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Texas Instruments SN74ALS638A-1N
- Part No.:
- SN74ALS638A-1N
- Manufacturer:
- Texas Instruments
- Package:
- 20-DIP (0.300", 7.62mm)
- Datasheet:
-
SN74ALS638A-1N.pdf
- Description:
- IC TRANSCEIVER INVERT 5.5V 20DIP
- Quantity:
- Payment:
- Shipping:
Inventory:3,051
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Product details
Overview
SN74ALS638A-1N from Texas Instruments is an octal bus transceiver with inverting logic, designed for asynchronous bidirectional communication between open-collector A-bus and 3-state B-bus interfaces. It features 20-pin PDIP (N) package, 0°C to 70°C operating range, 48 mA max IOL (–1 version), and supports isolation via OE control. Used in legacy industrial backplane and parallel bus interface systems requiring level translation and bus contention management.
For engineers reviewing the SN74ALS638A-1N datasheet, SN74ALS638A-1N pinout, SN74ALS638A-1N application, or SN74ALS638A-1N equivalent, key selection criteria include open-collector A-port drive capability, 3-state B-port compatibility, DIR/OE control timing, inverting data path behavior, and PDIP packaging for through-hole prototyping and repair.
Technical Context
This device implements a dual-directional bus interface with direction-controlled signal flow: when DIR = L, data transfers from B bus to A bus; when DIR = H, data flows from A bus to B bus. The A-bus outputs are open-collector, requiring external pull-ups, while B-bus outputs are active 3-state with high-impedance disable under OE = H.
It operates within 4.5 V to 5.5 V VCC, delivers VOL ≤ 0.5 V at IOL = 48 mA (–1 version), and exhibits propagation delays as low as 2 ns (A→B) and 8 ns (B→A). All control inputs (DIR, OE) meet ALS family thresholds: VIH ≥ 2 V, VIL ≤ 0.8 V.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Range | 4.5 V to 5.5 V - Ensures compatibility with standard 5 V TTL/ALS logic rails and noise margin stability. |
| IOL (A/B ports) | 48 mA - Enables direct driving of multiple TTL loads or higher-current pull-down on open-collector buses. |
| tPLH / tPHL (A→B) | 2 ns to 12 ns - Supports high-speed bidirectional data transfer in synchronous bus arbitration schemes. |
| VOH (B ports) | ≥2.4 V at IOH = –15 mA - Guarantees valid HIGH-level signaling into standard TTL inputs under load. |
| VOL (A/B ports) | ≤0.5 V at IOL = 48 mA - Maintains low saturation voltage for reliable LOW-level assertion across temperature. |
| Operating Temp | 0°C to 70°C - Qualified for commercial-grade embedded control, instrumentation, and legacy computing systems. |
| Logic Function | Inverting - Output complements input; required for polarity-matched bus bridging in specific legacy architectures. |
Pinout & Package
SN74ALS638A-1N uses a 20-pin plastic DIP (PDIP-N) package with 300-mil width, through-hole mounting, and standard 0.1-inch pin pitch. Pin 1 is marked by a notch or dot; package height is 4.95 mm max.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (DIR) | Direction Control Input | Active-HIGH: enables A→B data flow; LOW enables B→A flow - critical for bus arbitration state machine design. |
| 2 (A1) | A-Bus Data Input/Output | Open-collector output/input - requires external pull-up; used for wired-OR bus topology or interrupt sharing. |
| 3–9 (A2–A8) | A-Bus Data I/Os | Seven additional open-collector channels - supports full 8-bit parallel bus bridging with shared pull-up network. |
| 10 (GND) | Ground Reference | Power return path for all internal logic and output stages - must be low-impedance for noise immunity. |
| 11–18 (B1–B8) | B-Bus Data Outputs | 3-state outputs - driven HIGH/LOW when enabled; high-Z when OE = HIGH for bus isolation. |
| 19 (OE) | Output Enable Input | Active-LOW: disables all B-port outputs and isolates B bus; A ports remain functional as inputs. |
| 20 (VCC) | Supply Voltage | +5 V nominal power rail - decoupling capacitor (0.1 µF) required near pin for switching noise suppression. |
Key Features
| Feature | Design Value |
|---|---|
| Inverting data path | Ensures signal polarity alignment with legacy bus protocols requiring complemented handshake or status lines. |
