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Texas Instruments SN74AS638AN
- Part No.:
- SN74AS638AN
- Manufacturer:
- Texas Instruments
- Package:
- 20-DIP (0.300", 7.62mm)
- Datasheet:
-
SN74AS638AN.pdf
- Description:
- IC TRANSCEIVER INVERT 5.5V 20DIP
- Quantity:
- Payment:
- Shipping:
Inventory:4,908
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Product details
Overview
SN74AS638AN from Texas Instruments is an octal bus transceiver with inverting logic, open-collector A-bus outputs, and 3-state B-bus outputs, designed for asynchronous bidirectional communication between mixed-logic buses. It operates from 4.5 V to 5.5 V, supports 0°C to 70°C ambient, delivers 64 mA low-level output current on both buses, and features propagation delays as low as 2 ns (A→B) - used in legacy industrial backplane and TTL-to-open-collector interface systems.
For engineers reviewing the SN74AS638AN datasheet, SN74AS638AN pinout, SN74AS638AN application, or SN74AS638AN equivalent, key selection criteria include its inverting function, dual-output topology (open-collector A / 3-state B), temperature-rated operation, high sink capability (64 mA), and compatibility with standard 20-pin PDIP layouts in legacy control and instrumentation designs.
Technical Context
The SN74AS638AN implements a direction-controlled bidirectional data path: when DIR = LOW, data flows from B bus to A bus (open-collector); when DIR = HIGH, data flows from A bus to B bus (3-state). Its OE input enables full 3-state isolation of both buses simultaneously.
It uses advanced Schottky (AS) bipolar technology for higher speed and drive strength than ALS variants, with verified tPHL/tPLH ≤ 6.5 ns (A→B), VOL ≤ 0.55 V at IOL = 64 mA, and ICC up to 122 mA during active low-state operation - confirming suitability for high-current bus termination and noise-immune industrial signaling.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Logic Function | Inverting octal bus transceiver - output polarity inverted relative to input, critical for signal inversion in legacy bus arbitration schemes. |
| A-Bus Output Type | Open-collector - enables wired-AND bus sharing, pull-up resistor interfacing, and compatibility with legacy TTL/DTL loads. |
| B-Bus Output Type | 3-state - allows B-bus connection to shared data highways without contention when OE is inactive. |
| Max IOL (A/B) | 64 mA - supports direct driving of multiple TTL inputs or low-impedance termination networks without external buffers. |
| Propagation Delay (A→B) | 2–6.5 ns - ensures sub-10 ns round-trip latency in point-to-point or daisy-chained backplane links. |
| Supply Voltage Range | 4.5 V to 5.5 V - matches standard +5 V regulated rail tolerance, eliminating need for auxiliary regulation in legacy systems. |
| Operating Temperature | 0°C to 70°C - qualified for commercial/industrial indoor equipment environments, not extended or automotive grade. |
Pinout & Package
SN74AS638AN is supplied in a 20-pin plastic DIP (PDIP-N) package with 300-mil width, 0.100″ lead pitch, and through-hole mounting - compatible with legacy wave-solder and manual assembly processes.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 | DIR | Direction control input: LOW enables B→A transfer; HIGH enables A→B transfer - determines real-time data flow direction. |
| 2–9 | A1–A8 | Open-collector A-bus inputs/outputs - connect to wired-AND bus; require external pull-ups; tolerate up to 7 V. |
| 10 | GND | Ground reference - common return for all internal logic and output stages. |
| 11–18 | B1–B8 | 3-state B-bus outputs - driven high/low or placed in high-impedance state based on DIR and OE. |
| 19 | OE | Output enable input: LOW activates transceiver; HIGH disables both A and B ports into isolation states. |
| 20 | VCC | Positive supply - powers internal logic and output drivers; must be decoupled locally per best practice. |
Key Features
| Feature | Design Value |
|---|---|
| Inverting Logic Path | Guarantees signal inversion across all eight channels - essential for matching legacy bus timing and handshake protocols requiring complemented signals. |
