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Texas Instruments SN74ALS638ADW
- Part No.:
- SN74ALS638ADW
- Manufacturer:
- Texas Instruments
- Package:
- 20-SOIC (0.295", 7.50mm Width)
- Datasheet:
-
SN74ALS638ADW.pdf
- Description:
- IC TXRX INVERT 5.5V 20SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:2,046
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Product details
Overview
SN74ALS638ADW from Texas Instruments is an octal bidirectional bus transceiver with inverting logic, open-collector A-bus outputs, and 3-state B-bus outputs, operating at 4.5–5.5 V over 0°C to 70°C. It supports asynchronous two-way communication between mixed-logic buses and features 24 mA low-level output current (IOL), 12 ns max A→B propagation delay, and 25 ns max OE→A disable time.
For engineers reviewing the SN74ALS638ADW datasheet, SN74ALS638ADW pinout, SN74ALS638ADW application, or SN74ALS638ADW equivalent, this page delivers verified electrical specs, true pin mapping for the DW package, functional role in legacy TTL bus isolation, and validated drop-in alternatives for industrial control and backplane interface designs.
Technical Context
The SN74ALS638ADW implements dual-bus direction control via DIR input and full 3-state isolation via OE, enabling dynamic data flow arbitration between open-collector (A) and tri-state (B) buses. Its internal architecture separates A-side pull-up requirements from B-side active drive capability, requiring external pull-ups on A-bus lines.
It operates strictly within ALS logic family voltage thresholds: VIH = 2 V min, VIL = 0.8 V max, VOH = VCC – 2 V (A ports), and VOL = 0.4 V max at IOL = 24 mA. Propagation delays are asymmetric-A→B is faster (2–12 ns) than B→A (8–30 ns)-reflecting inherent open-collector vs. totem-pole output timing differences.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Range | 4.5 V to 5.5 V - Compatible with standard 5 V TTL/ALS power rails; not 3.3 V tolerant. |
| IOL (A/B ports) | 24 mA - Supports driving multiple TTL loads or terminating stubs on open-collector buses. |
| tPLH/tPHL (A→B) | 2–12 ns - Enables high-speed bidirectional data transfer in legacy backplane systems. |
| VOL @ IOL=24 mA | 0.4 V max - Ensures solid low-level noise margin when sinking current into B-bus receivers. |
| Operating Temp | 0°C to 70°C - Rated for commercial-grade industrial equipment, not extended or automotive temp ranges. |
| Logic Function | Inverting - Output polarity is inverted relative to input on both A→B and B→A paths. |
| Package | SOIC-20 (DW) - 0.300-inch body width, gull-wing leads, RoHS-compliant CU NIPDAU finish. |
Pinout & Package
SN74ALS638ADW uses a 20-pin SOIC (DW) package with 0.300-inch body width and standard gull-wing lead form. Pin 1 is located at top-left corner (Q1 quadrant), with pin numbering counterclockwise.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (DIR) | Direction Control Input | Low = B→A data flow; High = A→B data flow; controls internal bus switch path. |
| 2 (A1) | A-Bus Data Input/Output | Open-collector I/O; requires external pull-up; connects to legacy TTL bus segments. |
| 3–9 (A2–A8) | A-Bus Data I/O Group | Seven additional open-collector bidirectional ports, identical to A1 in function and drive. |
| 10 (GND) | Ground Reference | Primary signal return for all logic and output stages; must be low-impedance. |
| 11–18 (B1–B8) | B-Bus Data I/O Group | Eight 3-state outputs; actively driven high/low; no external pull-up required. |
| 19 (OE) | Output Enable Input | Active-low; disables all B outputs and isolates A outputs (open-collector high-Z). |
| 20 (VCC) | Power Supply | +5 V supply; decoupling capacitor (0.1 µF) required near pin for noise suppression. |
Key Features
| Feature | Design Value |
|---|---|
| Inverting Logic Path | Ensures consistent signal polarity inversion across both A→B and B→A directions for predictable bus-level timing alignment. |
| Asymmetric Bus Interface | Enables direct interconnection between legacy open-collector (e.g., I²C-like) and modern 3-state (e.g., microcontroller) buses without level shifters. |
| High Sink Current (24 mA) | Drives up to 10 standard TTL loads per A-port, supporting dense backplane termination schemes. |
| Fast A→B Propagation | Sub-12 ns delay allows real-time handshake protocols in synchronous bus arbitration applications. |
| Full 3-State Isolation | OE assertion places B outputs in high-impedance state while A ports float open-collector, preventing bus contention. |
Applications
| Industrial Backplane Interfacing | Legacy System Bus Expansion |
|---|---|
|
Use Scenario: Connecting a microcontroller-based module to an existing 8-bit parallel bus using open-collector signaling. IC Role / Device Role / Timing Role: Bidirectional protocol translator that arbitrates direction and isolates buses during idle cycles. Use Value: Eliminates need for discrete MOSFET switches or dual transceivers, reducing board area and BOM count by 50%. |
Use Scenario: Adding new peripheral cards to a vintage industrial PLC rack with mixed ALS/TTL logic levels. IC Role / Device Role / Timing Role: Level- and topology-adaptive bus repeater with direction-controlled data mirroring. Use Value: Maintains signal integrity across 20 cm traces at 20 MHz edge rates due to matched 12 ns A→B delay. |
| Test Equipment Signal Routing | Embedded Diagnostic Interface |
|
Use Scenario: Dynamic reconfiguration of stimulus/response paths in automated test fixtures. IC Role / Device Role / Timing Role: Hardware-controlled bus gate with sub-25 ns OE disable latency for precise timing windows. Use Value: Enables glitch-free bus switching during live system diagnostics without CPU intervention. |
Use Scenario: Isolating debug UART or JTAG signals from main system bus during firmware updates. IC Role / Device Role / Timing Role: Directionally gated diagnostic channel with hardware-enforced read/write separation. Use Value: Prevents accidental bus conflicts when host and target share common address/data lines. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar octal bidirectional bus transceiver applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74ALS639ADW | True (non-inverting) logic function; otherwise identical pinout, timing, and drive specs. | Required where signal polarity preservation is mandatory across bus translation layers. | Select SN74ALS639ADW only if system-level timing analysis confirms zero-phase inversion tolerance. |
| SN74AS638ADW | Faster propagation (A→B: 2–7 ns), higher IOL (64 mA), but increased ICC (up to 122 mA) and tighter VIH/VIL thresholds. | Suitable for high-speed backplanes (>25 MHz) where ALS speed limits cause setup violations. | Choose SN74AS638ADW only when measured tPLH < 7 ns is required and power budget allows +80 mA worst-case draw. |
Compared with SN74ALS638ADW, SN74ALS639ADW preserves signal phase but offers no speed or drive advantage, while SN74AS638ADW delivers measurable timing improvement at the cost of significantly higher static and dynamic power consumption.
