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Texas Instruments SN74LS645NG4
- Part No.:
- SN74LS645NG4
- Manufacturer:
- Texas Instruments
- Package:
- 20-DIP (0.300", 7.62mm)
- Datasheet:
-
SN74LS645NG4.pdf
- Description:
- IC TXRX NON-INVERT 5.25V 20DIP
- Quantity:
- Payment:
- Shipping:
Inventory:4,519
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Product details
Overview
SN74LS645NG4 from Texas Instruments is an octal bus transceiver with 3-state outputs, designed for bidirectional data flow control in TTL-level parallel bus systems. It features 8-bit non-inverting data paths, independent transmit/receive direction control via DIR pin, and output-enable (OE) logic for bus isolation. Used in legacy industrial controllers and retro-computing backplanes requiring robust 5V TTL compatibility.
For engineers reviewing the SN74LS645NG4 datasheet, SN74LS645NG4 pinout, SN74LS645NG4 application, or SN74LS645NG4 equivalent, this page delivers verified electrical parameters, package-specific terminal mapping, real-world use cases, and validated alternative parts - all grounded in TI's SDLS189 production data sheet and official packaging addendum.
Technical Context
The SN74LS645NG4 implements dual 8-bit bidirectional buffers with separate DIR (direction) and OE (output enable) inputs, enabling full-duplex or half-duplex bus arbitration. Its TTL-compatible input thresholds (VIH = 2.0 V min, VIL = 0.8 V max) and guaranteed VOH/VOL levels ensure interoperability with LS-TTL and standard TTL logic families.
It operates strictly within 4.75 V to 5.25 V supply range at 0 °C to +70 °C ambient, with propagation delays of tPLH/tPHL ≤ 25 ns (max) and output drive capability of IOH = –15 mA / IOL = 24 mA per pin - sufficient for direct fan-out to 20 LS-TTL loads without buffering.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Logic Family | LS-TTL - ensures compatibility with legacy 74LS series systems and predictable noise margins. |
| Supply Voltage | 4.75 V to 5.25 V - requires stable 5V rail; not tolerant of undervoltage or overvoltage operation. |
| Propagation Delay | ≤25 ns (max) - supports bus clock rates up to ~20 MHz in point-to-point configurations. |
| Output Drive | IOL = 24 mA / IOH = –15 mA - drives 20 LS-TTL loads directly; eliminates need for external buffer stages. |
| Input Thresholds | VIH ≥ 2.0 V, VIL ≤ 0.8 V - guarantees reliable switching across temperature and voltage variations. |
| Operating Temperature | 0 °C to +70 °C - qualified for commercial-grade applications only; not rated for extended or industrial temp ranges. |
| Package Type | PDIP-20 (N) - through-hole mounting compatible with manual soldering and legacy PCB assembly processes. |
Pinout & Package
SN74LS645NG4 is supplied in a 20-pin plastic dual in-line package (PDIP-N), with 0.3-inch body width and standard 0.1-inch pin pitch. The package conforms to JEDEC MS-001 and supports wave or hand soldering per TI's recommended profiles.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (OE) | Output Enable (active-low) | When low, enables A↔B bidirectional data transfer; when high, places both A and B ports in high-impedance state. |
| 2–9 (A1–A8) | Port A data lines | Connect to local subsystem bus (e.g., microprocessor data bus); driven by or sourced from A-side logic. |
| 10 (GND) | Ground reference | Primary signal return path; must be low-inductance connection to minimize ground bounce during switching. |
| 11–18 (B1–B8) | Port B data lines | Interface to remote bus segment (e.g., peripheral expansion bus); direction determined by DIR pin state. |
| 19 (DIR) | Direction control input | Low = A→B transmission; High = B→A transmission - defines data flow polarity for active transceiver mode. |
| 20 (VCC) | Positive supply | 5V power input; requires local 0.1 µF ceramic decoupling capacitor placed within 10 mm of pin 20. |
Key Features
| Feature | Design Value |
|---|---|
| Octal bidirectional data path | Enables single-chip replacement of four 74LS245 transceivers in space-constrained legacy designs. |
| 3-state outputs with common OE | Allows multiple SN74LS645NG4 devices to share same bus segment without contention when OE is deasserted. |
| Separate DIR and OE controls | Permits independent management of data direction and bus access - critical for DMA and multi-master arbitration schemes. |
| TTL-compatible input/output levels | Guarantees interoperability with existing 74LS, 74S, and standard TTL logic without level-shifting circuitry. |
| High sink current (24 mA) | Supports direct driving of LEDs, relays, or other moderate-current loads on B-port side in diagnostic or control modes. |
Applications
| Industrial PLC Backplane Interface | Retrocomputing Memory Expansion Bus |
|---|---|
Use Scenario: Isolating CPU data bus from modular I/O card slots in programmable logic controllers. IC Role / Device Role / Timing Role: Bidirectional bus transceiver managing data flow between Z80/8085-based CPU module and field I/O modules. Use Value: Enables hot-swap-capable slot architecture using DIR/OE sequencing; 24 mA sink capability drives optocoupler inputs directly. |
Use Scenario: Extending 8-bit data bus between vintage computer mainboard and third-party RAM expansion cards. IC Role / Device Role / Timing Role: Level-matched bus repeater ensuring signal integrity across 15 cm backplane traces at 4 MHz. Use Value: 25 ns max propagation delay preserves setup/hold timing margins; PDIP-20 footprint matches original design constraints. |
| Legacy Test Equipment Data Mux | Embedded Instrumentation Signal Router |
Use Scenario: Multiplexing sensor data streams from eight analog front-end channels into shared ADC interface. IC Role / Device Role / Timing Role: Controlled-direction data concentrator synchronizing channel reads under microcontroller command. Use Value: DIR pin allows dynamic reversal to feed calibration references back to sensors; OE enables clean bus release during idle cycles. |
Use Scenario: Routing debug trace signals between FPGA JTAG port and multiple embedded processor cores on a test board. IC Role / Device Role / Timing Role: Isolated bidirectional signal bridge preventing cross-talk between debug domains. Use Value: 3-state outputs eliminate need for external bus switches; TTL thresholds match FPGA I/O bank settings in 5V-tolerant mode. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar octal bus transceiver applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74LS245N | Identical pinout and function but lacks DIR-controlled direction reversal; uses separate A→B/B→A enable pins. | Requires additional logic to sequence direction control; less flexible in dynamic bus arbitration scenarios. | Select SN74LS245N only if fixed unidirectional paths or simpler control logic is acceptable. |
| SN74ALS645N | Advanced LS variant: lower power (20 mW vs 55 mW), faster speed (tPD = 15 ns), identical pinout and voltage specs. | Direct drop-in replacement where reduced power dissipation and tighter timing margins are required. | Choose SN74ALS645N for new designs needing improved performance without layout changes. |
Compared with SN74LS645NG4, SN74LS245N offers simpler control at the cost of inflexible data flow direction, while SN74ALS645N delivers measurable speed and power advantages with full mechanical and functional compatibility - making it the preferred upgrade path for performance-sensitive deployments.
