Texas Instruments SN74LS646DWR

Overview

SN74LS646DWR from Texas Instruments is an octal bus transceiver with 3-state outputs and internal 32-bit bidirectional bus interface logic, featuring TTL-compatible inputs, 5-V supply operation, and ±24-mA output drive capability. It supports bidirectional data flow between two 8-bit buses in microprocessor systems with address/data multiplexing.

For engineers reviewing the SN74LS646DWR datasheet, SN74LS646DWR pinout, SN74LS646DWR application, or SN74LS646DWR equivalent, key selection criteria include 3-state control timing (OE/DIR setup/hold), bus-hold capability absence, SOIC-24 thermal limits, and compatibility with LS-family logic voltage thresholds and propagation delays.

Technical Context

The SN74LS646DWR integrates eight non-inverting transceivers with direction control (DIR) and output-enable (OE) inputs, enabling synchronous bidirectional data transfer without external gating. Its TTL-compatible input thresholds (V_IL = 0.8 V, V_IH = 2.0 V) and guaranteed 5-V operation ensure interoperability with legacy LS/TTL systems.

Each transceiver provides symmetrical output drive (–24 mA sink / +15 mA source at V_CC = 5 V), with typical propagation delay of 15 ns (A→B) and 17 ns (B→A) under standard load conditions. The device lacks bus-hold or weak pull-up circuitry, requiring external termination for floating-bus stability.

Key Specifications

Pinout & Package

SN74LS646DWR is housed in a 24-pin SOIC (DW) package with 300-mil body width, gull-wing leads, and JEDEC MS-013 compliant dimensions (7.5 mm × 15.4 mm). Thermal resistance θ_JA = 90°C/W (SOIC on FR-4, 1 oz Cu).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS245N and SN74ALS646DW, the SN74LS646DWR delivers optimal balance of drive strength, timing predictability, and integration density for legacy-compatible 8-bit bus architectures-especially where board space and deterministic direction control are critical.

Availability

SN74LS646DWR is available at Aetrix Electronics and suitable for industrial control backplanes, test equipment interfaces, and legacy system upgrades requiring stable component supply and long-term obsolescence management.

Supply support for SN74LS646DWR 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 for industrial, automotive, and communications markets.

The SN74LS646DWR belongs to TI's 74LS logic family, designed specifically for robust, low-cost interfacing in 5-V TTL-based systems where timing determinism and bus drive strength outweigh ultra-low power requirements.

FAQ

What is the function of the DIR pin on the SN74LS646DWR?

The DIR (Direction) pin on the SN74LS646DWR is an asynchronous level-sensitive control that determines data flow direction: when DIR = HIGH, data passes from Bus A to Bus B; when DIR = LOW, data flows from Bus B to Bus A. This pin operates independently of OE and does not require clocking. The SN74LS646DWR uses this single control to eliminate external direction logic needed in dual-transceiver configurations.

Does the SN74LS646DWR support bus-hold or weak pull-up functionality?

No, the SN74LS646DWR does not include bus-hold or weak pull-up circuitry on any I/O pin. All inputs and outputs rely on external termination or driven signals to maintain defined logic states. Floating bus lines will result in undefined operation and potential increased power consumption due to input stage metastability. External pull-up/down resistors or active drivers are required for unused or tri-stated bus segments.

What is the maximum operating frequency supported by the SN74LS646DWR in a bidirectional bus application?

The SN74LS646DWR does not specify a maximum clock frequency, as it is not a clocked device. Its usable data rate depends on propagation delay (15–17 ns) and system timing margins. In practice, it supports reliable operation in 8-MHz bus systems with proper setup/hold timing for OE and DIR. For higher rates, designers must verify timing closure using worst-case t_pd, t_su, and t_h values from the SN74LS646DWR datasheet.

Can the SN74LS646DWR be used with 3.3-V logic systems?

No, the SN74LS646DWR is strictly a 5-V TTL device with V_CC range 4.75–5.25 V and TTL input thresholds (V_IH ≥ 2.0 V, V_IL ≤ 0.8 V). It cannot reliably interface with 3.3-V CMOS logic without level translation, as its inputs may misinterpret 3.3-V HIGH signals and its 5-V outputs exceed 3.3-V I/O absolute maximum ratings. Use dedicated level translators or 3.3-V-compatible transceivers instead.

What does the "DWR" suffix indicate in SN74LS646DWR?

The "DWR" suffix in SN74LS646DWR denotes a tape-and-reel packaging variant of the SOIC-24 (DW) package, with 2000 units per reel and RoHS-compliant NiPdAu lead finish. It is functionally identical to the tube-packaged SN74LS646DW but optimized for automated SMT assembly. The SN74LS646DWR shares identical electrical specifications, timing, and pinout with all other SN74LS646DW variants.