Availability
SN74ALS638ADW is available at Aetrix Electronics and suitable for industrial backplane interfacing, legacy system bus expansion, test equipment signal routing, and embedded diagnostic interface applications requiring stable component supply and long-term obsolescence management.
Supply support for SN74ALS638ADW 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, logic, and embedded processing technologies with broad industrial and automotive portfolio coverage.
The SN74ALS638ADW belongs to TI's legacy ALS logic family, designed specifically for reliable bidirectional bus bridging in commercial-grade industrial control systems and instrumentation platforms.
FAQ
What is the maximum sink current supported by SN74ALS638ADW on its A-bus outputs?
The SN74ALS638ADW supports a guaranteed low-level output current (IOL) of 24 mA per A-bus port under recommended operating conditions (VCC = 4.5–5.5 V, TA = 0°C to 70°C). This rating applies to both A and B ports and ensures compatibility with standard TTL fan-out requirements. The -1 version increases IOL to 48 mA, but SN74ALS638ADW is the standard variant.
Does SN74ALS638ADW support 3.3 V logic interfaces?
No, SN74ALS638ADW does not support 3.3 V logic interfaces. Its absolute maximum input voltage is 7 V, but recommended operating conditions specify VCC = 4.5–5.5 V and input thresholds (VIH = 2 V min, VIL = 0.8 V max) aligned with 5 V TTL/ALS families. Direct connection to 3.3 V systems risks violating VIH/VIL margins and may cause unreliable operation or damage under sustained overvoltage.
What is the function of the DIR pin on SN74ALS638ADW?
The DIR (Direction) pin on SN74ALS638ADW controls data flow direction: when DIR = Low, data transfers from B-bus to A-bus; when DIR = High, data flows from A-bus to B-bus. This single-pin control enables hardware-managed bidirectional arbitration without software overhead, making SN74ALS638ADW ideal for real-time bus isolation in industrial controllers.
Can SN74ALS638ADW replace SN74ALS639ADW in a circuit?
SN74ALS638ADW cannot directly replace SN74ALS639ADW without circuit modification because SN74ALS638ADW provides inverting logic while SN74ALS639ADW is true (non-inverting). Swapping them introduces unintended signal inversion, potentially breaking handshake protocols or data integrity. Functional replacement requires verification of system-level polarity tolerance and possible logic inversion elsewhere in the signal chain.
What package type does SN74ALS638ADW use, and is it RoHS compliant?
SN74ALS638ADW uses the SOIC-20 (DW) package - a 20-pin plastic small-outline integrated circuit with 0.300-inch body width and gull-wing leads. It is RoHS compliant with Green (RoHS & no Sb/Br) eco plan, featuring CU NIPDAU lead finish and JEDEC MSL Level-1 rating (260°C peak reflow, unlimited floor life).
SN74ALS638ADW 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
SN74ALS638ADW FAQ
1.How can I place an order for SN74ALS638ADW through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74ALS638ADW 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 SN74ALS638ADW reliable?
The price and inventory of SN74ALS638ADW are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74ALS638ADW is usually 5 days.
3.What payment methods are accepted for SN74ALS638ADW?
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4.How is shipping managed for SN74ALS638ADW?
SN74ALS638ADW orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74ALS638ADW 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 SN74ALS638ADW?
For technical support, including SN74ALS638ADW datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74ALS638ADW requirements.
6.How does Aetrix verify that SN74ALS638ADW is sourced from the original manufacturer or authorized distributors?
All SN74ALS638ADW 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 SN74ALS638ADW meets industry standards.
7.What is the process for return or replacement of SN74ALS638ADW?
All SN74ALS638ADW units undergo pre-shipment inspection (PSI). If there is an issue with SN74ALS638ADW, 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 SN74ALS638ADW part is unused and in its original packaging.
Return procedure for SN74ALS638ADW:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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