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Texas Instruments SN74LS646DW
- Part No.:
- SN74LS646DW
- Manufacturer:
- Texas Instruments
- Package:
- 24-SOIC (0.295", 7.50mm Width)
- Datasheet:
-
SN74LS646DW.pdf
- Description:
- IC TXRX NON-INVERT 5.25V 24SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:3,767
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Product details
Overview
SN74LS646DW from Texas Instruments is an octal bus transceiver with 3-state outputs and internal bus-hold circuitry, designed for bidirectional data transmission in TTL-level systems. It features 8-bit parallel data flow, ±24-mA output drive capability, 15 ns typical propagation delay (tPLH/tPHL), and operates over 0°C to 70°C. It is used in legacy industrial control backplanes and microprocessor address/data bus interfaces.
For engineers reviewing the SN74LS646DW datasheet, SN74LS646DW pinout, SN74LS646DW application, or SN74LS646DW equivalent, key selection considerations include its bidirectional 3-state control logic (OE\ and DIR), bus-hold functionality for floating-input immunity, SOIC-24 package compatibility, and LS-family DC/AC electrical specifications.
Technical Context
The SN74LS646DW integrates dual-function control: DIR selects data direction (A→B or B→A), while active-low OE\ enables/disables both A- and B-side 3-state outputs. Its bus-hold circuitry maintains last-valid logic state on un-driven I/O pins without external pull-ups.
Designed for TTL-compatible systems, it supports 5-V VCC, meets standard LS logic thresholds (VIH = 2.0 V min, VIL = 0.8 V max), and delivers ±24-mA sink/source current per output - sufficient to drive multiple LS loads or interface directly with 74ALS devices.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Supply | 5 V nominal; ensures compatibility with standard TTL logic families and legacy 5-V system rails. |
| Propagation Delay | 15 ns typical (tPLH/tPHL); enables reliable operation in 33-MHz bus cycles with margin. |
| Output Drive | ±24 mA per pin; drives up to 15 LS TTL loads or interfaces directly with 74ALS without buffers. |
| Operating Temp | 0°C to +70°C; suitable for commercial-grade embedded controllers and non-extended-temperature instrumentation. |
| Bus-Hold Current | ±1 mA typical; actively retains logic state on unused or disconnected I/Os, eliminating external pull resistors. |
| Input Hysteresis | 0.5 V typical; improves noise immunity on shared bus lines in electrically noisy industrial environments. |
Pinout & Package
SN74LS646DW is housed in a 24-pin SOIC (DW) package with 300-mil body width, JEDEC MS-013 compliant, and RoHS-compliant NiPdAu lead finish. Pin pitch is 1.27 mm; package thickness is 2.35 mm.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| A1–A8 | Data inputs/outputs (Side A) | Bidirectional I/O port connected to local subsystem (e.g., CPU data bus). |
| B1–B8 | Data inputs/outputs (Side B) | Bidirectional I/O port connected to peripheral or memory bus segment. |
| DIR | Direction control input | High = A→B transfer; Low = B→A transfer - enables full-duplex bus arbitration. |
| OE\ | Output enable (active low) | When high, all A/B outputs enter high-impedance state - essential for multi-drop bus sharing. |
| GND | Ground reference | Signal return for logic and output drivers; requires low-inductance connection to minimize switching noise. |
| VCC | Positive supply | 5-V power rail; must be decoupled locally with 0.1-µF ceramic capacitor near pin 24. |
Key Features
| Feature | Design Value |
|---|---|
| Bus-hold circuitry | Maintains valid logic level on floating A/B pins without external biasing - reduces BOM count and layout area. |
| 3-state dual-output architecture | Independent high-impedance control of both A and B ports via single OE\ signal - simplifies bus arbitration logic. |
| LS-family compatible thresholds | Guaranteed VIH ≥ 2.0 V and VIL ≤ 0.8 V - ensures interoperability with 74LS, 74ALS, and 74F logic in mixed-family designs. |
| ±24-mA output drive | Supports fan-out of 15 LS loads per pin - eliminates need for line drivers in moderate-speed backplane applications. |
Applications
| Industrial Backplane Interface | Microprocessor Address Latching |
|---|---|
Use Scenario: Interfacing a Z80 or 8085 CPU with multiple peripheral cards across a 20-slot passive backplane. IC Role / Device Role / Timing Role: Bidirectional data transceiver managing shared 8-bit data bus between CPU and peripherals under DIR/OE\ control. Use Value: Bus-hold prevents floating states during slot insertion/removal; ±24-mA drive sustains signal integrity over 30-cm ribbon cable runs. |
Use Scenario: Latching 16-bit address lines split into high/low bytes for memory-mapped I/O decoding in legacy PLC controllers. IC Role / Device Role / Timing Role: Direction-controlled address buffer isolating CPU address bus from decoder logic during write cycles. Use Value: 15-ns propagation delay aligns with 4-MHz Z80 timing budgets; 3-state outputs prevent address contention during refresh cycles. |
| Legacy Test Equipment Bus | EPROM Programming Adapter |
Use Scenario: Signal conditioning and level translation between GPIB controller and modular instrument modules in 1980s-era automated test systems. IC Role / Device Role / Timing Role: Isolating and buffering handshake/control signals (e.g., DAV, NRFD) across isolated bus segments. Use Value: Input hysteresis rejects EMI-induced glitches on long bus traces; bus-hold avoids false triggers during module hot-swap. |
Use Scenario: Adapting 8-bit parallel data from PC LPT port to 27C256 EPROM programming voltage and timing requirements. IC Role / Device Role / Timing Role: Level-shifting and driving EPROM data/address pins during VPP=21-V programming pulses. Use Value: ±24-mA sink capability safely handles EPROM input leakage during high-voltage programming; OE\ enables safe idle state. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar octal bidirectional transceiver applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74LS245N | No bus-hold circuitry; identical SOIC-20 pinout but 8-pin smaller; ±15-mA output drive. | Lacks floating-input protection; requires external pull-up/down resistors in unconnected states. | Select when board space is constrained and bus-hold is managed externally. |
| SN74ALS646DW | Faster 10-ns propagation delay; higher 50-mA output drive; ALS family, not LS - higher VIL threshold (0.5 V max). | Not drop-in compatible due to different input thresholds and increased power consumption (25 mW vs 19 mW). | Choose for speed-critical upgrades where VCC noise margin allows tighter input thresholds. |
Compared with SN74LS245N and SN74ALS646DW, the SN74LS646DW uniquely combines bus-hold functionality, LS-family compatibility, and SOIC-24 packaging - making it the only choice for maintaining legacy system reliability without redesigning termination networks or requalifying timing margins.