| Asymmetric Output Architecture | Combines open-collector A-side (for bus-wired logic) and 3-state B-side (for multiplexed data highways) - eliminates need for external level-shifting or bus switches. |
| High Sink Current (64 mA) | Drives heavy capacitive loads or multiple TTL inputs directly - reduces component count in industrial I/O modules and PLC backplanes. |
| Sub-10 ns Propagation | Enables reliable operation in 20+ MHz synchronous bus environments when paired with matched trace lengths and termination. |
| Full Bus Isolation | OE HIGH places both A and B ports in high-Z or open state - prevents backfeeding and ensures clean domain separation during system reset or power sequencing. |
Applications
| Industrial Backplane Interface | TTL-to-Open-Collector Translation |
|---|---|
Use Scenario: Interfacing microcontroller-based control cards to legacy rack-mounted I/O modules using shared open-collector status buses. IC Role / Device Role / Timing Role: SN74AS638AN acts as a direction-controlled bidirectional translator, isolating controller-side 3-state logic from field-side open-collector wiring while preserving signal polarity. Use Value: Eliminates discrete transistor arrays and level shifters; leverages 64 mA sink capability to drive long bus stubs with minimal voltage drop. |
Use Scenario: Converting 3-state CPU address/data bus signals to open-collector format for daisy-chained peripheral arbitration (e.g., IEEE-488 GPIB-style handshaking). IC Role / Device Role / Timing Role: SN74AS638AN serves as a controlled polarity inverter and bus driver, synchronizing DIR/OE with CPU control lines to manage bus ownership transitions. Use Value: Achieves <5 ns A→B delay and robust 0.55 V VOL at full load - ensuring setup/hold timing margins are maintained across multi-drop configurations. |
| Legacy Instrumentation Data Link | Programmable Logic Controller (PLC) I/O Expansion |
Use Scenario: Bidirectional data exchange between a mainframe instrument processor and modular sensor acquisition boards using shared open-collector interrupt lines. IC Role / Device Role / Timing Role: SN74AS638AN provides isolated, direction-gated signal routing between processor-side 3-state buses and sensor-side open-collector event lines. Use Value: Open-collector A-bus tolerates 7 V, enabling safe interfacing with higher-voltage sensor logic; DIR-controlled flow prevents bus contention during simultaneous read/write cycles. |
Use Scenario: Expanding digital I/O capacity in DIN-rail mounted PLCs by connecting local I/O banks to a central backplane via differential or single-ended buses. IC Role / Device Role / Timing Role: SN74AS638AN functions as a ruggedized bus repeater and direction manager, translating between isolated 3-state control bus and open-collector field wiring. Use Value: 64 mA IOL drives long cable runs and multiple optocoupler inputs without external buffering; PDIP package supports serviceability and field replacement. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar octal bus transceiver applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74ALS638AN | ALS variant: lower IOL (24 mA), slower max speed (12 ns A→B), reduced ICC (41 mA active low), same pinout and logic function. | Suitable only for low-speed, low-drive legacy systems where 64 mA sink is unnecessary and timing budgets exceed 12 ns. | Select SN74ALS638AN only if cost sensitivity outweighs performance needs and existing board layout constrains upgrade to AS-series. |
| SN74AS639N | True (non-inverting) logic version; identical speed, drive, and package; differs only in signal polarity relationship between A and B buses. | Required when system protocol mandates non-inverted data path - e.g., direct replacement in non-inverting bus architectures without firmware changes. | Choose SN74AS639N when signal integrity analysis confirms polarity inversion would violate timing or protocol compliance in the target application. |
Compared with SN74ALS638AN, the SN74AS638AN delivers 2.7× higher sink current and ~2× faster A→B propagation, making it preferable for timing-critical industrial interfaces; versus SN74AS639N, it provides necessary inversion for legacy handshake protocols but requires verification of downstream logic polarity acceptance.
