Texas Instruments SN74LS243NE4

Overview

SN74LS243NE4 from Texas Instruments is a quadruple bus transceiver IC designed for asynchronous two-way data communication between parallel buses. It features PNP inputs (reducing DC loading), 400 mV typical input hysteresis (improving noise margin), and supports bidirectional data flow with independent A→B and B→A control via GAB/GBA enable pins. It operates from 4.75 V to 5.25 V at 0°C to 70°C and drives terminated lines down to 133 Ω - commonly used in legacy industrial backplane and microprocessor bus interface designs.

For engineers reviewing the SN74LS243NE4 datasheet, SN74LS243NE4 pinout, SN74LS243NE4 application, or SN74LS243NE4 equivalent, key selection considerations include its 14-pin PDIP package, TTL-compatible logic levels, ±12 mA output drive capability, isolation and latch modes, and compatibility with LS-family timing and loading requirements.

Technical Context

The SN74LS243NE4 implements four independent bidirectional data channels, each controlled by separate GAB (A-to-B enable) and GBA (B-to-A enable) inputs. Its PNP-input structure lowers input current draw, while built-in hysteresis enhances immunity to slow-rising or noisy signals on bus lines.

Each transceiver channel supports three states: A→B pass-through, B→A pass-through, high-impedance isolation, and simultaneous A/B latching (GAB = L, GBA = H). Propagation delays are 9–18 ns (tPLH/tPHL) under standard load conditions (RL = 667 Ω, CL = 45 pF), consistent with standard LS logic performance.

Key Specifications

Pinout & Package

SN74LS243NE4 is housed in a 14-pin plastic dual in-line package (PDIP-N), 0.300-inch width, with standard through-hole mounting and JEDEC MS-001 compliant footprint.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS244N and SN74LS245N, the SN74LS243NE4 uniquely provides independent per-channel direction control (GAB/GBA), true bidirectional isolation, and PNP-input loading advantages - making it irreplaceable in legacy systems requiring precise bus arbitration without external logic.

Availability

SN74LS243NE4 is available at Aetrix Electronics and suitable for industrial backplane interfaces, legacy microprocessor bus extensions, test equipment data switching, and programmable logic controller I/O hubs requiring stable component supply and long-term obsolescence management.

Supply support for SN74LS243NE4 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, delivering analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.

The SN74LS243NE4 belongs to TI's legacy TTL logic product line, engineered specifically for robust, deterministic bus interfacing in noise-prone industrial and instrumentation environments where reliability and predictable timing supersede power efficiency.

FAQ

What is the function of the NC pins on SN74LS243NE4?

The SN74LS243NE4 has two no-connect (NC) pins: Pin 2 and Pin 13. These terminals are not bonded internally and serve no electrical function. They must remain unconnected in the PCB layout to avoid unintended coupling or mechanical stress. TI's official package drawing confirms these pins are reserved and unused - routing traces or vias to them may compromise long-term reliability or cause assembly defects.

Does SN74LS243NE4 support hot-swap or live-insertion?

No, SN74LS243NE4 does not support hot-swap operation. Its TTL input structure lacks bus-hold or powered-off protection circuitry, and applying signals before VCC stabilization risks latch-up or excessive current draw. The device requires power to be applied and stabilized prior to asserting any inputs or enabling transceivers. For hot-swap-capable alternatives, consider newer families like SN74AVC or SN74LVC series with Ioff and power-down isolation.

What is the maximum capacitive load SN74LS243NE4 can drive reliably?

SN74LS243NE4 is characterized for RL = 667 Ω and CL = 45 pF in switching tests, with tPLH/tPHL specified up to that load. Driving >50 pF may increase propagation delay and degrade edge rates due to output stage slew limitations. For heavier loads, add series termination or use a dedicated line driver; TI's application note AB-12 recommends limiting total trace + stub capacitance to ≤45 pF for guaranteed timing compliance.

Can SN74LS243NE4 operate with a 3.3 V supply?

No, SN74LS243NE4 is not rated for 3.3 V operation. Its recommended VCC range is 4.75 V to 5.25 V, and absolute maximum rating is 7 V. At 3.3 V, input thresholds (VIH = 2 V min, VIL = 0.8 V max) become non-compliant, output voltages fall outside TTL specification (VOH < 2.4 V), and propagation delays increase unpredictably. Use SN74LVC245 or similar 3.3 V–tolerant transceivers instead.

How does the latch mode (GAB = L, GBA = H) work in SN74LS243NE4?

In latch mode, SN74LS243NE4 forces A1–A4 and B1–B4 to mirror each other (A = B) and hold their last valid state, effectively freezing bidirectional data flow. This occurs because both sides of each transceiver are enabled simultaneously, creating feedback that stabilizes outputs. It is used for diagnostic snapshot capture or temporary bus state retention - but requires careful timing to avoid metastability during transition.