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Texas Instruments SN74ALS621A-1NS
- Part No.:
- SN74ALS621A-1NS
- Manufacturer:
- Texas Instruments
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- -
- Datasheet:
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SN74ALS621A-1NS.pdf
- Description:
- IC TXRX BUS OCTAL 20SO
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Product details
Overview
SN74ALS621A-1NS 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-compatible systems. It features dual output-enable controls (OEAB/OEBA), local bus-latch capability, 20-pin SOIC (NS) package, and supports 48 mA low-level output current in the -1 version at VCC = 4.75–5.25 V. It operates across 0°C to 70°C and is used in legacy industrial backplane and memory expansion interfaces.
For engineers reviewing the SN74ALS621A-1NS datasheet, SN74ALS621A-1NS pinout, SN74ALS621A-1NS application, or SN74ALS621A-1NS equivalent, this page delivers verified functional identity, confirmed open-collector drive strength, validated timing parameters (tPLH/tPHL ≤ 33 ns), exact package mapping (SOP-20/NS), and two technically documented alternative parts for bus interface redesigns.
Technical Context
The SN74ALS621A-1NS implements a dual-bus transceiver architecture with independent OEAB (A-to-B enable) and OEBA (B-to-A enable) inputs, enabling flexible direction control and simultaneous bus isolation or latched storage. Its open-collector outputs require external pull-up resistors and support wired-AND logic, unlike 3-state variants.
Unlike the inverting SN74ALS620A or 3-state SN74ALS623A, the SN74ALS621A-1NS provides true (non-inverted) data transmission in both directions and achieves 48 mA IOL only in the -1 version under tightly regulated VCC (4.75–5.25 V), distinguishing it from standard SN74ALS621A's 24 mA rating.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Function | Octal bidirectional bus transceiver with true logic and open-collector outputs |
| Supply Voltage | 4.5 V to 5.5 V - compatible with standard 5 V TTL systems |
| Low-Level Output Current | 48 mA (–1 version only, at VCC = 4.75–5.25 V) - enables direct driving of higher-load buses without buffers |
| Propagation Delay | tPLH/tPHL ≤ 33 ns (A↔B), tPLH/tPHL ≤ 39 ns (OE→output) - ensures timing compliance in 20–25 MHz bus cycles |
| Operating Temperature | 0°C to 70°C - qualified for commercial-grade industrial control and instrumentation environments |
| Input Voltage Thresholds | VIH = 2 V, VIL = 0.8 V - matches standard TTL logic levels for seamless integration |
| Output Structure | Open-collector - requires external pull-up; supports wired-AND, level translation, and shared-bus arbitration |
Pinout & Package
SN74ALS621A-1NS is housed in a 20-pin SOP (Small Outline Package) with NS suffix, 0.300-inch body width, 12.6 mm × 7.4 mm footprint, and 1.27 mm lead pitch. Pin 1 is located at the top-left corner with notch or beveled edge marking.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 | OEAB | Active-low output-enable for A-to-B data path; disables A-side drivers when high |
| 2–9 | A1–A8 | Input/output terminals for A bus; open-collector outputs sink current when enabled |
| 10 | GND | Ground reference for all internal circuitry and I/O |
| 11–18 | B1–B8 | Input/output terminals for B bus; open-collector outputs sink current when enabled |
| 19 | OEBA | Active-low output-enable for B-to-A data path; disables B-side drivers when high |
| 20 | VCC | +5 V supply input; decoupling capacitor required near pin for noise immunity |
Key Features
| Feature | Design Value |
|---|---|
| True logic polarity | Non-inverting data transfer in both A→B and B→A directions - eliminates need for external inversion in symmetric bus designs |
| Dual output-enable control | Independent OEAB and OEBA pins allow selective direction activation, full isolation, or simultaneous latching of both buses |
| Open-collector outputs | Enable wired-AND bus sharing, level shifting via pull-up voltage selection, and fault-tolerant arbitration in multi-driver systems |
| Enhanced sink current (–1 version) | 48 mA IOL rating supports direct connection to heavier capacitive loads (e.g., >100 pF traces or multiple TTL inputs) without external drivers |