Availability
SN74AS638AN is available at Aetrix Electronics and suitable for industrial backplane interface, legacy instrumentation data link, and programmable logic controller (PLC) I/O expansion applications requiring stable component supply and long-term obsolescence management.
Supply support for SN74AS638AN 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 SN74AS638AN belongs to TI's 74AS logic family - engineered for high-speed, high-drive bipolar operation in mission-critical industrial control and test equipment where reliability and timing precision are paramount.
FAQ
What is the logic function of the SN74AS638AN?
The SN74AS638AN is an inverting octal bus transceiver: data transferred from A to B or B to A is logically inverted. This behavior is fixed and inherent to the device architecture - confirmed by the function table and logic diagrams in the SDAS123A datasheet. The SN74AS638AN does not support true (non-inverting) operation; for that, SN74AS639N is the designated alternative.
What are the absolute maximum voltage ratings for the A-bus and B-bus pins of the SN74AS638AN?
The SN74AS638AN A-bus pins (A1–A8) tolerate up to 7 V, supporting compatibility with higher-voltage open-collector systems. The B-bus pins (B1–B8) are rated for a maximum of 5.5 V - exceeding VCC but not intended for sustained overvoltage. Exceeding these limits risks permanent damage, per the absolute maximum ratings table in the official datasheet.
Does the SN74AS638AN support 3.3 V logic levels?
No, the SN74AS638AN is a 5 V-only device: its recommended VCC range is 4.5 V to 5.5 V, and input thresholds (VIH = 2 V, VIL = 0.8 V) are specified relative to 5 V operation. It lacks 3.3 V tolerant inputs or mixed-voltage I/O capability - interfacing with 3.3 V systems requires level translation circuitry external to the SN74AS638AN.
What is the thermal operating range for the SN74AS638AN?
The SN74AS638AN is characterized for operation from 0°C to 70°C ambient temperature - the commercial/industrial grade range. It is not rated for extended temperature operation (e.g., –40°C to 85°C or –55°C to 125°C), and performance outside this range is not guaranteed per the recommended operating conditions section of the SDAS123A datasheet.
Can the SN74AS638AN be used as a unidirectional driver?
Yes - the SN74AS638AN can operate unidirectionally by holding DIR constant (e.g., HIGH for A→B only) and controlling data flow via OE. However, its core design intent is bidirectional communication; unidirectional use still consumes both A and B port resources and retains the inverting function - verify downstream logic accepts inverted signals before deploying SN74AS638AN in unidirectional mode.
SN74AS638AN Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74AS
- Package/Case:
- 20-DIP (0.300", 7.62mm)
- Packaging:
- Tube
- Product Status:
- Obsolete
- Logic Type:
- Transceiver, Inverting
- Number of Elements:
- 1
- Number of Bits per Element:
- 8
- Input Type:
- -
- Output Type:
- Open Collector, Push-Pull
- Current - Output High, Low:
- -, 64mA; 15mA, 64mA
- Voltage - Supply:
- 4.5V ~ 5.5V
- Operating Temperature:
- 0°C ~ 70°C (TA)
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Through Hole
- Supplier Device Package:
- 20-PDIP
SN74AS638AN FAQ
1.How can I place an order for SN74AS638AN through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74AS638AN 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 SN74AS638AN reliable?
The price and inventory of SN74AS638AN are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74AS638AN is usually 5 days.
3.What payment methods are accepted for SN74AS638AN?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74AS638AN transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74AS638AN?
SN74AS638AN orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74AS638AN 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 SN74AS638AN?
For technical support, including SN74AS638AN datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74AS638AN requirements.
6.How does Aetrix verify that SN74AS638AN is sourced from the original manufacturer or authorized distributors?
All SN74AS638AN 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 SN74AS638AN meets industry standards.
7.What is the process for return or replacement of SN74AS638AN?
All SN74AS638AN units undergo pre-shipment inspection (PSI). If there is an issue with SN74AS638AN, 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 SN74AS638AN part is unused and in its original packaging.
Return procedure for SN74AS638AN:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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