Texas Instruments SN74LS148DR

Overview

SN74LS148DR from Texas Instruments is a TTL 8-line-to-3-line priority encoder IC performing octal binary encoding with active-low inputs and outputs. It features priority decoding to encode the highest-order asserted input (0–7), three encoded outputs (A0–A2), group select (GS), and enable in/out (EI/EO) for cascading. It operates at 5 V, delivers 15 ns typical propagation delay, and draws 12–20 mA supply current - used in keyboard scanning, interrupt prioritization, and digital control panels.

For engineers reviewing the SN74LS148DR datasheet, SN74LS148DR pinout, SN74LS148DR application, or SN74LS148DR equivalent, this page provides verified functional specifications, validated SOIC-16 pin mapping, real-world use cases in legacy industrial control and embedded I/O subsystems, and two confirmed alternative encoders with documented interface and timing differences.

Technical Context

The SN74LS148DR implements combinational priority logic where only the highest-priority low-level input (0–7) determines output state; lower-priority inputs are masked. Its internal structure uses NAND/NOR gates to generate active-low 3-bit binary (A0–A2), GS (group select, low when any input is active), and EO (enable output, low when EI is low and all inputs high).

Cascading is supported via EI (active-low enable input) and EO (active-low enable output): EO of a lower-priority stage connects to EI of a higher-priority stage, allowing expansion to 16+ inputs without external logic. All inputs and outputs are TTL-compatible with 0.8 V/2.0 V logic thresholds and drive capability of 4 mA sink per output.

Key Specifications

Pinout & Package

SN74LS148DR is housed in a 16-pin SOIC (D) package (JEDEC MS-012AC, 3.9 mm body width, 1.75 mm height), RoHS-compliant with NiPdAu lead finish and MSL Level-1 rating.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS148DR, SN74LS147N provides BCD output for decimal systems but lacks cascading capability, while 74HC148PW offers CMOS efficiency and flexibility at the cost of native TTL compatibility - making SN74LS148DR the optimal choice for maintaining signal integrity and timing in established 5 V TTL environments.

Availability

SN74LS148DR is available at Aetrix Electronics and suitable for industrial control panels, legacy equipment repair, and embedded I/O expansion modules requiring stable component supply and long-term obsolescence mitigation.

Supply support for SN74LS148DR 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 company founded in 1930, specializing in analog and embedded processing technologies with broad industrial, automotive, and consumer product portfolios.

The SN74LS148DR belongs to TI's legacy 74LS logic family, engineered for robust 5 V TTL systems requiring predictable timing, noise immunity, and direct compatibility with earlier 74xx and later 74F devices in industrial control and instrumentation.

FAQ

What is the function of the GS (Group Select) pin on the SN74LS148DR?

The GS (Group Select) pin on the SN74LS148DR is an active-low output that goes low whenever at least one of the data inputs (0–7) is low, indicating valid encoding activity. It remains high when all inputs are high (no active request) or when EI is high (encoder disabled). This signal is used to strobe or gate the A0–A2 outputs in synchronous systems, preventing metastability during input transients.

Can SN74LS148DR be used in a 3.3 V system?

No, SN74LS148DR is not rated for 3.3 V operation. Its recommended supply range is 4.75–5.25 V, and absolute maximum VCC is 5.5 V. Operating below 4.75 V risks incorrect logic thresholds, increased propagation delay, and potential output failure. For 3.3 V systems, consider CMOS alternatives like 74HC148 or level-shifted interfaces.

How does cascading work between two SN74LS148DR devices?

To cascade two SN74LS148DR devices, connect the EO (Enable Output) of the lower-priority unit to the EI (Enable Input) of the higher-priority unit. When the higher-priority unit has no active input, its EO goes low, enabling the lower-priority unit. The GS outputs of both units can be OR'd (via wired-AND or external gate) to indicate overall activity, while A0–A2 outputs are combined with priority-encoded MSB generation in external logic.

What is the maximum fan-out of SN74LS148DR outputs driving other 74LS inputs?

Each SN74LS148DR output can drive up to 10 standard 74LS inputs (IIL = −0.4 mA max per LS input, IOL = 4 mA min). This is derived from the 4 mA low-level output current specification under VOL ≤ 0.4 V. Exceeding this load may cause VOL to exceed 0.4 V, risking logic errors in downstream devices.

Why does SN74LS148DR have active-low inputs and outputs?

SN74LS148DR uses active-low signaling to align with TTL design conventions of the 1970s–1980s: it minimizes DC current draw in idle states (inputs pulled high via resistors), improves noise immunity on long traces, and allows wired-AND logic for cascading and status aggregation. Active-low outputs also simplify interfacing with common cathode LED displays and open-collector interrupt lines in legacy systems.