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Home > Products > Integrated Circuits (ICs) > Logic > Buffers, Drivers, Receivers, Transceivers

onsemi DM74AS651NT

Name: DM74AS651NT
Brand: onsemi
SKU: DM74AS651NT
Price: 2.40 USD
Availability: InStock
Rating: 5 (2 reviews)

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Part No.:: DM74AS651NT

Manufacturer:: onsemi

Category:: Buffers, Drivers, Receivers, Transceivers

Package:: 24-DIP (0.300", 7.62mm)

Datasheet:: DM74AS651NT.pdf

Description:: IC TXRX NON-INVERT 5.5V 24DIP

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Product details

Overview

DM74AS651NT from Fairchild Semiconductor is an octal bus transceiver and register IC with totem-pole 3-STATE outputs, edge-triggered D-type flip-flops, and dual-bus bidirectional data routing capability. It operates across 4.5V–5.5V supply, supports up to 90 MHz clock frequency, and guarantees industrial-grade performance from 0°C to +70°C in a 24-pin PDIP package. It enables real-time or registered data transfer between Bus A and Bus B in microprocessor system buses.

For engineers reviewing the DM74AS651NT datasheet, pinout, applications, or equivalent options, key selection criteria include its 3-STATE bus-driving capability, simultaneous A/B register storage control, make-before-break select logic, clocked edge-triggered latching, and compatibility with TTL-level systems requiring low-glitch bidirectional buffering.

Technical Context

The DM74AS651NT integrates two independent 8-bit data paths with separate clock (CAB/CBA) and select (SAB/SBA) controls, enabling either real-time pass-through or registered data transfer between Bus A and Bus B. Its dual-edge-triggered D-type registers store input data on the LOW-to-HIGH transition of their respective clocks, while SAB/SBA determine whether output reflects live bus data or stored register contents.

Four operational modes are defined by GAB/GBA enable inputs: A→B real-time, B→A real-time, A/B→register storage, or register→A/B output. The "make before break" select architecture prevents glitches during mode transitions, and 3-STATE outputs drive bus lines directly without pull-ups or interface components.

Key Specifications

Parameter	Value and Actual Design Meaning
Supply Voltage	4.5 V to 5.5 V - ensures compatibility with standard TTL logic rails and stable operation under varying board-level regulation.
Max Clock Frequency	90 MHz - supports high-speed synchronous bus transfers in 1980s–1990s microprocessor and memory subsystem designs.
Output Drive Current	−30 mA (source), 48 mA (sink) - sufficient to drive heavily loaded PCB traces or multiple TTL inputs without external buffers.
Propagation Delay	1 ns to 11 ns - enables sub-10 ns timing margins for critical control-path signals in bus arbitration logic.
Operating Temperature	0°C to +70°C - validated for commercial-grade embedded systems and industrial control panels with passive cooling.
Input Thresholds	V_IL = 0.8 V, V_IH = 2.0 V - matches standard TTL voltage levels for seamless integration into legacy logic families.

Pinout & Package

DM74AS651NT is housed in a 24-lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300" wide, with through-hole mounting and standard 0.1" pin pitch. Thermal resistance θ_JA is 41.1°C/W.

Pin/Terminal	Circuit Role	Design Meaning
1, 2, 3, 4, 5, 6, 7, 8	Bus A I/O	Octal bidirectional data lines connected to Bus A; driven high/low or placed in high-impedance state per GAB/SAB control.
9, 10, 11, 12, 13, 14, 15, 16	Bus B I/O	Octal bidirectional data lines connected to Bus B; functionally symmetric to Bus A pins under GBA/SBA control.
17	GAB	Enable control for A→B data path; active-low, determines whether Bus A drives Bus B or internal register feeds Bus B.
18	GBA	Enable control for B→A data path; active-low, determines whether Bus B drives Bus A or internal register feeds Bus A.
19	SAB	Select control for A→B path; HIGH selects stored register data, LOW selects real-time Bus A data.
20	SBA	Select control for B→A path; HIGH selects stored register data, LOW selects real-time Bus B data.
21	CAB	Positive-edge clock for storing Bus A data into A-side register; triggers on LOW-to-HIGH transition.
22	CBA	Positive-edge clock for storing Bus B data into B-side register; triggers on LOW-to-HIGH transition.
23	V_CC	Power supply input (4.5–5.5 V); decoupling capacitor required near pin for noise suppression.
24	GND	Ground reference for all logic and I/O; must be low-inductance connection to minimize switching noise.

