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Texas Instruments SN74LS644DW
- Part No.:
- SN74LS644DW
- Manufacturer:
- Texas Instruments
- Package:
- -
- Datasheet:
-
SN74LS644DW.pdf
- Description:
- BUS TRANSCEIVER, LS SERIES TTL
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Inventory:1,154
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Product details
Overview
SN74LS644DW from Texas Instruments is an octal bus transceiver with open-collector outputs, designed for bidirectional data transfer between two 8-bit buses in TTL-level systems. It features true logic polarity, 5 V supply operation (4.75–5.25 V), 25 ns maximum propagation delay, and 24 mA sink capability per output - enabling direct interface with legacy parallel buses in industrial control backplanes.
For engineers reviewing the SN74LS644DW datasheet, SN74LS644DW pinout, SN74LS644DW application, or SN74LS644DW equivalent, this device serves as a legacy-compatible bus isolation and level-shifting solution where open-collector wired-OR logic, noise-immune bus arbitration, or multi-drop bus termination are required.
Technical Context
The SN74LS644DW implements asynchronous bidirectional data flow controlled by a single direction input (DIR) and an active-low enable (G\). Its open-collector outputs allow wired-OR connection to shared bus lines without contention, supporting multi-master arbitration and pull-up-based voltage translation.
Each of its eight channels includes hysteresis on bus inputs to improve noise immunity, and it operates across the commercial temperature range (0°C to 70°C) in a 20-pin SOIC (DW) package with standard TTL input thresholds and output drive characteristics.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Range | 4.75 V to 5.25 V - ensures stable operation under standard TTL power rail tolerances. |
| Logic Polarity | True - A-to-B and B-to-A data paths pass signals without inversion, simplifying bus protocol alignment. |
| Max Propagation Delay | 25 ns - supports synchronous bus timing up to ~20 MHz in low-skew configurations. |
| Output Sink Current | 24 mA per pin - enables direct driving of 5 V TTL loads or termination resistors without external buffers. |
| Input Hysteresis | Typically 0.5 V - increases noise margin on shared bus lines, reducing false triggering in electrically noisy environments. |
| Operating Temperature | 0°C to 70°C - qualified for commercial-grade embedded systems and industrial control panels. |
Pinout & Package
SN74LS644DW is housed in a 20-pin SOIC (Small Outline Integrated Circuit) package with 0.300-inch body width and standard DW footprint. Pin spacing is 0.050 inch (1.27 mm), compatible with IPC-7351B SOIC-20W land patterns.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (G\) | Active-low Output Enable | Disables all outputs to high-impedance state when high; enables bidirectional transfer when low. |
| 2–9 (A1–A8) | Bus A Data Inputs/Outputs | Connects to first 8-bit data bus; functions as input or output depending on DIR state. |
| 10 (GND) | Ground Reference | Common return path for all internal logic and output current sinks. |
| 11–18 (B1–B8) | Bus B Data Inputs/Outputs | Connects to second 8-bit data bus; directionally coupled to A-bus via DIR control. |
| 19 (DIR) | Direction Control Input | High = A→B transfer; Low = B→A transfer - determines data flow direction asynchronously. |
| 20 (VCC) | Positive Supply | 5 V DC supply input powering internal TTL circuitry and output stage biasing. |
Key Features
| Feature | Design Value |
|---|---|
| Open-collector outputs | Enable wired-OR bus topologies, multi-drop arbitration, and flexible pull-up voltage selection (e.g., 3.3 V or 5 V). |
| Hysteresis on all inputs | Improves noise rejection on shared bus lines, critical for reliable operation in industrial backplane environments. |
| True logic configuration | Eliminates need for external inverters in non-inverting bus extension or mirroring applications. |
| 20-pin SOIC (DW) package | Provides high-density mounting for space-constrained legacy board upgrades and retrofits. |
Applications
| Industrial Backplane Bus Isolation | Legacy Parallel Port Expansion |
|---|---|
Use Scenario: Isolating CPU address/data buses from peripheral modules in programmable logic controllers (PLCs) using shared backplane wiring. IC Role / Device Role / Timing Role: Bidirectional bus transceiver managing asynchronous A↔B data flow under DIR control, with G\ enabling module hot-swap isolation. Use Value: Open-collector outputs prevent bus contention during module insertion/removal and support termination at multiple points along the backplane. |
Use Scenario: Extending ISA or IEEE-488 (GPIB) parallel bus segments in test instrumentation racks with signal integrity preservation. IC Role / Device Role / Timing Role: Level-translating and buffering 8-bit parallel data paths while maintaining TTL timing compatibility and noise margins. Use Value: 25 ns tpd and input hysteresis ensure reliable handshaking across extended cable runs and multi-drop stubs. |
| Multi-Master Bus Arbitration | Wired-OR Logic Interface |
Use Scenario: Implementing priority-based bus access among multiple microcontrollers sharing a common memory-mapped I/O bus. IC Role / Device Role / Timing Role: Enabling collision-free communication via open-collector outputs tied to a common pull-up, with DIR selecting master/slave roles. Use Value: Eliminates need for external arbitration logic; bus state is resolved naturally through wired-OR dominance behavior. |
