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Texas Instruments SN74ALS638ADWE4
- Part No.:
- SN74ALS638ADWE4
- Manufacturer:
- Texas Instruments
- Package:
- 20-SOIC (0.295", 7.50mm Width)
- Datasheet:
-
SN74ALS638ADWE4.pdf
- Description:
- IC TXRX INVERT 5.5V 20SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:1,770
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Product details
Overview
SN74ALS638ADWE4 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 SOIC (DW) packaging, operates from 0°C to 70°C, supports 5 V supply, and delivers 24 mA low-level output current on both buses - used in legacy industrial backplane interfaces and TTL-compatible data routing.
For engineers reviewing the SN74ALS638ADWE4 datasheet, SN74ALS638ADWE4 pinout, SN74ALS638ADWE4 application, or SN74ALS638ADWE4 equivalent, key selection criteria include direction-control (DIR) and output-enable (OE) timing behavior, A-bus open-collector drive capability, B-bus 3-state voltage compliance (VOH ≥ 2.4 V at –15 mA), and propagation delays (tPLH/tPHL ≤ 12 ns A→B).
Technical Context
The SN74ALS638ADWE4 implements dual-bus isolation using separate control paths: DIR selects data flow direction (A↔B), while OE enables/disables all outputs. Its A-bus outputs are open-collector with –1.5 V clamp (VIK), allowing wired-AND configurations; B-bus outputs are active-pull 3-state with VOH = VCC – 2 V typical and VOL ≤ 0.4 V at 24 mA.
It uses ALS (Advanced Low-Power Schottky) logic architecture, delivering lower power than standard AS variants while maintaining TTL-compatible input thresholds (VIH = 2 V, VIL = 0.8 V) and supporting 50 pF load capacitance. Propagation delay asymmetry is intentional: B→A path (8–25 ns) is slower than A→B (2–12 ns), reflecting internal driver strength differences aligned with bus role specialization.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Range | 4.5 V to 5.5 V - ensures compatibility with standard 5 V TTL/CMOS systems without regulation overhead |
| Operating Temperature | 0°C to 70°C - qualified for commercial-grade industrial control and instrumentation environments |
| IOL (A/B ports) | 24 mA - sufficient to drive multiple TTL loads or pull down 4.7 kΩ termination resistors on open-collector buses |
| VOH (B ports) | ≥2.4 V at –15 mA - meets minimum high-level output requirement for interfacing with 74LS/74ALS inputs |
| VOL (A/B ports) | ≤0.4 V at 24 mA - guarantees solid logic-low recognition across temperature and process variation |
| tPLH / tPHL (A→B) | 2 ns to 12 ns - enables sub-100 MHz burst-mode data transfer in synchronous bus arbitration schemes |
| Input Thresholds | VIH = 2 V, VIL = 0.8 V - matches standard TTL switching points for seamless integration into mixed-logic systems |
Pinout & Package
SN74ALS638ADWE4 is housed in a 20-pin SOIC (DW) package measuring 12.83 mm × 7.5 mm × 2.65 mm max height, with 1.27 mm pitch, RoHS-compliant NiPdAu lead finish, and moisture sensitivity level (MSL) Level-1 (unlimited floor life).
