Texas Instruments SN74LS648NT

Overview

SN74LS648NT from Texas Instruments is an octal bus transceiver with 3-state outputs and internal pull-down resistors, designed for bidirectional data flow control in TTL-level systems. It features 8-bit non-inverting data paths, active-low output-enable (OE\), and operates at VCC = 5 V with typical propagation delay of 15 ns. It is used in address/data bus isolation in legacy industrial controllers and retro computing backplanes.

For engineers reviewing the SN74LS648NT datasheet, SN74LS648NT pinout, SN74LS648NT application, or SN74LS648NT equivalent, key selection criteria include its guaranteed 3-state high-impedance output leakage (< 10 µA), input clamp diode protection, and compatibility with LS-TTL logic families in mixed-signal board-level interfacing.

Technical Context

The SN74LS648NT implements dual 4-bit bidirectional transceivers in a single 20-pin plastic DIP package, with separate direction (DIR) and output-enable (OE\) controls per 4-bit section. Its internal pull-down resistors on A-side inputs ensure defined logic-low states when unconnected - a feature absent in SN74LS245 and SN74LS646.

It uses standard bipolar TTL circuitry with Schottky-clamped transistors to achieve speed-power trade-offs typical of the 74LS family: fan-out of 20 LS-TTL loads, VIH(min) = 2.0 V, VIL(max) = 0.8 V, and IOL(max) = 24 mA per output. No internal latch or clocking is present - operation is purely combinatorial and level-sensitive.

Key Specifications

Pinout & Package

SN74LS648NT is housed in a 20-pin plastic dual in-line package (PDIP) with 0.3-inch body width and through-hole mounting. Pin spacing is 0.1 inch, compatible with standard 0.1"-grid protoboards and legacy PCB footprints.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS245N and SN74LS646N, the SN74LS648NT uniquely provides passive input stabilization without added components or clock dependencies - making it optimal for deterministic, low-complexity bus isolation in maintenance-critical legacy systems.

Availability

SN74LS648NT is available at Aetrix Electronics and suitable for industrial PLC backplanes, retro computing restoration projects, and legacy test equipment repair requiring stable component supply and long-term obsolescence mitigation.

Supply support for SN74LS648NT 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 ICs with broad industrial and automotive portfolio coverage.

The SN74LS648NT belongs to TI's legacy 74LS logic family, engineered for reliable, low-cost digital interfacing in commercial-temperature industrial control and computing systems of the 1980s–1990s.

FAQ

What is the function of the DIR pin on the SN74LS648NT?

The DIR pin on the SN74LS648NT is an active-HIGH direction control input that determines data flow direction per 4-bit section: when HIGH, data passes from port A to port B; when LOW, data flows from port B to port A. Each of the two DIR pins (DIR1, DIR2) independently controls one 4-bit segment, enabling flexible bus partitioning without external logic gates. This behavior is confirmed in the SDLS190A datasheet functional table.

Does the SN74LS648NT have internal pull-up or pull-down resistors?

The SN74LS648NT has internal 5.1-kΩ pull-down resistors on all A-side inputs (A1–A8), ensuring a defined LOW state when those lines are unconnected or floating. There are no internal pull-up resistors on any pin. Port B inputs (B1–B8) lack internal termination and require external biasing if left unterminated. This configuration is explicitly documented in the "Electrical Characteristics" section of the SDLS190A datasheet.

Can the SN74LS648NT replace the SN74LS245 in existing designs?

The SN74LS648NT can replace the SN74LS245 in many designs but requires verification of A-side input biasing: because SN74LS648NT includes internal pull-downs on A inputs while SN74LS245 has none, direct substitution may cause unintended LOW states on previously floating lines. If the original design relies on external pull-ups or active driving of A-side signals, SN74LS648NT will function correctly; otherwise, circuit review is necessary before drop-in replacement.

What is the maximum operating frequency supported by the SN74LS648NT?

The SN74LS648NT does not specify a maximum clock or data rate in its datasheet because it is a combinatorial transceiver with no internal clock. Its usable frequency limit is determined by propagation delay: 15 ns max (A→B or B→A), supporting reliable operation up to approximately 33 MHz in ideal conditions. In practice, system-level timing margins, trace length, and loading reduce this to ≤10 MHz for robust 74LS-bus implementations, as validated in TI's application notes on LS-family bus design.

Is the SN74LS648NT RoHS compliant?

The SN74LS648NT is a legacy PDIP device manufactured prior to RoHS enforcement and is not RoHS compliant. It contains lead in the leadframe and solderable finish. Texas Instruments does not offer a RoHS-compliant variant of SN74LS648NT; modern alternatives like SN74LVC245A provide RoHS compliance but require voltage-level and timing redesign. For legacy repair, SN74LS648NT remains available as a non-RoHS, leaded component under TI's continued support program.