| Local bus-latch capability | Simultaneous assertion of OEAB and OEBA holds last valid state on both A and B buses - useful for temporary data retention during bus handoff |
Applications
| Industrial Backplane Interface | Legacy Memory Expansion |
|---|---|
Use Scenario: Interfacing microprocessor address/data buses to modular I/O cards in programmable logic controllers (PLCs). IC Role / Device Role / Timing Role: Bidirectional bus transceiver managing data flow between CPU and peripheral slots; open-collector outputs enable shared interrupt lines. Use Value: 48 mA sink current drives long backplane traces and multiple TTL loads; dual-OE control allows precise slot arbitration without glue logic. |
Use Scenario: Expanding RAM or ROM capacity in 1980s–1990s embedded systems using parallel address/data buses. IC Role / Device Role / Timing Role: True-logic octal transceiver isolating memory banks during read/write cycles; latch mode retains address during bank switching. Use Value: Matched TTL thresholds and sub-35 ns propagation ensure compatibility with Z80, 8086, and 68000-based systems without timing violations. |
| Test Equipment Bus Coupling | Industrial Data Acquisition Hub |
Use Scenario: Connecting DUT (device-under-test) signal lines to automated test equipment (ATE) controller via shared test bus. IC Role / Device Role / Timing Role: Open-collector transceiver enabling multiple DUTs to share a common data line with collision detection via pull-up monitoring. Use Value: Wired-AND capability simplifies bus arbitration; 0°C to 70°C rating supports operation inside temperature-controlled test racks. |
Use Scenario: Aggregating sensor data from multiple analog front-ends onto a central microcontroller bus in factory-floor monitoring systems. IC Role / Device Role / Timing Role: Bidirectional interface buffering sensor module data streams while maintaining electrical isolation between subsystems. Use Value: Dual-OE latching preserves sensor readings during microcontroller interrupt servicing; SOP-20 package fits dense PCB layouts. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar octal bus transceiver applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74ALS621AN | Same function and pinout, but in 20-pin PDIP (N) package; rated for 24 mA IOL (not 48 mA) | Suitable for through-hole prototyping or legacy board rework where SOIC footprint is unavailable | Select SN74ALS621AN only if PDIP assembly is required and 24 mA drive suffices. |
| SN74ALS623ANSR | 3-state (not open-collector) outputs, true logic, SOP-20 (NS) package; rated for 24 mA IOL | Requires separate bus-direction logic and cannot perform wired-AND; better for high-speed isolated bus segments | Choose SN74ALS623ANSR only when 3-state control and absence of pull-ups are preferred over wired-AND functionality. |
Compared with SN74ALS621A-1NS, SN74ALS621AN lacks the enhanced 48 mA drive and uses a larger through-hole package, while SN74ALS623ANSR replaces open-collector flexibility with 3-state isolation - making SN74ALS621A-1NS uniquely suited for shared-bus arbitration and heavy-sink legacy interfaces.
Availability
SN74ALS621A-1NS is available at Aetrix Electronics and suitable for industrial backplane interfaces, legacy memory expansion systems, and test equipment bus coupling requiring stable component supply and long-term obsolescence management.
Supply support for SN74ALS621A-1NS 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, specializing in analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.
The SN74ALS621A-1NS belongs to TI's legacy ALS (Advanced Low-Power Schottky) logic family, engineered for high-speed, low-power TTL-compatible bus interfacing in 1980s–1990s industrial and computing systems.
FAQ
What is the key functional distinction of SN74ALS621A-1NS versus standard SN74ALS621A?
The SN74ALS621A-1NS is the –1 version of the SN74ALS621A, featuring an increased low-level output current rating of 48 mA (vs. 24 mA in the standard part), but only when VCC is tightly regulated between 4.75 V and 5.25 V. This enhancement enables direct driving of higher-capacitance or multi-load buses without external buffers, while retaining identical pinout, logic function, and open-collector architecture. All other electrical and timing specifications remain unchanged from the base SN74ALS621A.
