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Texas Instruments SN74ALS621A-1N
- Part No.:
- SN74ALS621A-1N
- Manufacturer:
- Texas Instruments
- Package:
- 20-DIP (0.300", 7.62mm)
- Datasheet:
-
SN74ALS621A-1N.pdf
- Description:
- IC TXRX NON-INVERT 5.5V 20DIP
- Quantity:
- Payment:
- Shipping:
Inventory:1,568
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Product details
Overview
SN74ALS621A-1N 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 supports 48 mA low-level output current in the -1 version. It operates across 0°C to 70°C and is used in legacy industrial control backplanes and parallel I/O expansion interfaces.
For engineers reviewing the SN74ALS621A-1N datasheet, SN74ALS621A-1N pinout, SN74ALS621A-1N application, or SN74ALS621A-1N equivalent, key selection considerations include its open-collector drive strength (48 mA IOL), dual-enable bus isolation behavior, latch-mode operation, and compatibility with 5 V TTL logic families in space-constrained DIP-20 layouts.
Technical Context
The SN74ALS621A-1N implements a bidirectional 8-bit transceiver architecture where data flows either A→B or B→A based on OEAB and OEBA logic states. Its open-collector outputs require external pull-up resistors and support wired-AND bus configurations, unlike 3-state variants.
When both OEAB and OEBA are asserted (low), the device enters bus-latch mode: outputs reinforce inputs, maintaining prior bus states across both A and B ports while other drivers are high-impedance. This enables transparent data storage without additional latches.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Range | 4.5 V to 5.5 V - compatible with standard 5 V TTL supply rails and tolerant of typical power rail variation. |
| IOL (max) | 48 mA - enables direct driving of higher-current loads (e.g., LEDs, relays) or termination into lower-impedance buses without external buffers. |
| Propagation Delay | 5–33 ns (A↔B) - ensures sub-35 ns bidirectional latency suitable for synchronous parallel bus handshaking at ≤20 MHz effective rates. |
| VOH / VOL | VOH = 5.5 V (open-collector, pulled externally); VOL = 0.5 V @ 48 mA - defines low-voltage drop under full load for reliable logic-low assertion. |
| Operating Temperature | 0°C to 70°C - validated for commercial/industrial ambient environments, excluding extended temperature or automotive use. |
| Input Thresholds | VIH = 2.0 V, VIL = 0.8 V - matches standard TTL input compatibility, ensuring noise margin and interoperability with 74LS/ALS/AS families. |
Pinout & Package
SN74ALS621A-1N is supplied in a 300-mil plastic dual in-line package (PDIP-20), with 2.54 mm lead pitch and standard through-hole mounting. The package is RoHS-compliant with NiPdAu lead finish and rated for manual and wave soldering.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1, 19 | OEAB, OEBA | Active-low output-enable controls: OEAB enables A→B direction; OEBA enables B→A direction; both low enables bus-latch mode. |
| 2–9 | A1–A8 | Input/output port A - bidirectional I/O pins with open-collector outputs; require external pull-ups for logic-high assertion. |
| 11–18 | B1–B8 | Input/output port B - identical electrical behavior to A port; supports independent or mirrored bus communication. |
| 10 | GND | Ground reference for all internal circuitry and I/O paths. |
| 20 | VCC | Positive supply terminal - must be decoupled locally with 0.1 µF ceramic capacitor near pin 20. |
Key Features
| Feature | Design Value |
|---|---|
| Open-collector outputs | Supports wired-AND bus arbitration, level translation via pull-up voltage selection, and direct interface to higher-voltage peripherals (e.g., 12 V logic). |
| Dual output-enable architecture | Enables independent directional control (A→B or B→A) or simultaneous isolation - critical for multi-master bus systems and hot-swap protection. |
| Local bus-latch capability | Eliminates need for external latches when OEAB and OEBA are both low; maintains stable bus states during arbitration or interrupt service routines. |
| True logic polarity | Preserves signal phase across transceiver path - essential for parity checking, CRC validation, and non-inverting data path integrity. |
