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Texas Instruments SN74ALS638ANS
- Part No.:
- SN74ALS638ANS
- Manufacturer:
- Texas Instruments
- Package:
- 20-SOIC (0.209", 5.30mm Width)
- Datasheet:
-
SN74ALS638ANS.pdf
- Description:
- IC TXRX INVERT 5.5V 20SO
- Quantity:
- Payment:
- Shipping:
Inventory:2,440
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Product details
Overview
SN74ALS638ANS 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 systems. It features 20-pin PDIP (N) package, 0°C to 70°C operating range, 24 mA low-level output current (IOL), and supports direction control via DIR and output enable via OE.
For engineers reviewing the SN74ALS638ANS datasheet, SN74ALS638ANS pinout, SN74ALS638ANS application, or SN74ALS638ANS equivalent, key selection criteria include open-collector A-port drive capability, 3-state B-port isolation timing, DIR/OE control logic compatibility, and legacy TTL-level interface robustness in industrial backplane and memory expansion designs.
Technical Context
The SN74ALS638ANS implements dual-bus bidirectional data routing using separate control paths: DIR selects data flow direction (A→B or B→A), while OE enables/disables all outputs for bus isolation. Its A-bus outputs are open-collector with 7 V absolute max rating, enabling wired-AND configurations; B-bus outputs are 3-state with 5.5 V max voltage and –15 mA high-level output current.
Propagation delays are asymmetric: A→B transitions are faster (2–12 ns) than B→A (8–25 ns), reflecting internal driver asymmetry optimized for master-slave bus arbitration. Switching performance is specified under 50 pF load with RL = 680 Ω for A-ports and R1/R2 = 500 Ω for B-ports, confirming suitability for medium-speed parallel interconnects up to ~40 MHz effective throughput.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Logic Function | Inverting octal bus transceiver with DIR/OE control |
| Supply Voltage | 4.5 V to 5.5 V - compatible with standard 5 V TTL systems |
| A-port Output Type | Open-collector - supports wired-AND, level translation, and pull-up flexibility |
| B-port Output Type | 3-state - enables shared bus arbitration without contention |
| IOL (A/B ports) | 24 mA - drives standard TTL loads and multiple fanouts |
| tPHL/tPLH (A→B) | 2–12 ns - ensures fast upstream-to-downstream data transfer |
| Operating Temperature | 0°C to 70°C - rated for commercial/industrial ambient environments |
Pinout & Package
SN74ALS638ANS is supplied in a 20-pin plastic DIP (N) package with 300-mil width, 0.100" lead pitch, and through-hole mounting. Pin 1 is marked by a notch or beveled corner at the top-left edge when viewed from top with pins down.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (DIR) | Direction Control Input | Low = B→A data flow; High = A→B data flow |
| 2 (A1) | A-bus Data Input/Output | Open-collector I/O - requires external pull-up for logic HIGH |
| 3–9 (A2–A8) | A-bus Data I/Os | Seven additional open-collector bidirectional lines |
| 10 (GND) | Ground Reference | Power return path for all internal logic and I/O stages |
| 11–18 (B1–B8) | B-bus Data I/Os | Eight 3-state outputs - driven HIGH/LOW or high-impedance per OE |
| 19 (OE) | Output Enable Input | Low = outputs active; High = all outputs disabled (high-Z) |
| 20 (VCC) | Positive Supply | +5 V power rail - must be decoupled locally per best practice |
Key Features
| Feature | Design Value |
|---|---|
| Inverting Logic Path | Ensures signal polarity consistency across A↔B translation in legacy TTL bus architectures |
| Asymmetric Propagation Delay | Optimized 2–12 ns A→B delay improves timing margin in master-initiated read cycles |
| Open-Collector A-Ports | Enables direct connection to shared interrupt lines or daisy-chained control buses |
| 3-State B-Ports | Allows seamless integration into multiplexed data/address buses without external bus buffers |
| Robust Input/Output Ratings | 7 V max A-port voltage tolerance prevents damage during hot-swap or fault conditions |
Applications
| Industrial Backplane Interface | Legacy Memory Expansion |
|---|---|
Use Scenario: Interfacing microcontroller local bus to multi-slot ISA-style backplane with shared address/data lines. IC Role / Device Role / Timing Role: Bidirectional level-shifting transceiver managing A-bus (open-collector control signals) and B-bus (3-state data lines) under DIR-controlled arbitration. Use Value: Eliminates need for discrete buffer arrays while maintaining TTL-compatible timing and fault-tolerant open-collector signaling on control lines. | Use Scenario: Adding external SRAM or EPROM to 8-bit microprocessor systems with limited address/data bus drive capability. IC Role / Device Role / Timing Role: Octal transceiver isolating CPU data bus from memory module during write cycles and enabling read-through during fetch operations. Use Value: Provides 24 mA sink current per A-port to drive memory chip select and output enable lines reliably across long PCB traces. |
| Parallel Printer Port Controller | Test Equipment Bus Coupling |
Use Scenario: Isolating host controller from Centronics-compatible printer port with bidirectional handshaking lines. IC Role / Device Role / Timing Role: Translating CPU-side 3-state data bus to printer-side open-collector status/control lines (BUSY, ACK, STROBE). Use Value: Enables direct wired-AND of multiple peripheral status signals without external diodes or pull-ups on A-port side. | Use Scenario: Coupling automated test equipment (ATE) digital I/O channels to device-under-test (DUT) with mixed-signal bus requirements. IC Role / Device Role / Timing Role: Providing galvanically isolated bidirectional data path between ATE controller and DUT's open-collector diagnostic bus. Use Value: Delivers sub-15 ns propagation delay and 0.5 V VOL at 24 mA to meet tight timing budgets in production functional test fixtures. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar octal bus transceiver applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74ALS639AN | True (non-inverting) logic function; identical pinout, timing, and electrical specs otherwise | Required where signal polarity preservation is mandatory across bus translation | Select when system-level logic design mandates non-inverted data path between A and B buses |
| SN74AS638AN | Faster switching (2–7 ns A→B), higher IOL (64 mA), but increased ICC (75–122 mA) and tighter VIH/VIL thresholds | Suitable for high-speed backplanes requiring <10 ns propagation, but demands stricter supply regulation | Choose only if timing budget requires sub-10 ns A→B delay and board layout supports higher power dissipation |
Compared with SN74ALS638ANS, SN74ALS639AN preserves signal polarity at identical speed and drive strength, while SN74AS638AN trades higher power and tighter input margins for 40% faster A→B propagation - making SN74ALS638ANS optimal for cost-sensitive, noise-tolerant commercial systems where inverting logic is acceptable.
