Texas Instruments SN74ALS243AD

Overview

SN74ALS243AD from Texas Instruments is a quadruple 2-bit bus transceiver with dual output-enable controls (OEAB and OEBA), designed for asynchronous two-way data communication between A and B buses in TTL-level systems. It operates at 4.5 V to 5.5 V, supports 0°C to 70°C ambient, delivers ±12 mA output drive, and features pnp-input structure to reduce DC loading on driving sources. It enables bidirectional data flow, bus isolation, or latched state retention when both enables are active.

For engineers reviewing the SN74ALS243AD datasheet, SN74ALS243AD pinout, SN74ALS243AD application, or SN74ALS243AD equivalent, key selection criteria include its dual-enable 3-state control logic, 11 ns max propagation delay (VCC = 4.5–5.5 V, CL = 50 pF), 0.4 V max VOL at 12 mA, and SOIC-14 package compatibility with legacy DIP-based bus architectures.

Technical Context

The SN74ALS243AD implements independent A↔B bidirectional data paths using two separate 3-state enable inputs: OEAB controls A-to-B transmission, OEBA controls B-to-A transmission. Its dual-enable latch mode (OEAB = L, OEBA = L) allows simultaneous high-impedance isolation of both buses while retaining prior logic states across all eight I/O lines.

It uses ALS (Advanced Low-Power Schottky) bipolar technology, delivering TTL-compatible voltage thresholds (VIH = 2.0 V min, VIL = 0.8 V max), low power consumption (ICC = 20–30 mA active, 21–32 mA disabled), and robust noise immunity via pnp input stages that minimize input current draw (IIH = 20 µA max, IIL = −0.1 mA max).

Key Specifications

Pinout & Package

SN74ALS243AD is housed in a 14-pin plastic small-outline integrated circuit (SOIC) package (Package Drawing D), with 300-mil width and standard JEDEC MS-012AC footprint. Pin 1 is located at the top-left corner with polarity notch or dot marking.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS243N and SN74ALS244AN, the SN74ALS243AD provides faster switching, lower power, and unique dual-enable latch capability - making it optimal for dynamic bus sharing, hot-plug interfaces, and space-constrained designs requiring bidirectional control in a single package.

Availability

SN74ALS243AD is available at Aetrix Electronics and suitable for industrial PLC backplanes, legacy microprocessor bus extensions, automated test fixture arbitration, and signal routing matrices requiring stable component supply and long-term obsolescence management.

Supply support for SN74ALS243AD 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, logic, and power management ICs for industrial, automotive, and communications markets.

The SN74ALS243AD belongs to TI's legacy 74ALS logic family, engineered for high-speed, low-power TTL-compatible bus interfacing in systems where reliability, timing predictability, and backward compatibility with 74LS designs are critical.

FAQ

What is the maximum operating temperature range for the SN74ALS243AD?

The SN74ALS243AD is characterized for operation from 0°C to 70°C - the commercial temperature range. This differs from the SN54ALS243A military variant, which supports −55°C to 125°C. The SN74ALS243AD datasheet specifies absolute maximum ratings up to 150°C storage, but functional operation is guaranteed only within the 0°C to 70°C ambient range. Always verify thermal derating in high-power-density layouts.

Does the SN74ALS243AD support true bidirectional data flow on each pin pair?

Yes, the SN74ALS243AD supports true bidirectional data flow: A1↔B1, A2↔B2, A3↔B3, and A4↔B4. Direction is controlled exclusively by the logic states of OEAB and OEBA - low on OEAB enables A→B transmission, low on OEBA enables B→A transmission. Both can be low simultaneously to enter latch mode, preserving logic states on both buses without external clocking or storage.

What is the purpose of the NC pins on the SN74ALS243AD SOIC package?

The SN74ALS243AD SOIC-14 package has no NC (no-connect) pins - all 14 pins are assigned per the datasheet pinout. Earlier ceramic DIP (J) and LCCC (FK) variants included NC pins, but the D-package version uses pins 13 and 14 as NC only in certain drawings; however, TI's official SN74ALS243AD D-package marking and SOIC drawing confirm pins 13 and 14 are NC and must remain unconnected. Connecting them risks undefined behavior or damage.

Can the SN74ALS243AD replace a 74LS243 in an existing design without modification?

Yes, the SN74ALS243AD is a direct functional and pin-compatible replacement for the 74LS243 in most cases: same SOIC-14 or PDIP-14 footprint, identical pinout, TTL-compatible voltage thresholds, and improved specs (faster speed, lower power, latch mode). However, verify that the host system's timing budget accommodates the SN74ALS243AD's shorter propagation delay and that VCC ripple remains within 4.5–5.5 V, as ALS devices are less tolerant of undershoot than LS.

What does "pnp inputs reduce dc loading" mean for the SN74ALS243AD?

"Pnp inputs reduce dc loading" means the SN74ALS243AD uses PNP bipolar transistors at its input stage instead of the NPN-based inputs found in standard TTL. This reduces input current draw: IIH is only 20 µA (vs. ~20 µA for LS but with better noise margin) and IIL is just −0.1 mA (vs. −0.4 mA for 74LS243). As a result, the SN74ALS243AD places significantly less DC fan-out burden on upstream drivers - critical when driving multiple transceivers from a single microcontroller port or FPGA I/O bank.