Availability
SN74LS646DW is available at Aetrix Electronics and suitable for industrial backplane interfaces, legacy microprocessor bus expansion, and EPROM programming adapters requiring stable component supply and long-term obsolescence support.
Supply support for SN74LS646DW 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 innovator founded in 1930, with leadership in analog, embedded processing, and logic technologies spanning over nine decades.
The SN74LS646DW belongs to TI's legacy 74LS logic family, engineered specifically for robust, low-power, medium-speed digital interconnect in industrial control, test equipment, and computer peripheral systems of the 1980s–1990s era.
FAQ
What is the function of the DIR pin on the SN74LS646DW?
The DIR (direction) pin on the SN74LS646DW controls data flow direction between the A and B ports: when DIR is high, data passes from A to B; when DIR is low, data flows from B to A. This bidirectional capability allows the SN74LS646DW to serve as a flexible bus interface in systems where data must move in either direction under processor control, such as in shared-memory architectures or peripheral expansion slots.
Does the SN74LS646DW require external pull-up resistors on its I/O pins?
No, the SN74LS646DW does not require external pull-up resistors because it includes integrated bus-hold circuitry on all A- and B-side I/O pins. This feature actively maintains the last-valid logic state when inputs are floating - a critical advantage in hot-plug backplane or modular systems. The SN74LS646DW's bus-hold current is ±1 mA typical, eliminating design overhead associated with discrete biasing components.
What is the maximum clock frequency supported by the SN74LS646DW?
The SN74LS646DW is not a clocked device and has no internal clock; it is a combinatorial transceiver. Its usable data rate depends on propagation delay (15 ns typical) and system setup/hold timing. In practice, it reliably supports asynchronous bus cycles up to approximately 33 MHz - verified by tPLH/tPHL specs and LS-family AC characterization in the SDLS190A datasheet for the SN74LS646DW.
Is the SN74LS646DW pin-compatible with the SN74LS245?
No, the SN74LS646DW is not pin-compatible with the SN74LS245: the SN74LS646DW uses a 24-pin SOIC (DW) package with dedicated DIR and separate A/B I/O sets, whereas the SN74LS245 uses a 20-pin package and lacks bus-hold and DIR pin functionality. Their pin counts, layouts, and control signal assignments differ - direct substitution would require PCB revision and firmware logic updates for direction control.
What is the operating temperature range of the SN74LS646DW?
The SN74LS646DW is specified for commercial-grade operation from 0°C to +70°C ambient temperature, as confirmed in TI's official packaging addendum and SDLS190A datasheet. This range suits indoor industrial controls, laboratory instruments, and legacy computing equipment but excludes extended-temperature applications such as automotive under-hood or outdoor telecom enclosures.
SN74LS646DW Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74LS
- Package/Case:
- 24-SOIC (0.295", 7.50mm Width)
- Packaging:
- Bulk
- Product Status:
- Active
- Logic Type:
- Transceiver, Non-Inverting
- Number of Elements:
- 1
- Number of Bits per Element:
- 8
- Input Type:
- -
- Output Type:
- 3-State
- Current - Output High, Low:
- 15mA, 24mA
- Voltage - Supply:
- 4.75V ~ 5.25V
- Operating Temperature:
- 0°C ~ 70°C (TA)
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 24-SOIC
SN74LS646DW FAQ
1.How can I place an order for SN74LS646DW through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74LS646DW 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 SN74LS646DW reliable?
The price and inventory of SN74LS646DW are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74LS646DW is usually 5 days.
3.What payment methods are accepted for SN74LS646DW?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74LS646DW transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74LS646DW?
SN74LS646DW orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74LS646DW 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 SN74LS646DW?
For technical support, including SN74LS646DW datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74LS646DW requirements.
6.How does Aetrix verify that SN74LS646DW is sourced from the original manufacturer or authorized distributors?
All SN74LS646DW 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 SN74LS646DW meets industry standards.
7.What is the process for return or replacement of SN74LS646DW?
All SN74LS646DW units undergo pre-shipment inspection (PSI). If there is an issue with SN74LS646DW, 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 SN74LS646DW part is unused and in its original packaging.
Return procedure for SN74LS646DW:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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