| -1 version enhanced drive | 48 mA IOL rating (vs. 24 mA in standard SN74ALS621A) allows robust driving of unterminated lines or multiple TTL inputs without fanout limitation. |
Applications
| Industrial Backplane Interface | Legacy Parallel I/O Expansion |
|---|---|
Use Scenario: Interfacing microcontroller-based PLC modules to shared 8-bit data backplanes with multiple slot-addressable peripherals. IC Role / Device Role / Timing Role: Bidirectional data bridge between CPU bus and peripheral slots; provides direction control and bus contention isolation via OEAB/OEBA sequencing. Use Value: Enables deterministic read/write arbitration using simple enable timing, reducing FPGA or CPLD resource usage in retrofit designs. |
Use Scenario: Adding parallel printer or scanner ports to embedded x86 or 8051-based systems using ISA-style expansion headers. IC Role / Device Role / Timing Role: Level-shifting and bus-driving transceiver between host controller and peripheral data bus; open-collector outputs simplify handshake line sharing (e.g., BUSY, ACK). Use Value: Eliminates need for discrete transistor arrays on control lines while supporting 48 mA sink for LED status indicators and relay drivers. |
| Test Equipment Bus Coupling | TTL Logic Analyzer Front-End |
Use Scenario: Isolating and conditioning signals between DUT (device under test) and automated test system controllers in benchtop ATE fixtures. IC Role / Device Role / Timing Role: Directionally configurable buffer that prevents back-driving during test mode transitions; latch mode holds stimulus patterns during measurement windows. Use Value: Reduces fixture complexity by integrating direction control, isolation, and pattern hold in one DIP-20 IC - simplifying PCB layout and debug trace routing. |
Use Scenario: Capturing parallel address/data traces from vintage microprocessor systems (e.g., Z80, 6502) for protocol analysis and firmware reverse engineering. IC Role / Device Role / Timing Role: High-fidelity signal conditioner with minimal propagation skew (<33 ns) and TTL-compatible thresholds to preserve timing margins during capture. Use Value: Maintains setup/hold integrity for logic analyzer sampling clocks, enabling accurate reconstruction of instruction cycles and bus transactions. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar octal bus transceiver applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74ALS623A-1N | 3-state (not open-collector) outputs; true logic; same pinout and timing but requires separate bus drivers for wired-AND. | Suitable where bus isolation without external pull-ups is preferred; not usable in wired-AND or mixed-voltage termination schemes. | Select SN74ALS623A-1N only if system uses 3-state bus arbitration and does not require open-collector flexibility. |
| SN74AS623N | Faster propagation (1–9 ns vs. 5–33 ns); higher IOL (64 mA); AS family speed grade; same pinout and function but different AC characteristics. | Used in high-speed backplanes requiring <10 ns latency; consumes more power (ICC up to 189 mA vs. 48 mA) and has tighter layout constraints. | Choose SN74AS623N only when timing budget demands sub-10 ns delays and power/performance trade-off is acceptable. |
Compared with SN74ALS621A-1N, SN74ALS623A-1N offers simpler bus termination but lacks wired-AND capability, while SN74AS623N delivers significantly faster switching at the cost of higher supply current and reduced noise margin - making SN74ALS621A-1N optimal for robust, moderate-speed industrial interfacing where open-collector versatility is required.
Availability
SN74ALS621A-1N is available at Aetrix Electronics and suitable for industrial backplane interfaces, legacy parallel I/O expansion, test equipment bus coupling, and TTL logic analyzer front-ends requiring stable component supply across long-lifecycle programs.
Supply support for SN74ALS621A-1N 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 with deep heritage in TTL and advanced logic families.
The SN74ALS621A-1N belongs to TI's 74ALS advanced low-power Schottky logic product line, engineered for reliable, low-noise bidirectional data transfer in industrial control, instrumentation, and retro-computing infrastructure.