Availability
SN74LS645NG4 is available at Aetrix Electronics and suitable for industrial PLC backplanes, retrocomputing expansions, legacy test equipment interfaces, and embedded instrumentation signal routing requiring stable component supply and long-term obsolescence mitigation.
Supply support for SN74LS645NG4 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 deep heritage in TTL and bipolar logic families.
The SN74LS645NG4 belongs to TI's classic 74LS logic product line, engineered for reliability and interoperability in industrial control, instrumentation, and computing systems deployed from the 1980s onward.
FAQ
What is the maximum clock rate supported by SN74LS645NG4 in a bus application?
The SN74LS645NG4 does not operate on a clock signal - it is an asynchronous transceiver. Its usable data rate depends on propagation delay (≤25 ns) and system setup/hold timing. In practice, it reliably supports bus cycles up to 20 MHz in point-to-point configurations with proper termination and layout. For SN74LS645NG4 implementations, ensure trace lengths remain under 15 cm and maintain controlled impedance where possible.
Can SN74LS645NG4 be used with 3.3V logic systems?
No - SN74LS645NG4 is strictly a 5V TTL device. Its input thresholds (VIH ≥ 2.0 V, VIL ≤ 0.8 V) and output levels (VOH ≈ 3.4 V, VOL ≈ 0.5 V at IOL = 24 mA) are incompatible with 3.3V CMOS logic without level translation. Direct connection risks marginal switching or excessive input current. For mixed-voltage systems, use dedicated level translators or select 3.3V-compatible alternatives like SN74LVC245A instead of SN74LS645NG4.
Is SN74LS645NG4 RoHS compliant?
Yes - SN74LS645NG4 carries "Yes" in TI's official packaging addendum for RoHS compliance, with lead finish specified as NIPDAU (nickel/palladium/gold). It meets EU RoHS Directive 2011/65/EU requirements and is suitable for commercial applications requiring lead-free assembly. Always verify latest status via TI's product folder, as compliance applies to specific date codes and manufacturing lots.
Does SN74LS645NG4 support hot insertion or live bus swapping?
SN74LS645NG4 itself has no built-in hot-swap protection, but its 3-state outputs enable safe bus insertion when OE is held high during plug-in. To implement robust hot-swap capability, external circuitry - such as series current-limiting resistors, TVS diodes on all data lines, and controlled power-rail sequencing - must accompany SN74LS645NG4. TI's application notes on bus isolation recommend this approach for systems requiring field-replaceable modules.
How does the DIR pin function in SN74LS645NG4 compared to similar transceivers?
In SN74LS645NG4, the DIR pin determines data flow direction: DIR = Low enables A→B transmission; DIR = High enables B→A transmission. Unlike SN74LS245, which uses separate A→B and B→A enables, SN74LS645NG4 provides single-pin polarity control - simplifying microcontroller GPIO usage and reducing external logic count. This makes SN74LS645NG4 especially valuable in firmware-driven bus arbitration schemes where direction changes occur dynamically.
SN74LS645NG4 Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74LS
- Package/Case:
- 20-DIP (0.300", 7.62mm)
- Packaging:
- Tube
- Product Status:
- Discontinued at Digi-Key
- 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:
- Through Hole
- Supplier Device Package:
- 20-PDIP
SN74LS645NG4 FAQ
1.How can I place an order for SN74LS645NG4 through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74LS645NG4 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 SN74LS645NG4 reliable?
The price and inventory of SN74LS645NG4 are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74LS645NG4 is usually 5 days.
3.What payment methods are accepted for SN74LS645NG4?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74LS645NG4 transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74LS645NG4?
SN74LS645NG4 orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74LS645NG4 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 SN74LS645NG4?
For technical support, including SN74LS645NG4 datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74LS645NG4 requirements.
6.How does Aetrix verify that SN74LS645NG4 is sourced from the original manufacturer or authorized distributors?
All SN74LS645NG4 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 SN74LS645NG4 meets industry standards.
7.What is the process for return or replacement of SN74LS645NG4?
All SN74LS645NG4 units undergo pre-shipment inspection (PSI). If there is an issue with SN74LS645NG4, 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 SN74LS645NG4 part is unused and in its original packaging.
Return procedure for SN74LS645NG4:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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