Texas Instruments SN74ALS645ANS

Overview

SN74ALS645ANS from ON Semiconductor is an octal bus transceiver with true (non-inverting) 3-state outputs, designed for asynchronous bidirectional data transfer between two 8-bit buses. It operates at 5 V supply, supports 0°C to 70°C ambient temperature, delivers ±24 mA output drive, and features propagation delays as low as 8 ns (A↔B) and 15 ns enable/disable times - used in legacy industrial backplane and memory-mapped I/O interface designs.

For engineers reviewing the SN74ALS645ANS datasheet, SN74ALS645ANS pinout, SN74ALS645ANS application, or SN74ALS645ANS equivalent, key selection criteria include direction-controlled bidirectional flow, 3-state isolation timing (tPZL/tPLZ), guaranteed 5 V TTL compatibility, and PDIP-20 package mechanical fit for through-hole board upgrades.

Technical Context

This device implements dual-bus communication using a single DIR input to select A→B or B→A data flow, while G enables/disables all outputs simultaneously. Its TTL-compatible inputs accept VIH ≥ 2.0 V and VIL ≤ 0.6 V, and outputs meet standard LS voltage levels: VOH ≥ 2.4 V at IOH = –3 mA, VOL ≤ 0.4 V at IOL = 24 mA.

AC performance is characterized at TA = 25°C, VCC = 5.0 V, CL = 45 pF, RL = 667 Ω: tPLH/tPHL = 8–11 ns (A↔B), tPZL/tPZH = 23–40 ns (G/DIR to output enable), and tPLZ/tPHZ = 15–25 ns (G/DIR to high-impedance transition).

Key Specifications

Pinout & Package

SN74ALS645ANS is housed in a 20-pin plastic dual in-line package (PDIP–20, N suffix, Case 738), with 0.300-inch body width and 0.100-inch lead pitch. Pin 1 is located at the left end of the top row when viewed from above with notch or dot oriented upward.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS645N and SN74F645N, the SN74ALS645ANS delivers optimal balance of speed (8 ns), power (ICC = 70 mA), and noise immunity (VIK = –1.5 V), making it preferred for mid-speed industrial control where reliability and thermal stability outweigh raw speed.

Availability

SN74ALS645ANS is available at Aetrix Electronics and suitable for industrial backplane interfaces, legacy microprocessor bus expansion, and programmable logic controller I/O modules requiring stable component supply and long-term obsolescence management.

Supply support for SN74ALS645ANS 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

ON Semiconductor (now onsemi) is a global semiconductor manufacturer specializing in power management, analog, sensor, and logic solutions for automotive, industrial, and cloud infrastructure markets.

The SN74ALS645ANS belongs to the advanced low-power Schottky (ALS) logic family, engineered for improved speed-power trade-offs in legacy 5 V TTL systems where backward compatibility and thermal efficiency are critical.

FAQ

What is the function of the DIR pin on the SN74ALS645ANS?

The DIR (direction) pin on the SN74ALS645ANS controls data flow direction: when DIR = LOW, data transfers from Bus B to Bus A; when DIR = HIGH, data flows from Bus A to Bus B. This single-pin control eliminates the need for separate transmit/receive control logic, simplifying interface design in bidirectional systems like microprocessor data buses or test equipment capture paths. The SN74ALS645ANS uses true (non-inverting) logic for both directions.

Can SN74ALS645ANS replace SN74LS645N in an existing design?

Yes, SN74ALS645ANS is a direct functional and pinout-compatible replacement for SN74LS645N, sharing identical PDIP-20 packaging, pin assignments, and truth table behavior. Key improvements include faster propagation delay (8 ns vs 11 ns typical), lower supply current (70 mA vs 90 mA), and enhanced noise immunity (VIK = –1.5 V). No PCB changes are required, though decoupling and thermal layout should be verified for the reduced power dissipation of the SN74ALS645ANS.

What is the maximum output current rating for SN74ALS645ANS?

The SN74ALS645ANS guarantees IOL = 24 mA per output at VOL ≤ 0.4 V (VCC = 4.75 V, TA = 70°C), and IOH = –3.0 mA at VOH ≥ 2.4 V. These ratings allow driving up to 10 standard LS-TTL inputs or terminating short PCB traces without external buffers. Sustained operation at full 24 mA requires adequate PCB copper area and ambient temperature control to maintain junction temperature within limits.

Does SN74ALS645ANS support hot insertion or live bus swapping?

No, SN74ALS645ANS is not designed for hot insertion. Its inputs lack powered-off protection, and outputs do not feature bus-hold or weak pull-up circuitry. Applying signals to A or B pins while VCC is unpowered may forward-bias internal clamp diodes, risking latch-up or damage. For live-swap applications, external protection circuitry or hot-swap controllers must be added - the SN74ALS645ANS itself provides no built-in hot-plug capability.

What are the recommended decoupling practices for SN74ALS645ANS?

Each SN74ALS645ANS should have a 0.1 µF ceramic capacitor placed between VCC (pin 19) and GND (pin 10), with leads ≤ 5 mm long and mounted directly adjacent to the package. For boards with multiple devices, add a bulk 4.7 µF–10 µF tantalum or aluminum electrolytic capacitor per 5–10 ICs near the power entry point. Avoid daisy-chaining VCC/GND traces; use star or grid routing to minimize shared impedance and ground bounce during simultaneous output switching.