| Asymmetric port types | A-bus open-collector + B-bus 3-state enables mixed-technology bus interfacing (e.g., TTL to NMOS or microcontroller GPIO). |
| 48 mA A/B-port sink current | Supports direct connection to up to 12 standard TTL loads without external buffers - reduces component count. |
| Isolation via OE control | Allows hot-swap or dynamic bus segmentation - B bus electrically disconnected while A bus remains monitored. |
| ALS family speed-power balance | Delivers 12 ns max A→B delay with ICC < 41 mA - optimized for low-noise, medium-speed industrial control applications. |
Applications
| Industrial Backplane Interface | Legacy Microprocessor Bus Extension |
|---|---|
|
Use Scenario: Interfacing a microcontroller's 3-state data bus to a shared open-collector system bus carrying multiple peripheral interrupts and status flags. IC Role / Device Role / Timing Role: SN74ALS638A-1N acts as a bidirectional level-shifting and contention-managed bridge, translating between CMOS-compatible 3-state outputs and wired-OR interrupt lines. Use Value: Enables single-chip arbitration of up to eight interrupt sources without external diodes or pull-up resistors per line, reducing board space and improving reliability. |
Use Scenario: Extending the data/address bus of an 8085-based controller to add memory-mapped I/O peripherals using open-collector handshaking signals. IC Role / Device Role / Timing Role: SN74ALS638A-1N serves as a controlled-direction transceiver, synchronizing data flow during RD/WR cycles while maintaining bus isolation during idle states. Use Value: Provides deterministic 8-bit data transfer with sub-15 ns propagation delay and guaranteed 48 mA sink capability - essential for meeting 8085's 300 ns minimum setup/hold timing. |
| Test Equipment Signal Routing | Repair-Friendly Parallel Port Adapter |
|
Use Scenario: Configurable signal routing in automated test equipment where DUT I/O lines must be dynamically connected to either stimulus or measurement channels. IC Role / Device Role / Timing Role: SN74ALS638A-1N functions as a manually or firmware-controlled bidirectional switch matrix node, leveraging DIR/OE for real-time path reconfiguration. Use Value: Delivers repeatable, low-leakage isolation (ICC < 30 mA disabled) and robust 5 V tolerance - critical for protecting sensitive DUT pins during test sequence transitions. |
Use Scenario: Retrofitting a vintage PC parallel port (DB-25) to interface with modern microcontrollers using 3-state GPIO, preserving original cable and connector form factor. IC Role / Device Role / Timing Role: SN74ALS638A-1N provides inverting, open-collector-to-3-state conversion for STROBE, ACK, BUSY, and DATA lines in IEEE 1284-compliant mode. Use Value: Eliminates need for discrete transistor arrays or level shifters - simplifies PCB layout and ensures drop-in compatibility with legacy BIOS and DOS driver timing expectations. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar octal bus transceiver applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74ALS639A-1N | True (non-inverting) logic function; otherwise identical pinout, timing, and drive specs. | Used where bus polarity must be preserved end-to-end - e.g., connecting two identical 3-state subsystems without inversion. | Select SN74ALS639A-1N only if system-level signal integrity requires non-inverted data path; not interchangeable without logic redesign. |
| SN74AS638AN | Faster propagation (2–7 ns A→B), higher IOL (64 mA), but increased ICC (up to 122 mA) and tighter VCC tolerance (4.75–5.25 V). | Suitable for high-speed bus extensions requiring sub-10 ns latency, but demands stricter power regulation and thermal management. | Choose SN74AS638AN only when measured timing margins fall below 12 ns; verify power delivery and layout for AS-family noise sensitivity. |
Compared with SN74ALS639A-1N, SN74ALS638A-1N provides intentional inversion for handshake synchronization, while SN74AS638AN trades power efficiency for speed - making SN74ALS638A-1N optimal for stable, moderate-speed industrial bus bridging where thermal headroom and supply simplicity are prioritized.