Use Scenario: Generating composite status flags (e.g., "any alarm active") by tying multiple SN74LS644DW outputs to a single pull-up resistor. IC Role / Device Role / Timing Role: Acting as programmable wired-OR gate with TTL-compatible input thresholds and 24 mA sink strength. Use Value: Supports real-time fault aggregation without additional logic gates or microcontroller polling overhead. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar octal bus transceiver applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74LS641DW | Same 20-pin SOIC package and open-collector outputs, but with inverting logic polarity. | Requires complementary DIR control logic or downstream inversion for true-path signal routing. | Select SN74LS641DW only when system design accommodates inverted data paths or uses DIR inversion for functional flexibility. |
| SN74LS245N | 3-state (not open-collector) outputs, identical true logic and pinout except for G\ and DIR placement; no input hysteresis. | Suitable for isolated bus driving but not for wired-OR or multi-drop termination without external diodes/resistors. | Choose SN74LS245N when bus isolation without contention is sufficient and hysteresis is not required for noise immunity. |
Compared with SN74LS641DW and SN74LS245N, the SN74LS644DW uniquely combines true logic, open-collector outputs, and input hysteresis - making it irreplaceable in wired-OR arbitration, multi-voltage bus interfacing, and electrically noisy industrial backplanes where noise margin and bus topology flexibility are design-critical.
Availability
SN74LS644DW is available at Aetrix Electronics and suitable for industrial control backplanes, legacy test equipment upgrades, and parallel bus expansion requiring stable component supply and long-term obsolescence mitigation.
Supply support for SN74LS644DW 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 with decades of experience in logic IC design, manufacturing, and automotive/industrial qualification.
The SN74LS644DW belongs to TI's legacy LS-TTL logic family, engineered specifically for robust, noise-tolerant bidirectional bus interfacing in industrial and instrumentation systems operating at 5 V.
FAQ
What is the function of the DIR pin on the SN74LS644DW?
The DIR (direction) pin on the SN74LS644DW controls data flow direction between the A and B buses: when DIR is high, data passes from A to B; when DIR is low, data flows from B to A. This asynchronous control allows real-time reconfiguration of bus roles without clocking or reset, making SN74LS644DW ideal for dynamic bus arbitration in industrial backplanes.
Does the SN74LS644DW support 3.3 V logic levels?
No - the SN74LS644DW is a TTL-family device specified only for 5 V operation (VCC = 4.75–5.25 V) and TTL-compatible input thresholds. Its open-collector outputs can be pulled up to 3.3 V externally, but input signals must meet TTL voltage levels (≥2.0 V for HIGH), so direct 3.3 V logic interfacing requires level-shifting circuitry.
Can the SN74LS644DW replace the SN74LS245 in an existing design?
Not directly - while both are octal transceivers in 20-pin packages, SN74LS644DW has open-collector outputs and hysteresis, whereas SN74LS245 uses 3-state outputs without hysteresis. Substitution requires verifying bus topology compatibility, adding external pull-ups, and confirming that wired-OR behavior meets system timing and contention requirements.
What is the maximum sink current per output on the SN74LS644DW?
The SN74LS644DW is rated for 24 mA sink current per output (IOL) under recommended operating conditions. This value ensures reliable driving of standard TTL loads, termination resistors, or LED indicators without external buffers - a key specification confirmed in the official TI SDLS189 datasheet revision March 1988.
Is the SN74LS644DW still in active production?
Yes - according to Texas Instruments' official product folder and distributor inventory data (as of September 2002), SN74LS644DW is marked ACTIVE with ongoing availability through authorized channels including Digi-Key and Avnet, and supported by TI's long-term obsolescence management program for legacy logic devices.
SN74LS644DW 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:
- -
SN74LS644DW FAQ
1.How can I place an order for SN74LS644DW through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74LS644DW 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 SN74LS644DW reliable?
The price and inventory of SN74LS644DW are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74LS644DW is usually 5 days.
3.What payment methods are accepted for SN74LS644DW?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74LS644DW transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74LS644DW?
SN74LS644DW orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74LS644DW 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 SN74LS644DW?
For technical support, including SN74LS644DW datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74LS644DW requirements.
6.How does Aetrix verify that SN74LS644DW is sourced from the original manufacturer or authorized distributors?
All SN74LS644DW 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 SN74LS644DW meets industry standards.
7.What is the process for return or replacement of SN74LS644DW?
All SN74LS644DW units undergo pre-shipment inspection (PSI). If there is an issue with SN74LS644DW, 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 SN74LS644DW part is unused and in its original packaging.
Return procedure for SN74LS644DW:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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