Texas Instruments SN74ALS243ANS

Overview

SN74ALS243ANS from Texas Instruments is a quadruple 2-bit bus transceiver IC designed for asynchronous two-way data communication between parallel buses in TTL-compatible systems. It features dual output-enable controls (OEAB and OEBA), bidirectional data flow (A↔B), latch capability when both enables are low, and operates at 4.5–5.5 V with propagation delays as low as 4 ns. It is used in legacy industrial control backplanes and vintage computer bus arbitration circuits.

For engineers reviewing the SN74ALS243ANS datasheet, SN74ALS243ANS pinout, SN74ALS243ANS application, or SN74ALS243ANS equivalent, key selection considerations include its 3-state bidirectional operation, ±12 mA output drive, 0°C to 70°C temperature range, DIP-14 package compatibility, and ALS logic family timing characteristics.

Technical Context

The SN74ALS243ANS implements a dual-control 3-state transceiver architecture using bipolar ALS (Advanced Low-Power Schottky) technology. Its function table defines four distinct modes: A-to-B transmission (OEAB=L, OEBA=H), B-to-A transmission (OEAB=H, OEBA=L), full isolation (OEAB=H, OEBA=H), and transparent latching (OEAB=L, OEBA=L), where both bus sets retain state when isolated from external drivers.

Each of the four bit-pairs (A1/B1 through A4/B4) operates independently under shared enable logic, with pnp input stages reducing DC loading on driving sources. The device supports 50 pF capacitive loads and exhibits guaranteed VOL ≤ 0.4 V at IOL = 12 mA and VOH ≥ 2.4 V at IOH = −12 mA under recommended operating conditions.

Key Specifications

Pinout & Package

SN74ALS243ANS is supplied in a 14-pin plastic dual in-line package (PDIP), designated "N" by Texas Instruments, with 300-mil row spacing and through-hole mounting. Pin 7 is GND and Pin 14 is VCC; all other pins serve dedicated A/B data or control functions with no internal connections (NC).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS243N and SN74F243N, the SN74ALS243ANS delivers optimal balance of speed (4–11 ns), drive (±12 mA), and power (≤32 mA disabled), making it the preferred choice for mid-performance 5 V bus isolation where ALS compatibility is maintained.

Availability

SN74ALS243ANS is available at Aetrix Electronics and suitable for industrial backplane arbitration, legacy computer bus interfacing, test equipment signal routing, and embedded system debug interfaces requiring stable component supply and long-term obsolescence management.

Supply support for SN74ALS243ANS 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 solutions for industrial, automotive, and communications markets.

The SN74ALS243ANS belongs to TI's legacy 74ALS logic family, engineered for high-speed, low-power bidirectional bus communication in commercial-grade 5 V systems where reliability and pin compatibility with earlier 74xx series are critical.

FAQ

What is the maximum clock frequency supported by SN74ALS243ANS in a synchronous bus application?

The SN74ALS243ANS does not contain an internal clock; it is an asynchronous transceiver. Its usable data rate depends on propagation delay and system timing margins. With a maximum tPLH/tPHL of 11 ns and setup/hold allowances, it reliably supports bus cycles down to ~25 MHz in well-terminated 5 V TTL systems. For precise timing analysis, engineers must calculate total path delay including driver, trace, and receiver parameters - the SN74ALS243ANS itself contributes ≤11 ns per direction.

Can SN74ALS243ANS operate with a 3.3 V supply?

No, SN74ALS243ANS is specified only for 4.5 V to 5.5 V operation per its recommended conditions. Applying 3.3 V violates the minimum VCC requirement and will result in undefined logic levels, increased propagation delay, and potential failure to drive TTL-compatible loads. For 3.3 V systems, consider modern alternatives like SN74LVC245A, but note that SN74ALS243ANS is not 3.3 V tolerant or compatible.

Does SN74ALS243ANS support hot-swap or live-insertion into a powered bus?

SN74ALS243ANS is not rated for hot-swap operation. Its absolute maximum input voltage is 7 V, but applying signals before VCC is stable may cause latch-up or excessive current flow through internal protection diodes. To safely insert SN74ALS243ANS into a live bus, power sequencing must ensure VCC is established and stable prior to asserting any A/B or OE signals - this requires external control circuitry not provided by the SN74ALS243ANS itself.

How does the latch mode (OEAB = L, OEBA = L) function in SN74ALS243ANS?

In latch mode, both OEAB and OEBA are low, causing all eight I/O terminals (A1–A4, B1–B4) to enter a feedback configuration where each output reinforces its corresponding input. When all external drivers are in high-impedance state, the A and B buses retain their last valid logic states - effectively storing 4 bits bidirectionally. This mode is not a true register but a passive bus-hold behavior dependent on maintaining isolation from competing drivers, and is documented in the SN74ALS243ANS function table as "A = B".

Is SN74ALS243ANS RoHS compliant?

SN74ALS243ANS was manufactured prior to RoHS enforcement and is not RoHS compliant. TI's packaging addendum lists it as obsolete with no Pb-Free (RoHS) conversion plan defined ("TBD"). For new designs requiring RoHS compliance, engineers should select modern replacements such as SN74LVC245A or consult TI's current logic portfolio - the SN74ALS243ANS remains available for legacy repair and continuity use only.