Availability
SN74ALS638A-1N is available at Aetrix Electronics and suitable for industrial backplane interfaces, legacy microprocessor bus extensions, and test equipment signal routing requiring stable component supply, long-lifecycle support, and through-hole manufacturability.
Supply support for SN74ALS638A-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 emphasis on reliability, longevity, and industrial-grade qualification.
SN74ALS638A-1N belongs to the 74ALS advanced low-power Schottky logic family, engineered for balanced speed, power, and noise immunity in commercial-grade control and interface applications from the 1980s onward.
FAQ
What is the key functional difference between SN74ALS638A-1N and SN74ALS639A-1N?
The SN74ALS638A-1N implements an inverting data path: A-bus inputs are complemented before appearing on B-bus outputs. In contrast, SN74ALS639A-1N is true (non-inverting). Both share identical pinout, timing, drive strength, and package - so substitution requires logic-level verification in the target circuit to avoid unintended signal polarity reversal.
Does SN74ALS638A-1N support 3.3 V logic levels on its control inputs (DIR, OE)?
No. SN74ALS638A-1N is an ALS-family device designed for 5 V operation only. Its input thresholds require VIH ≥ 2.0 V and VIL ≤ 0.8 V referenced to 5 V VCC. Direct 3.3 V logic driving DIR or OE may result in indeterminate states; level-shifting or buffer circuits are required for mixed-voltage system integration.
What does the "–1" suffix indicate in SN74ALS638A-1N?
"–1" denotes the enhanced-output-current variant: SN74ALS638A-1N guarantees IOL = 48 mA (vs. 24 mA for standard SN74ALS638A) when VCC is between 4.75 V and 5.25 V. This allows direct driving of heavier bus loads without external drivers, confirmed in the January 1995 revision of datasheet SDAS123A.
Can SN74ALS638A-1N be used with open-collector buses operating above 5.5 V?
No. Although A-bus I/O ports tolerate up to 7 V absolute maximum, the B-bus 3-state outputs are rated for VO ≤ 5.5 V. Driving an A-bus pulled to >5.5 V risks violating B-port voltage limits during B→A transfer. For >5.5 V open-collector buses, external clamping or isolation is mandatory to protect SN74ALS638A-1N B-port outputs.
Is SN74ALS638A-1N RoHS compliant and lead-free?
Yes. Per Texas Instruments' Packaging Information Addendum (July 2026), SN74ALS638A-1N carries "Yes" for RoHS compliance and uses NIPDAU (nickel/palladium/gold) lead finish. It is exempt from MSL rating due to its PDIP (N) package type, confirming suitability for standard through-hole reflow and wave soldering processes.
SN74ALS638A-1N Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74ALS
- Package/Case:
- 20-DIP (0.300", 7.62mm)
- Packaging:
- Bulk
- 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:
- -, 24mA; 15mA, 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
SN74ALS638A-1N FAQ
1.How can I place an order for SN74ALS638A-1N through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74ALS638A-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 SN74ALS638A-1N reliable?
The price and inventory of SN74ALS638A-1N are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74ALS638A-1N is usually 5 days.
3.What payment methods are accepted for SN74ALS638A-1N?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74ALS638A-1N transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74ALS638A-1N?
SN74ALS638A-1N orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74ALS638A-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 SN74ALS638A-1N?
For technical support, including SN74ALS638A-1N datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74ALS638A-1N requirements.
6.How does Aetrix verify that SN74ALS638A-1N is sourced from the original manufacturer or authorized distributors?
All SN74ALS638A-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 SN74ALS638A-1N meets industry standards.
7.What is the process for return or replacement of SN74ALS638A-1N?
All SN74ALS638A-1N units undergo pre-shipment inspection (PSI). If there is an issue with SN74ALS638A-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 SN74ALS638A-1N part is unused and in its original packaging.
Return procedure for SN74ALS638A-1N:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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