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (DIR) | Direction Control Input | Low = B→A data transfer; High = A→B data transfer - determines signal flow path through internal logic gates |
| 2 (A1) | A-Bus Data Input/Output | Open-collector I/O - requires external pull-up; sinks current when driven low; floats when high or disabled |
| 3–9 (A2–A8) | A-Bus Data I/Os | Seven additional open-collector bidirectional ports - identical electrical behavior to A1 |
| 10 (GND) | Ground Reference | Primary return path for all internal logic and output currents - must be low-impedance for noise immunity |
| 11–18 (B1–B8) | B-Bus Data Outputs | 3-state totem-pole outputs - actively drive high/low or enter high-impedance state when OE = high |
| 19 (OE) | Output Enable Input | Low = outputs enabled; High = all outputs (A and B) placed in high-Z - isolates both buses simultaneously |
| 20 (VCC) | Power Supply | +5 V supply rail - powers internal logic and B-bus drivers; decoupling capacitor required near pin |
Key Features
| Feature | Design Value |
|---|---|
| Inverting Logic Function | Outputs inverted relative to inputs - simplifies polarity alignment in differential bus architectures and avoids external inverters |
| Asymmetric Bus Interface | A-bus open-collector / B-bus 3-state - enables direct connection to legacy wired-AND backplanes while driving modern 3-state-controlled subsystems |
| Independent Control Inputs | Separate DIR and OE pins - allows concurrent bus direction management and system-level output gating without timing conflict |
| TTL-Compatible Thresholds | VIH = 2 V, VIL = 0.8 V - eliminates need for level-shifting when interfacing with 74LS, 74ALS, or microcontroller GPIOs |
| Low Propagation Skew | tPLH/tPHL matching ≤ 10 ns across all eight channels - preserves data integrity in parallel byte-wide transfers |
Applications
| Industrial Backplane Interface | Legacy System Bus Expansion |
|---|---|
Use Scenario: Interfacing a microcontroller-based master module to a multi-slot industrial backplane with open-collector address/data lines. IC Role / Device Role / Timing Role: SN74ALS638ADWE4 acts as a direction-controlled bus buffer, translating 3-state CPU data to open-collector backplane signals and returning status responses. Use Value: Enables reliable handshaking over long traces by providing 24 mA sink capability on A-bus and clean 3-state isolation during idle cycles. | Use Scenario: Adding peripheral slots to a vintage minicomputer system using TTL-era bus protocols. IC Role / Device Role / Timing Role: SN74ALS638ADWE4 serves as a bidirectional data transceiver between CPU bus and expansion card edge connectors, handling both read and write cycles. Use Value: Matches original system timing (≤12 ns A→B delay) and voltage levels, avoiding redesign of timing-critical control logic. |
| Test Equipment Signal Routing | Programmable Logic Interface |
Use Scenario: Configurable signal path switching in automated test equipment where DUT interface voltages vary between 3.3 V and 5 V domains. IC Role / Device Role / Timing Role: SN74ALS638ADWE4 functions as a programmable bus gate, controlled via FPGA I/O, to route stimulus/response data between test head and DUT. Use Value: Open-collector A-bus accepts pulled-up 3.3 V signals safely (VIK = –1.5 V), while B-bus drives 5 V logic cleanly (VOH ≥ 2.4 V). | Use Scenario: Glue logic between a CPLD and legacy TTL peripherals requiring bidirectional data exchange. IC Role / Device Role / Timing Role: SN74ALS638ADWE4 bridges the CPLD's 3-state outputs to TTL-compatible open-collector bus segments under DIR/OE control from CPLD firmware. Use Value: Eliminates need for discrete transistor arrays or additional level translators due to native dual-bus topology and TTL threshold alignment. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar octal bus transceiver applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74ALS639ADW | True (non-inverting) logic function; otherwise identical pinout, timing, and electrical specs | Used where signal polarity must be preserved end-to-end without external inversion | Select SN74ALS639ADW only if system-level logic requires non-inverted data path - no change to PCB layout or power delivery |
| SN74AS638ADW | Faster propagation (A→B: 2–7 ns vs. 2–12 ns); higher IOL (64 mA vs. 24 mA); increased ICC (75 mA vs. 41 mA) | Suitable for high-speed backplanes requiring tighter timing margins and stronger drive | Choose SN74AS638ADW when speed or drive strength outweighs power efficiency - verify thermal dissipation and supply stability |
Compared with SN74ALS638ADWE4, SN74ALS639ADW offers identical form-factor and timing but preserves signal polarity, while SN74AS638ADW trades higher power for faster switching and greater sink current - both require no PCB changes but differ in functional behavior and thermal design implications.
Availability
SN74ALS638ADWE4 is available at Aetrix Electronics and suitable for industrial backplane interfaces, legacy system upgrades, and test equipment signal routing requiring stable component supply across extended production lifecycles.