Key Features

Feature	Design Value
3-STATE Totem-Pole Outputs	Direct bus-line driving without pull-up resistors or level-shifting components, reducing component count and layout complexity.
Make-Before-Break Select Logic	Eliminates output glitches during SAB/SBA transitions between real-time and registered data modes, ensuring clean bus handoffs.
Dual Independent Edge-Triggered Registers	Allows asynchronous capture of Bus A and Bus B data on separate clock edges, supporting staggered or simultaneous storage.
Four-Mode Enable Architecture	GAB/GBA combinations support isolated bidirectional transfer, register loading, and register-to-bus output-enabling flexible bus arbitration logic.

Applications

Microprocessor System Bus Interface	Memory-Mapped I/O Expansion
Use Scenario: Interfacing an 8-bit CPU data bus to peripheral address/data latches in a discrete logic-based embedded controller. IC Role / Device Role / Timing Role: Bidirectional data buffer and temporary register between CPU and peripheral chips; synchronizes transfers using CAB/CBA clocks. Use Value: Enables CPU to read/write peripheral registers without bus contention, using GAB/GBA to isolate buses during register load cycles.	Use Scenario: Expanding I/O capacity in an industrial PLC rack where multiple sensor modules share a common backplane bus. IC Role / Device Role / Timing Role: Octal transceiver isolating module-specific data lanes from shared backplane; SAB/SBA selects between live sensor reads or buffered command writes. Use Value: Prevents bus collisions during configuration updates by holding prior data in registers while new commands are loaded.
Digital Signal Path Isolation	Legacy TTL Bus Arbitration
Use Scenario: Isolating test equipment digital stimulus signals from measurement circuitry to prevent feedback coupling. IC Role / Device Role / Timing Role: Direction-controlled signal gate with register hold capability; used to freeze stimulus values during measurement acquisition windows. Use Value: Eliminates need for external latches and tri-state drivers, reducing propagation delay and board space in automated test fixtures.	Use Scenario: Managing shared address/data bus access among multiple DMA controllers in a vintage workstation design. IC Role / Device Role / Timing Role: Bus transceiver with clocked register staging; allows one DMA channel to pre-load data while another releases the bus. Use Value: Increases effective bus utilization by overlapping register load and transfer phases, improving throughput in multi-master systems.

Microprocessor System Bus Interface

Memory-Mapped I/O Expansion

Use Scenario: Interfacing an 8-bit CPU data bus to peripheral address/data latches in a discrete logic-based embedded controller.

IC Role / Device Role / Timing Role: Bidirectional data buffer and temporary register between CPU and peripheral chips; synchronizes transfers using CAB/CBA clocks.

Use Value: Enables CPU to read/write peripheral registers without bus contention, using GAB/GBA to isolate buses during register load cycles.

Use Scenario: Expanding I/O capacity in an industrial PLC rack where multiple sensor modules share a common backplane bus.

IC Role / Device Role / Timing Role: Octal transceiver isolating module-specific data lanes from shared backplane; SAB/SBA selects between live sensor reads or buffered command writes.

Use Value: Prevents bus collisions during configuration updates by holding prior data in registers while new commands are loaded.

Digital Signal Path Isolation

Legacy TTL Bus Arbitration

Use Scenario: Isolating test equipment digital stimulus signals from measurement circuitry to prevent feedback coupling.

IC Role / Device Role / Timing Role: Direction-controlled signal gate with register hold capability; used to freeze stimulus values during measurement acquisition windows.

Use Value: Eliminates need for external latches and tri-state drivers, reducing propagation delay and board space in automated test fixtures.

Use Scenario: Managing shared address/data bus access among multiple DMA controllers in a vintage workstation design.

IC Role / Device Role / Timing Role: Bus transceiver with clocked register staging; allows one DMA channel to pre-load data while another releases the bus.

Use Value: Increases effective bus utilization by overlapping register load and transfer phases, improving throughput in multi-master systems.

Equivalent & Alternatives

The following parts are listed as comparable options for similar octal bus transceiver and register applications.

Alternative Part	Technical Difference	Application Difference	Selection Advice
SN74AS861N	Single-directional (A→B only), no B→A path or B-side register; identical 24-pin PDIP package and 90 MHz max clock.	Lacks bidirectional register capability; suitable only for unidirectional buffered data paths with storage.	Select when full bidirectionality is unnecessary and cost reduction is prioritized over functional flexibility.
DM74AS652NT	Identical pinout, package, and electrical specs; differs only in internal logic - DM74AS652NT lacks the "Store A and B Data" mode (GAB=LOW, GBA=LOW, both clocks active).	Cannot simultaneously latch both buses on one clock edge; requires sequential clocking for dual-register capture.	Choose DM74AS652NT only if the simultaneous dual-bus store function is unused in the target design.

Compared with SN74AS861N and DM74AS652NT, the DM74AS651NT uniquely supports true bidirectional registered transfer with simultaneous A/B storage - critical for bus arbitration logic where deterministic timing and dual-latch coordination are required.

