit ports: when DIR is HIGH, data passes from A-side to B-side; when DIR is LOW, data flows from B-side to A-side. It operates asynchronously and does not require clocking. This pin must be held at a valid TTL logic level at all times except during power-up sequencing to prevent undefined bus states. The SN74LS647NT requires both DIR and OE to be asserted for functional data transfer.

Can the SN74LS647NT drive LEDs directly?

Yes, the SN74LS647NT can drive LEDs directly using its open-collector outputs in sink mode: connect the LED anode to VCC (5 V) and cathode to an SN74LS647NT output pin, with a series current-limiting resistor. Each output supports up to 24 mA sink current, sufficient for standard indicator LEDs. Do not attempt to source current from outputs-the 0.4 mA source capability is insufficient for reliable LED illumination. The SN74LS647NT must be configured with OE LOW and appropriate DIR setting to activate the selected port.

Is the SN74LS647NT pin-compatible with the SN74LS245N?

No, the SN74LS647NT is not pin-compatible with the SN74LS245N. Although both are 20-pin DIP bus transceivers, their pin assignments differ: SN74LS647NT places DIR on pin 12 and OE on pin 20, whereas SN74LS245N uses pin 1 for OE and pin 19 for DIR. Additionally, SN74LS245N has separate A/B enable pins and no open-collector outputs. Substituting one for the other requires PCB redesign and logic reconfiguration. The SN74LS647NT remains distinct in its wired-AND support and pin mapping.

What is the maximum operating frequency supported by the SN74LS647NT?

The SN74LS647NT does not specify a maximum clock frequency because it is an asynchronous device with no internal clock. Its usable data rate depends on propagation delay (15 ns max), system setup/hold timing, and bus capacitance. In practice, it reliably supports bidirectional data transfers up to approximately 30 MHz in well-designed 5-V TTL buses with ≤30 pF load. For higher-speed applications, designers should verify timing margins using worst-case tPLH/tPHL values from the SN74LS647NT datasheet and account for trace inductance and termination.

Does the SN74LS647NT include bus-hold or power-down circuitry?

No, the SN74LS647NT does not include bus-hold or power-down circuitry. When OE is HIGH, all outputs enter high-impedance (open-circuit) state, but inputs remain unclamped and will float if left unconnected. External pull-up or pull-down resistors must be used on unused inputs to prevent metastability or excessive supply current. TI's official SN74LS647NT datasheet confirms absence of internal bias networks-this distinguishes it from later families like 74LVC or 74ALVC. Proper input termination is mandatory for stable operation of the SN74LS647NT in real-world systems.