Texas Instruments SN74HC148DT

Overview

SN74HC148DT from Texas Instruments is an 8-line to 3-line priority encoder IC used in digital logic systems to compress eight active-low data inputs into a 3-bit binary-coded output (A2–A0), with active-low group select (GS) and enable output (EO) for cascading. It operates from 2V to 6V, delivers ±4 mA output drive at 5V, and achieves typical propagation delay of 38 ns at 6V. It is deployed in industrial control panels requiring compact, low-power address encoding.

For engineers reviewing the SN74HC148DT datasheet, SN74HC148DT pinout, SN74HC148DT application, or SN74HC148DT equivalent, this page provides verified functional mode behavior, cascading timing constraints, input/output voltage thresholds per VCC, and real-world design implications of its active-low encoding architecture and transient GS behavior during EI transitions.

Technical Context

The SN74HC148DT implements combinational priority encoding with strict input hierarchy: only the highest-order asserted (low) input among I0–I7 determines the output code, while all lower-priority inputs are masked. Its EI/EO signals enable direct multi-stage expansion without external gates - EO goes low when EI is low and all inputs are high, allowing daisy-chaining of encoders for 16+ inputs.

Input and output logic levels are active-low; all data inputs (I0–I7), outputs (A0–A2, GS, EO), and control pins (EI) interpret LOW as asserted. Propagation delays vary significantly with supply voltage: tpd ranges from 21 ns (6V) to 225 ns (2V) for I→A0/A1/A2 paths, and transition times (tt) shrink from 19 ns (6V) to 95 ns (2V), directly impacting maximum operating frequency in synchronous systems.

Key Specifications

Pinout & Package

SN74HC148DT uses a 16-pin SOIC (D) package measuring 9.90 mm × 3.90 mm with 1.27 mm pitch, optimized for automated assembly and thermal performance (RθJA = 73°C/W).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar priority encoder applications.

Compared with SN74HC148DT, 74HC148N trades thermal performance and surface-mount compatibility for prototyping flexibility, while CD74HC148E offers identical form-fit-function with independent manufacturing lineage - both preserve the same 2V–6V operation, 38 ns tpd at 4.5V, and EI/EO cascading capability.

Availability

SN74HC148DT is available at Aetrix Electronics and suitable for industrial control panels, microcontroller interrupt arbiters, keyboard matrix compressors, and legacy bus address decoders requiring stable component supply, long-term lifecycle support, and RoHS-compliant sourcing.

Supply support for SN74HC148DT 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 solutions, with over 50 years of innovation in industrial, automotive, and communications markets.

The SN74HC148DT belongs to TI's 74HC high-speed CMOS logic family, designed specifically for low-power, wide-voltage-range digital interfacing in space-constrained industrial and instrumentation systems.

FAQ

What is the logic polarity of SN74HC148DT inputs and outputs?

All inputs (I0–I7, EI) and outputs (A0–A2, GS, EO) of the SN74HC148DT are active-low. This means a logic LOW asserts the signal - for example, I7=LOW triggers encoding regardless of other inputs, and GS=LOW indicates at least one input is active. This polarity aligns with common open-collector interrupt architectures and simplifies connection to PLC input cards.

How does SN74HC148DT handle multiple simultaneous low inputs?

The SN74HC148DT resolves multiple simultaneous low inputs using strict priority order: I7 > I6 > I5 > I4 > I3 > I2 > I1 > I0. Only the highest-priority asserted input determines the output code (A2–A0); lower-priority inputs are ignored. This ensures deterministic behavior in interrupt arbitration or switch encoding where conflict resolution is mandatory.

Can SN74HC148DT be used in a 3.3V system?

Yes - the SN74HC148DT operates reliably across 2V to 6V, including standard 3.3V logic rails. At 3.3V, VIH is 2.31V min and VIL is 1.09V max per datasheet, ensuring clean noise margins with 3.3V CMOS outputs. Output drive remains ≥±4 mA, supporting direct interface to most 3.3V microcontrollers without level shifting.

What causes the transient low pulse on GS when EI changes?

A transient low on GS occurs when EI transitions from HIGH to LOW while all inputs (I0–I7) are HIGH. This is due to internal timing skew between EI propagation and input sampling - documented in TI's Application Information section. To avoid false triggers, synchronize EI changes with stable input states or use GS only as a qualified enable, not a standalone event flag.

Is SN74HC148DT pin-compatible with older 74LS148 encoders?

No - SN74HC148DT is not pin-compatible with 74LS148. While both are 8-to-3 priority encoders, the 74LS148 uses different pin assignments (e.g., LS148 places EO at pin 11, GS at pin 14), has TTL-level thresholds, and lacks the HC family's wide voltage range. Direct replacement requires PCB redesign and logic-level validation.