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Texas Instruments SN74AS639DW
- Part No.:
- SN74AS639DW
- Manufacturer:
- Texas Instruments
- Package:
- -
- Datasheet:
-
SN74AS639DW.pdf
- Description:
- BUS TRANSCEIVER
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Inventory:1,600
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Product details
Overview
SN74AS639DW from Texas Instruments is a true-logic octal bus transceiver in a 20-pin SOIC (DW) package, designed for asynchronous bidirectional data transfer between open-collector A-bus and 3-state B-bus interfaces. It supports 5-V operation, delivers 64 mA low-level output current on both buses, and operates across 0°C to 70°C with propagation delays as low as 2 ns (A→B) and 5 ns (B→A). It is used in legacy industrial backplane and parallel bus isolation systems.
For engineers reviewing the SN74AS639DW datasheet, SN74AS639DW pinout, SN74AS639DW application, or SN74AS639DW equivalent, key selection criteria include its true-logic direction control, asymmetric bus drive capability (open-collector A-side / 3-state B-side), OE/DIR dual-control architecture, and compatibility with TTL-level signaling in mixed-technology bus environments.
Technical Context
This device implements a dual-bus interface with independent enable (OE) and direction (DIR) inputs controlling data flow between two electrically distinct I/O domains: the A-bus uses open-collector outputs compatible with wired-OR and pull-up–based termination, while the B-bus employs active 3-state drivers for high-speed, low-impedance bus driving. The DIR input selects transmission direction without inversion - low enables B→A transfer, high enables A→B transfer.
Electrical behavior is defined by strict TTL-compatible thresholds (VIL = 0.8 V, VIH = 2 V), rail-to-rail A-port voltage tolerance (up to 7 V), and B-port voltage limit (5.5 V). Its switching performance is characterized under 50-pF load with 500-Ω terminations, delivering sub-10-ns propagation delays in both directions and sub-11-ns 3-state enable/disable times.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Range | 4.5 V to 5.5 V - ensures robust operation across standard 5-V supply tolerances in industrial logic systems. |
| IOL (A/B ports) | 64 mA - supports direct drive of multiple TTL loads or termination resistors without external buffers. |
| tPLH / tPHL (A→B) | 2 ns to 7 ns - enables high-speed parallel data transfer in time-critical backplane applications. |
| VIK (A-port clamp) | –1.2 V - provides negative transient protection on open-collector bus lines during hot-swap or fault conditions. |
| VOH (B-port) | 2.4 V min at –15 mA - guarantees TTL-compatible high-level output under full load, ensuring noise margin in noisy environments. |
| VOL (A/B ports) | 0.55 V max at 64 mA - maintains low logic-low voltage despite high sink current, critical for reliable level detection. |
| Operating Temp | 0°C to 70°C - qualified for commercial and industrial ambient environments without derating. |
Pinout & Package
SN74AS639DW is housed in a 20-pin SOIC (DW) package, 7.5 mm wide, 12.83 mm long, with 1.27 mm pitch and maximum height of 2.65 mm. Pin 1 is marked by a beveled corner or notch; the package is RoHS-compliant with NIPDAU lead finish and MSL Level-1 rating.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (DIR) | Direction control input | High = A→B data transfer; Low = B→A transfer; TTL-compatible threshold ensures reliable logic-level interpretation. |
| 2 (A1) | A-bus I/O terminal | Open-collector output/input - requires external pull-up; supports wired-OR bus arbitration and level translation. |
| 3–9 (A2–A8) | A-bus I/O terminals | Seven additional open-collector bidirectional ports - identical electrical behavior to A1 for full octal bus interface. |
| 10 (GND) | Ground reference | Common return path for all internal circuitry and I/O current sinks; must be low-impedance for noise immunity. |
| 11–18 (B1–B8) | B-bus I/O terminals | Octal 3-state outputs - actively driven high/low when enabled; high-impedance when OE = high for bus isolation. |
| 19 (OE) | Output enable input | Active-low control - drives all B-port outputs into high-impedance state when high, isolating B-bus from A-bus. |
| 20 (VCC) | Power supply | +5 V supply pin - decoupling capacitor (0.1 µF) required near pin for stable switching performance. |
Key Features
| Feature | Design Value |
|---|---|
| True-logic bidirectional transfer | Preserves signal polarity across both A→B and B→A paths - eliminates need for external inverters in symmetric bus designs. |
| Asymmetric bus interface | Combines open-collector A-side (wired-OR compatible) with 3-state B-side (high-drive, low-impedance) - enables interoperability between legacy and modern bus topologies. |
| High sink current capability | 64 mA per port supports direct connection to 50-Ω terminated lines or up to 10 standard TTL loads without buffering. |
| Fast 3-state control | tPZH/tPLZ ≤ 10.5 ns ensures minimal bus contention window during direction or enable transitions in real-time systems. |
| TTL-compatible input thresholds | VIL = 0.8 V / VIH = 2.0 V matches standard TTL logic families - simplifies integration into mixed-technology digital systems. |
Applications
| Industrial Backplane Interface | Legacy System Bus Isolation |
|---|---|
|
Use Scenario: Interfacing a microprocessor's 3-state data bus to an open-collector peripheral bus in programmable logic controllers. IC Role / Device Role / Timing Role: Bidirectional bus transceiver enabling controlled data exchange between CPU and field I/O modules using shared address/data lines. Use Value: Eliminates need for discrete transistors or additional logic gates to manage bus direction and isolation, reducing board space and component count. |
Use Scenario: Isolating a diagnostic port from a main system bus during firmware updates in telecom line cards. IC Role / Device Role / Timing Role: Direction-controlled buffer that prevents signal contention when OE disables B-side outputs during maintenance mode. Use Value: Enables safe hot-swap diagnostics without power cycling - OE-driven high-impedance state blocks backfeeding into live circuits. |
| Parallel Printer Port Expansion | Test Equipment Signal Routing |
|