Availability

DM74AS651NT is available at Aetrix Electronics and suitable for microprocessor bus interfaces, memory-mapped I/O expansion, and legacy TTL system upgrades requiring stable component supply and long-term obsolescence management.

Supply support for DM74AS651NT 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

Fairchild Semiconductor was a U.S.-based semiconductor manufacturer specializing in analog, power, and logic ICs before its acquisition by ON Semiconductor in 2016. Known for robust TTL and AS-family logic devices.

The DM74AS651NT belongs to Fairchild's advanced Schottky TTL (AS) logic family, designed for high-speed, low-glitch bus interfacing in commercial and industrial control systems of the 1980s–1990s.

FAQ

What is the maximum clock frequency supported by the DM74AS651NT?

The DM74AS651NT supports a maximum clock frequency of 90 MHz under recommended operating conditions (V_CC = 4.5 V to 5.5 V). This specification is guaranteed across the full commercial temperature range (0°C to +70°C) and measured with 50 pF load capacitance. Timing margins remain valid only when setup and hold times (t_SU = 6 ns, t_H = 0 ns) are met. The DM74AS651NT achieves this speed using Fairchild's oxide-isolated, ion-implanted Schottky TTL process.

Does the DM74AS651NT support simultaneous storage of data from both Bus A and Bus B?

Yes, the DM74AS651NT supports simultaneous storage of data from both Bus A and Bus B when GAB = LOW, GBA = LOW, and both CAB and CBA receive coincident LOW-to-HIGH clock transitions. This mode is explicitly defined in the Function Table as "Store A and B Data." The DM74AS652NT does not support this exact mode - it requires staggered clocks for dual-register capture. This capability makes the DM74AS651NT essential for synchronized bus snapshot applications.

What is the purpose of the "make before break" behavior on SAB and SBA inputs?

"Make before break" on SAB and SBA ensures that the selected data source (real-time bus or stored register) overlaps briefly during transitions, eliminating output glitches that would occur in conventional multiplexers. For the DM74AS651NT, this means no invalid or floating states appear on Bus A or Bus B outputs when switching between live and registered data - critical for maintaining bus integrity in systems without external bus keepers. This behavior is intrinsic to the internal logic design and requires no external timing constraints.

Can the DM74AS651NT drive standard TTL loads directly without pull-up resistors?

Yes, the DM74AS651NT features totem-pole 3-STATE outputs rated for −15 mA (source) and 48 mA (sink), enabling direct connection to and driving of standard TTL bus loads without external pull-up resistors or interface components. Its high-impedance OFF-state and strong drive capability allow multiple DM74AS651NT devices to share the same bus line safely. This eliminates board-level components and reduces signal path incompatibility in mixed-logic systems.

What is the operating temperature range specified for the DM74AS651NT?

DM74AS651NT is specified for operation from 0°C to +70°C - the commercial temperature grade. Although the datasheet notes that "64-grade" versions (e.g., DM74AS651WM) guarantee −40°C to +85°C, the DM74AS651NT variant (PDIP package, N24C) is explicitly characterized and warranted only across the 0°C to +70°C range per the Recommended Operating Conditions table. Designers requiring extended temperature operation must verify qualification status with original documentation or consider the WM variant.

DM74AS651NT Specifications

Product attributes: Attribute value

Manufacturer:: onsemi

Series:: 74AS

Package/Case:: 24-DIP (0.300", 7.62mm)

Packaging:: Tube

Product Status:: Obsolete

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, 48mA

Voltage - Supply:: 4.5V ~ 5.5V

Operating Temperature:: 0°C ~ 70°C (TA)

Grade:: -

Qualification:: -

Mounting Type:: Through Hole

Supplier Device Package:: 24-PDIP

DM74AS651NT FAQ

1.How can I place an order for DM74AS651NT through Aetrix?

Please submit a Request for Quotation (RFQ) for DM74AS651NT 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 DM74AS651NT reliable?

The price and inventory of DM74AS651NT are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for DM74AS651NT is usually 5 days.

3.What payment methods are accepted for DM74AS651NT?

We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for DM74AS651NT transactions.

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for DM74AS651NT?

DM74AS651NT orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.

Once your DM74AS651NT 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 DM74AS651NT?

For technical support, including DM74AS651NT datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your DM74AS651NT requirements.

6.How does Aetrix verify that DM74AS651NT is sourced from the original manufacturer or authorized distributors?

All DM74AS651NT 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 DM74AS651NT meets industry standards.

7.What is the process for return or replacement of DM74AS651NT?

All DM74AS651NT units undergo pre-shipment inspection (PSI). If there is an issue with DM74AS651NT, 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 DM74AS651NT part is unused and in its original packaging.

Return procedure for DM74AS651NT:

1.Submit a request within 90 days.

2.Obtain a Return Material Authorization (RMA) from Aetrix.

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