Supply support for SN74ALS638ADWE4 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 U.S.-based semiconductor company founded in 1930, specializing in analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.
The SN74ALS638ADWE4 belongs to TI's legacy 74ALS logic family, engineered for low-power, high-noise-immunity TTL-compatible operation in fixed-function digital systems where reliability and long-term availability are critical.
FAQ
What is the maximum recommended supply voltage for SN74ALS638ADWE4?
The absolute maximum supply voltage (VCC) for SN74ALS638ADWE4 is 7 V, but the recommended operating range is strictly 4.5 V to 5.5 V. Operating outside this range risks parametric shift or permanent damage. All published electrical characteristics - including VOH, VOL, and propagation delays - are guaranteed only within the 4.5–5.5 V window per the SDAS123A datasheet.
Does SN74ALS638ADWE4 support 3.3 V logic inputs?
SN74ALS638ADWE4 does not guarantee reliable operation with 3.3 V logic inputs. Its input thresholds are TTL-defined: VIH = 2.0 V minimum and VIL = 0.8 V maximum. While a 3.3 V signal may exceed VIH, its noise margin is reduced, and DC loading (IIH up to 20 µA) must be verified against the source. For robust 3.3 V interfacing, level translation or a 3.3 V–compatible transceiver is recommended.
Can SN74ALS638ADWE4 drive standard 4.7 kΩ pull-up resistors on an open-collector bus?
Yes. With IOL = 24 mA minimum and VOL ≤ 0.4 V at that current, SN74ALS638ADWE4 can reliably sink current through a 4.7 kΩ pull-up to 5 V (I = 1.06 mA), leaving ample margin. At worst-case VOL = 0.5 V, the residual voltage drop across the resistor remains well below TTL's VIL limit, ensuring solid low-level recognition across temperature and process corners.
Is SN74ALS638ADWE4 pin-compatible with SN74ALS639ADW?
Yes - SN74ALS638ADWE4 and SN74ALS639ADW share identical 20-pin SOIC (DW) packaging, pin assignments, and electrical timing specifications. The sole functional difference is logic polarity: SN74ALS638ADWE4 is inverting, while SN74ALS639ADW is true. No PCB modification is needed for substitution, but firmware or upstream logic must accommodate the inversion.
What is the thermal performance limitation of SN74ALS638ADWE4 in continuous operation?
SN74ALS638ADWE4 has no specified junction temperature limit beyond its operating ambient range (0°C to 70°C). Maximum power dissipation depends on output loading: at full 24 mA sink on all eight A-bus lines and 15 mA source on all eight B-bus lines, ICC peaks at ~41 mA (VCC = 5.5 V), yielding ~225 mW. With proper PCB copper area and still-air conditions, this remains within safe limits - derating is advised above 60°C ambient.
SN74ALS638ADWE4 Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74ALS
- Package/Case:
- 20-SOIC (0.295", 7.50mm Width)
- 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:
- -, 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-SOIC
SN74ALS638ADWE4 FAQ
1.How can I place an order for SN74ALS638ADWE4 through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74ALS638ADWE4 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 SN74ALS638ADWE4 reliable?
The price and inventory of SN74ALS638ADWE4 are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74ALS638ADWE4 is usually 5 days.
3.What payment methods are accepted for SN74ALS638ADWE4?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74ALS638ADWE4 transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74ALS638ADWE4?
SN74ALS638ADWE4 orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74ALS638ADWE4 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 SN74ALS638ADWE4?
For technical support, including SN74ALS638ADWE4 datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74ALS638ADWE4 requirements.
6.How does Aetrix verify that SN74ALS638ADWE4 is sourced from the original manufacturer or authorized distributors?
All SN74ALS638ADWE4 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 SN74ALS638ADWE4 meets industry standards.
7.What is the process for return or replacement of SN74ALS638ADWE4?
All SN74ALS638ADWE4 units undergo pre-shipment inspection (PSI). If there is an issue with SN74ALS638ADWE4, 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 SN74ALS638ADWE4 part is unused and in its original packaging.
Return procedure for SN74ALS638ADWE4:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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