Availability
SN74ALS638ANS is available at Aetrix Electronics and suitable for industrial backplane interfaces, legacy memory expansion modules, and parallel printer port controllers requiring stable component supply and long-term obsolescence management.
Supply support for SN74ALS638ANS 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 SN74ALS638ANS belongs to TI's legacy 74ALS logic family, engineered for reliable 5 V TTL-compatible operation in commercial temperature ranges with enhanced noise immunity and reduced power consumption versus standard 74LS devices.
FAQ
What is the maximum allowable voltage on the A-bus I/O ports of the SN74ALS638ANS?
The SN74ALS638ANS specifies an absolute maximum rating of 7 V for A-bus I/O ports. This allows safe interfacing with higher-voltage open-collector buses or pull-up networks without clamping diodes, provided VCC remains within 4.5 V–5.5 V. Exceeding 7 V risks permanent damage per the datasheet's stress ratings.
Does the SN74ALS638ANS support hot insertion or live bus swapping?
The SN74ALS638ANS does not include explicit hot-swap protection circuitry. While its 7 V A-port rating and 5.5 V B-port limit provide some margin, uncontrolled power sequencing during insertion may violate absolute maximum ratings. For live bus applications, external current-limiting resistors and controlled power ramping are recommended alongside SN74ALS638ANS use.
Can the SN74ALS638ANS be used with 3.3 V logic systems?
The SN74ALS638ANS is designed for 5 V operation only. Its VIH minimum is 2.0 V and VIL maximum is 0.8 V, making it incompatible with 3.3 V CMOS logic levels without level-shifting. Driving SN74ALS638ANS inputs from 3.3 V sources risks marginal high-level recognition; receiving outputs into 3.3 V inputs violates B-port 5.5 V max rating unless clamped.
What is the purpose of the DIR and OE pins on the SN74ALS638ANS?
The DIR pin controls data direction: logic LOW enables B→A transmission, HIGH enables A→B. The OE pin globally enables or disables all outputs - LOW activates transceiver operation, HIGH places both A and B ports in high-impedance state for bus isolation. Both are active-low referenced for TTL compatibility in SN74ALS638ANS.
Is there a surface-mount version of the SN74ALS638ANS?
No - SN74ALS638ANS specifically denotes the 20-pin plastic DIP (N) package. Texas Instruments offers SN74ALS638ADW in SOIC-20, but that is a distinct orderable part number. SN74ALS638ANS is exclusively through-hole mounted with 300-mil width and 0.100" lead pitch, verified by TI's packaging addendum.
SN74ALS638ANS Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74ALS
- Package/Case:
- 20-SOIC (0.209", 5.30mm Width)
- Packaging:
- Bulk
- Product Status:
- Active
- 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:
- Surface Mount
- Supplier Device Package:
- 20-SO
SN74ALS638ANS FAQ
1.How can I place an order for SN74ALS638ANS through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74ALS638ANS 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 SN74ALS638ANS reliable?
The price and inventory of SN74ALS638ANS are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74ALS638ANS is usually 5 days.
3.What payment methods are accepted for SN74ALS638ANS?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74ALS638ANS transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74ALS638ANS?
SN74ALS638ANS orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74ALS638ANS 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 SN74ALS638ANS?
For technical support, including SN74ALS638ANS datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74ALS638ANS requirements.
6.How does Aetrix verify that SN74ALS638ANS is sourced from the original manufacturer or authorized distributors?
All SN74ALS638ANS 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 SN74ALS638ANS meets industry standards.
7.What is the process for return or replacement of SN74ALS638ANS?
All SN74ALS638ANS units undergo pre-shipment inspection (PSI). If there is an issue with SN74ALS638ANS, 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 SN74ALS638ANS part is unused and in its original packaging.
Return procedure for SN74ALS638ANS:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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