FAQ
What is the maximum sink current supported by SN74ALS621A-1N?
The SN74ALS621A-1N supports a guaranteed low-level output current (IOL) of 48 mA per pin under recommended operating conditions (VCC = 4.75–5.25 V). This rating applies specifically to the -1 version and enables direct driving of higher-current loads such as LEDs, relays, or multiple TTL inputs without external buffering. Exceeding this value risks exceeding absolute maximum ratings and may degrade reliability.
Does SN74ALS621A-1N support bidirectional data flow without external control logic?
Yes - SN74ALS621A-1N enables bidirectional data flow (A↔B) using only two active-low control signals: OEAB governs A→B transmission, and OEBA governs B→A transmission. No additional direction-control logic is needed. When both are low, the device enters bus-latch mode, reinforcing inputs to maintain bus state - a self-contained, logic-only implementation.
Can SN74ALS621A-1N be used with 3.3 V logic systems?
No - SN74ALS621A-1N is specified only for 4.5–5.5 V VCC operation and TTL-compatible input thresholds (VIH = 2.0 V, VIL = 0.8 V). Its inputs are not 5 V-tolerant in 3.3 V mode, and its open-collector outputs require pull-ups to 5 V for proper VOH definition. Interfacing with 3.3 V systems requires level-shifting circuitry or a different logic family.
What is the purpose of the bus-latch feature in SN74ALS621A-1N?
The bus-latch feature activates when both OEAB and OEBA are low, causing each output to reinforce its corresponding input. With all other bus drivers in high-impedance, both A and B buses retain their last valid states - effectively storing 16 bits without external latches. This is used in interrupt-driven systems or bus arbitration to freeze data during critical sections.
Is SN74ALS621A-1N pin-compatible with SN74ALS620A or SN74ALS623A?
Yes - SN74ALS621A-1N shares identical PDIP-20 pinout and footprint with SN74ALS620A and SN74ALS623A, including matching A/B port assignments and OEAB/OEBA locations. However, functional differences exist: SN74ALS620A is inverting, SN74ALS623A uses 3-state (not open-collector) outputs, and SN74ALS621A-1N is true-logic open-collector with enhanced IOL - electrical substitution requires design verification.
SN74ALS621A-1N Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74ALS
- Package/Case:
- 20-DIP (0.300", 7.62mm)
- Packaging:
- Bulk
- Product Status:
- Active
- Logic Type:
- Transceiver, Non-Inverting
- Number of Elements:
- 1
- Number of Bits per Element:
- 8
- Input Type:
- -
- Output Type:
- Open Collector
- Current - Output High, Low:
- -, 24mA
- Voltage - Supply:
- 4.5V ~ 5.5V
- Operating Temperature:
- 0°C ~ 70°C (TA)
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Through Hole
- Supplier Device Package:
- 20-PDIP
SN74ALS621A-1N FAQ
1.How can I place an order for SN74ALS621A-1N through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74ALS621A-1N on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.
2.Are the price and stock information for SN74ALS621A-1N reliable?
The price and inventory of SN74ALS621A-1N are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74ALS621A-1N is usually 5 days.
3.What payment methods are accepted for SN74ALS621A-1N?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74ALS621A-1N transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74ALS621A-1N?
SN74ALS621A-1N orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74ALS621A-1N 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-1N?
For technical support, including SN74ALS621A-1N datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74ALS621A-1N requirements.
6.How does Aetrix verify that SN74ALS621A-1N is sourced from the original manufacturer or authorized distributors?
All SN74ALS621A-1N 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-1N meets industry standards.
7.What is the process for return or replacement of SN74ALS621A-1N?
All SN74ALS621A-1N units undergo pre-shipment inspection (PSI). If there is an issue with SN74ALS621A-1N, 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-1N part is unused and in its original packaging.
Return procedure for SN74ALS621A-1N:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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