Use Scenario: Extending a PC's parallel port to drive multiple dot-matrix printers via daisy-chained open-collector handshaking lines. IC Role / Device Role / Timing Role: True-logic transceiver translating bidirectional STROBE/ACK signals between host and peripherals with precise timing alignment. Use Value: Maintains signal integrity across longer traces due to 64 mA sink strength and sub-10 ns propagation - reduces handshake timeout errors. |
Use Scenario: Routing DUT signals between multiple test instruments (oscilloscope, logic analyzer, pattern generator) in automated test fixtures. IC Role / Device Role / Timing Role: Reconfigurable bus switch allowing dynamic signal path selection via DIR/OE control under microcontroller supervision. Use Value: Provides deterministic, low-latency signal routing with no added jitter - essential for timing-critical parametric measurements. |
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 | Lower IOL (24 mA vs. 64 mA); slower B→A delay (10–30 ns vs. 5–22 ns); ALS family lower power but reduced drive. | Suitable for low-noise, low-power legacy systems where bus loading is light and speed is secondary. | Select SN74ALS639ADW only if 24 mA sink current suffices and propagation delay budget allows ≥10 ns B→A latency. |
| SN74AS638DW | Inverting logic (vs. true logic); otherwise identical pinout, package, and electrical specs including 64 mA IOL and 5–22 ns B→A delay. | Required where system-level signal polarity must be inverted during bus transfer - e.g., matching legacy controller expectations. | Choose SN74AS638DW when inverting functionality is needed; otherwise SN74AS639DW is preferred for polarity-transparent designs. |
Compared with SN74ALS639ADW, SN74AS639DW offers 2.7× higher sink current and ~2× faster B→A switching, making it suitable for heavier-loaded or higher-speed buses; versus SN74AS638DW, it removes the need for external inversion logic in true-logic signal paths.
Availability
SN74AS639DW is available at Aetrix Electronics and suitable for industrial backplane interfaces, legacy system bus isolation, parallel printer port expansion, and test equipment signal routing requiring stable component supply and long-term obsolescence management.
Supply support for SN74AS639DW 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 broad industrial, automotive, and communications applications.
The SN74AS639DW belongs to TI's 74AS advanced Schottky logic family, engineered for high-speed, high-drive digital interfacing in legacy and upgrade systems where TTL compatibility and robust bus driving are essential.
FAQ
What is the logic function of the SN74AS639DW?
The SN74AS639DW is a true-logic octal bus transceiver: when DIR = high, data flows from A-bus (open-collector) to B-bus (3-state); when DIR = low, data flows from B-bus to A-bus. Unlike the SN74AS638DW, it does not invert signals in either direction. This behavior is confirmed in the FUNCTION TABLE and logic diagrams of the SDAS123A datasheet.
What is the maximum sink current supported by each port of the SN74AS639DW?
Each A- or B-port of the SN74AS639DW supports up to 64 mA of low-level output current (IOL), as specified in the "recommended operating conditions" table for SN74AS639 under IOL = 64 mA. This value is validated across the full 0°C to 70°C temperature range and 4.5 V to 5.5 V supply.
Can the SN74AS639DW operate with a 3.3-V supply?
No - the SN74AS639DW is not rated for 3.3-V operation. Its recommended VCC range is strictly 4.5 V to 5.5 V, and absolute maximum VCC is 7 V. Operation below 4.5 V risks failure to meet VOH/VOL specifications and may result in undefined logic states or increased propagation delay.
What package type does the SN74AS639DW use, and what are its key mechanical dimensions?
The SN74AS639DW uses a 20-pin SOIC (DW) package: 12.83 mm length, 7.5 mm width, 2.65 mm maximum height, 1.27 mm lead pitch, and RoHS-compliant NIPDAU finish. Its MSL rating is Level-1 (unlimited floor life), and it ships in tubes of 25 units per tube per TI's packaging addendum.
How does the OE input affect the A-bus and B-bus outputs of the SN74AS639DW?
When OE is high, all B-bus outputs (B1–B8) enter high-impedance (3-state) mode, electrically disconnecting the B-bus from the transceiver - the A-bus remains functional as open-collector I/O. When OE is low, B-bus outputs are enabled and respond to DIR-controlled data flow. A-bus outputs are never disabled by OE; they remain open-collector at all times.
SN74AS639DW Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- *
- Package/Case:
- -
- Packaging:
- Bulk
- Product Status:
- Active
- Logic Type:
- -
- Number of Elements:
- -
- Number of Bits per Element:
- -
- Input Type:
- -
- Output Type:
- -
- Current - Output High, Low:
- -
- Voltage - Supply:
- -
- Operating Temperature:
- -
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- -
- Supplier Device Package:
- -
SN74AS639DW FAQ
1.How can I place an order for SN74AS639DW through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74AS639DW 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 SN74AS639DW reliable?
The price and inventory of SN74AS639DW are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74AS639DW is usually 5 days.
3.What payment methods are accepted for SN74AS639DW?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74AS639DW transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74AS639DW?
SN74AS639DW orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74AS639DW 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 SN74AS639DW?
For technical support, including SN74AS639DW datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74AS639DW requirements.
6.How does Aetrix verify that SN74AS639DW is sourced from the original manufacturer or authorized distributors?
All SN74AS639DW 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 SN74AS639DW meets industry standards.
7.What is the process for return or replacement of SN74AS639DW?
All SN74AS639DW units undergo pre-shipment inspection (PSI). If there is an issue with SN74AS639DW, 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 SN74AS639DW part is unused and in its original packaging.
Return procedure for SN74AS639DW:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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