Texas Instruments SN74ALS621ADW

Overview

SN74ALS621ADW from Texas Instruments is an octal bus transceiver with open-collector outputs and true logic polarity, designed for asynchronous bidirectional data transfer between A and B buses in TTL-level systems. It features dual output-enable controls (OEAB/OEBA), local bus-latch capability via simultaneous enable, and operates across 0°C to 70°C with 5 V supply. Used in legacy industrial backplanes and memory-mapped I/O interfaces requiring level translation and bus isolation.

For engineers reviewing the SN74ALS621ADW datasheet, SN74ALS621ADW pinout, SN74ALS621ADW application, or SN74ALS621ADW equivalent, key selection criteria include its open-collector output architecture, 24 mA low-level drive (48 mA in -1 variant), 20-pin SOIC (DW) package, and compatibility with ALS logic families in mixed-technology bus systems.

Technical Context

The SN74ALS621ADW implements a dual-bus transceiver with independent OEAB (A-to-B enable) and OEBA (B-to-A enable) inputs, enabling flexible direction control and bus-latch behavior when both enables are asserted. Its open-collector outputs require external pull-up resistors and support wired-AND logic, current-sinking up to 24 mA per channel at VCC = 4.5–5.5 V.

Timing is characterized with CL = 50 pF and RL = 680 Ω, delivering propagation delays of 5–33 ns (A↔B) and enable/disable times of 10–39 ns. The device conforms to ALS family voltage thresholds: VIH = 2 V, VIL = 0.8 V, VOH = 5.5 V (open), VOL = 0.35–0.5 V at IOL = 24 mA.

Key Specifications

Pinout & Package

SN74ALS621ADW uses a 20-pin SOIC (DW) package, 7.5 mm wide, 12.83 mm long, 2.65 mm max height, with 1.27 mm pitch and JEDEC MS-013 compliant footprint.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74ALS621ADW, SN74ALS623ADW offers simplified 3-state control but lacks wired-AND functionality, while SN74AS623DW delivers significantly faster switching at the cost of higher ICC and thermal load-making SN74ALS621ADW optimal for cost-sensitive, moderate-speed industrial bus isolation with arbitration needs.

Availability

SN74ALS621ADW is available at Aetrix Electronics and suitable for industrial backplane interfaces, legacy memory-mapped I/O subsystems, wired-AND bus arbitration nodes, and TTL-to-discrete logic interconnects requiring stable component supply over extended production lifecycles.

Supply support for SN74ALS621ADW 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 specializing in analog, embedded processing, and logic solutions with decades of legacy product support.

The SN74ALS621ADW belongs to TI's Advanced Low-Power Schottky (ALS) logic family, engineered for reliable 5 V TTL-compatible bus interfacing in industrial control, instrumentation, and retro-computing systems where timing predictability and interoperability with older logic families are critical.

FAQ

What is the output configuration of the SN74ALS621ADW?

The SN74ALS621ADW features open-collector outputs on both A and B ports. This means each output can only sink current and requires external pull-up resistors to establish a high logic level. This architecture enables wired-AND bus structures, level translation, and safe bus sharing among multiple drivers - a key distinction from 3-state alternatives like SN74ALS623ADW. The SN74ALS621ADW does not provide active high-drive capability.

Does the SN74ALS621ADW support bus-latching behavior?

Yes, the SN74ALS621ADW supports local bus-latch functionality. When both output-enable inputs OEAB and OEBA are asserted low simultaneously, the transceiver enters a latched state: outputs reinforce their last driven states, and both A and B buses retain their prior logic levels as long as all other connected drivers remain in high-impedance mode. This behavior is explicitly documented in the functional description and enables temporary data hold without external latches.

What is the maximum sink current per output for SN74ALS621ADW?

The SN74ALS621ADW is rated for 24 mA low-level output current (IOL) per channel under recommended operating conditions (VCC = 4.5–5.5 V, TA = 0°C to 70°C). This value is confirmed in the "Recommended Operating Conditions" table for SN74ALS621A on page 5 of the datasheet. Note that the -1 variant (e.g., SN74ALS621A-1N) increases IOL to 48 mA, but SN74ALS621ADW is the standard version.

Is SN74ALS621ADW pin-compatible with SN74ALS623ADW?

Yes, SN74ALS621ADW and SN74ALS623ADW share identical 20-pin SOIC (DW) packages and pin assignments, including matching A1–A8, B1–B8, OEAB, OEBA, VCC, and GND locations. However, their output types differ: SN74ALS621ADW uses open-collector outputs, while SN74ALS623ADW uses 3-state totem-pole outputs. Direct substitution requires circuit review of pull-up requirements and bus contention handling.

What logic family does SN74ALS621ADW belong to, and how does it interface with modern components?

SN74ALS621ADW belongs to the Advanced Low-Power Schottky (ALS) logic family, a 5 V TTL-compatible series optimized for speed-power tradeoffs in legacy systems. It interfaces with modern components via level-shifting circuits or translators (e.g., TXS0108E) when connecting to 3.3 V or CMOS logic, as its VIH = 2 V and VIL = 0.8 V are incompatible with standard 3.3 V logic thresholds. Its open-collector outputs simplify integration with mixed-voltage buses using appropriate pull-up voltages.