Does SN74ALS621A-1NS support bidirectional data flow without external direction control logic?
Yes - the SN74ALS621A-1NS supports fully bidirectional data flow using its two dedicated output-enable inputs: OEAB controls A-to-B transmission, and OEBA controls B-to-A transmission. When both are asserted low, the device enters local bus-latch mode, holding the last valid states on both buses. No external direction register or control logic is needed, making it ideal for simple, deterministic bus handshaking in legacy systems.
What package type does SN74ALS621A-1NS use, and is it RoHS compliant?
SN74ALS621A-1NS uses the SOP-20 (NS) package: a surface-mount, 20-pin small-outline plastic package with 1.27 mm lead pitch, 12.6 mm × 7.4 mm body, and 2.65 mm max height. Per TI's official packaging documentation, SN74ALS621A-1NS is RoHS-compliant (lead-free NiPdAu finish) and rated MSL Level-1 (unlimited floor life at <30°C/85% RH), with peak reflow temperature of 260°C.
Can SN74ALS621A-1NS replace SN74ALS620A in an existing design?
No - SN74ALS621A-1NS cannot directly replace SN74ALS620A because they differ fundamentally in logic polarity and output structure: SN74ALS620A is inverting with 3-state outputs, while SN74ALS621A-1NS is true-logic with open-collector outputs. Swapping them would invert data and eliminate high-impedance tri-state behavior, breaking bus isolation and potentially causing contention. A redesign including pull-up resistors and logic inversion would be required.
What is the maximum capacitive load SN74ALS621A-1NS can drive while maintaining specified timing?
Per the datasheet's switching characteristics test conditions, SN74ALS621A-1NS is characterized for CL = 50 pF with RL = 680 Ω. While not explicitly rated beyond that, its 48 mA IOL and typical VOL of 0.35–0.5 V at 48 mA indicate reliable operation into ≥100 pF loads when paired with appropriately sized pull-up resistors (e.g., 1–2 kΩ for 5 V). For loads exceeding 100 pF, empirical validation under actual signal integrity conditions is recommended.
SN74ALS621A-1NS Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- *
- Package/Case:
- -
- Packaging:
- Bulk
- Product Status:
- Active
- Logic Type:
- -
- Number of Elements:
- -
- Number of Bits per Element:
- -
- Input Type:
- -
- Output Type:
- -
- Current - Output High, Low:
- -
- Voltage - Supply:
- -
- Operating Temperature:
- -
- Grade:
- -
- Qualification:
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SN74ALS621A-1NS FAQ
1.How can I place an order for SN74ALS621A-1NS through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74ALS621A-1NS on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.
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The price and inventory of SN74ALS621A-1NS are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74ALS621A-1NS is usually 5 days.
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Once your SN74ALS621A-1NS order is processed, you will receive an email with the shipment details and tracking number.
Note: Tracking information may take up to 24 hours to appear. Express delivery typically takes 3–5 business days.
5.How can I obtain technical support or documentation for SN74ALS621A-1NS?
For technical support, including SN74ALS621A-1NS datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74ALS621A-1NS requirements.
6.How does Aetrix verify that SN74ALS621A-1NS is sourced from the original manufacturer or authorized distributors?
All SN74ALS621A-1NS products on Aetrix are procured from qualified distributors and authorized channels. Our dedicated quality assurance team conducts strict verification, including traceability checks and, if necessary, third-party testing. This ensures that SN74ALS621A-1NS meets industry standards.
7.What is the process for return or replacement of SN74ALS621A-1NS?
All SN74ALS621A-1NS units undergo pre-shipment inspection (PSI). If there is an issue with SN74ALS621A-1NS, returns or replacements are accepted under the following conditions:
1.Quantity discrepancies, incorrect items, or visible external defects (such as breakage or corrosion), acknowledged by Aetrix.
2.The issue is reported within 90 days of delivery.
3.The SN74ALS621A-1NS part is unused and in its original packaging.
Return procedure for SN74ALS621A-1NS:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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