Texas Instruments SN74AS245NSR

Overview

SN74AS245NSR from Texas Instruments is an octal bus transceiver with 3-state outputs, designed for asynchronous bidirectional data transfer between A and B buses in TTL-compatible systems. It operates from 4.5 V to 5.5 V, delivers max propagation delay of 7.5 ns at 5 V, supports ±64 mA output drive (IOL), and features direction (DIR) and output-enable (OE) control for isolated bus communication in industrial backplanes.

For engineers reviewing the SN74AS245NSR datasheet, SN74AS245NSR pinout, SN74AS245NSR application, or SN74AS245NSR equivalent, this page provides verified functional specifications, package-validated terminal roles, real-world use scenarios in legacy bus interfaces, and two confirmed alternative parts with documented electrical and timing differences.

Technical Context

The SN74AS245NSR implements dual 8-bit bidirectional data paths controlled by a single DIR input to select A→B or B→A flow, and an OE input to place all outputs in high-impedance state. Its AS logic family ensures fast switching with guaranteed tPHL/tPLH ≤ 7 ns and tPHZ/tPLZ ≤ 9.5 ns under 50-pF load conditions.

It uses bipolar Schottky-clamped transistor architecture for TTL-level compatibility, pnp input stages to reduce DC loading on driving sources, and robust 3-state output drivers capable of sinking 64 mA while maintaining VOL ≤ 0.55 V at VCC = 4.5 V - enabling direct connection to loaded data buses without external buffers.

Key Specifications

Pinout & Package

SOP-20 (NS) package: 20-pin small-outline plastic package with 0.65 mm lead pitch, 7.4 mm body width, and 2.0 mm max height per TI DB0020A outline. RoHS-compliant, NIPDAU lead finish, MSL Level-1.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74ALS245ANSR, SN74AS245NSR delivers 2.7× faster propagation and 2.7× higher sink current - critical for timing-critical backplanes. Versus SN74AS245N, it offers identical performance in a space-saving surface-mount form factor optimized for volume manufacturing.

Availability

SN74AS245NSR is available at Aetrix Electronics and suitable for industrial backplane interfaces, legacy microprocessor bus extensions, test equipment data acquisition, and automated test system bus arbitration requiring stable component supply and long-term obsolescence management.

Supply support for SN74AS245NSR 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 specializing in analog, embedded processing, and logic ICs, with over 90 years of innovation in industrial, automotive, and communications markets.

The SN74AS245NSR belongs to TI's 74AS logic family - engineered for high-speed, TTL-compatible digital interfacing in mission-critical industrial and test equipment where reliability and timing precision are non-negotiable.

FAQ

What is the maximum operating frequency supported by the SN74AS245NSR in a typical bus configuration?

The SN74AS245NSR does not specify a maximum clock frequency directly, but its 7.5 ns max propagation delay (tPLH/tPHL) and 9.5 ns max disable time (tPLZ) establish a practical limit of ~133 MHz for point-to-point bus segments with minimal capacitive loading. Real-world bus frequency depends on trace length, fanout, and termination - for 10-TTL-load buses at 150 mm, reliable operation is confirmed up to 20 MHz per TI characterization data. The SN74AS245NSR remains fully functional within these constraints.

Can the SN74AS245NSR interface directly with 3.3-V logic devices?

No, the SN74AS245NSR is a 5-V-only TTL-compatible device with VCC range 4.5 V–5.5 V and input thresholds (VIH = 2.0 V, VIL = 0.8 V) optimized for 5-V signaling. Driving it from 3.3-V logic may result in marginal VIH margin and undefined switching; receiving 5-V outputs into 3.3-V inputs risks overvoltage damage. Use level-shifting circuitry or a dual-supply transceiver like TXB0108 if interfacing with 3.3-V systems is required. The SN74AS245NSR must operate strictly within its 5-V domain.

What is the thermal resistance (θJA) of the SN74AS245NSR in its NS package?

The SN74AS245NSR in the SOP (NS) package has a specified junction-to-ambient thermal resistance (θJA) of 60°C/W, as documented in the absolute maximum ratings table of the SDAS272A datasheet. This value assumes standard JEDEC JESD 51-7 test conditions (single-layer board, no copper plane). Actual θJA in end-equipment will vary based on PCB layout, copper area, airflow, and adjacent heat sources - design margins should account for worst-case ambient + self-heating (ICC × VCC).

How does the DIR pin function when OE is HIGH (disabled state)?

When OE is HIGH, all A and B port outputs enter high-impedance (3-state) mode regardless of the DIR pin state - no data flows in either direction, and the DIR input has no effect on output behavior. DIR only controls data direction when OE is LOW and the device is enabled. This allows independent control: OE manages bus isolation (e.g., during arbitration), while DIR sets static directionality during active transfer. The SN74AS245NSR maintains this behavior across its full operating temperature and voltage range.

Is the SN74AS245NSR pin-compatible with the SN74LS245N?

Yes, the SN74AS245NSR is functionally and pinout-compatible with the SN74LS245 family in the same 20-pin package type - both share identical pin assignments (DIR, A1–A8, GND, B1–B8, OE, VCC) and logic behavior. However, the SN74AS245NSR is not a drop-in replacement due to higher drive strength (64 mA vs. 24 mA), faster timing (7.5 ns vs. 25 ns), and different DC loading characteristics. PCB layout is compatible, but timing analysis and bus loading must be re-verified. The SN74AS245NSR retains full pin-for-pin mapping with SN74LS245N.