Texas Instruments SN74LS155ADR

Overview

SN74LS155ADR from Texas Instruments is a dual 2-line-to-4-line decoder/demultiplexer in standard TTL logic family, featuring two independent 2:4 decoding sections with active-low outputs and common enable control (G̅). It operates at V_CC = 5 V, supports fan-out of 20 LS-TTL loads, and delivers propagation delay t_pd = 22 ns typical at 5 V. It is used in address decoding for memory expansion and I/O selection in legacy industrial control systems.

For engineers reviewing the SN74LS155ADR datasheet, SN74LS155ADR pinout, SN74LS155ADR application, or SN74LS155ADR equivalent, key selection criteria include its dual-decoder architecture, active-low output polarity, single shared enable input, SOIC-16 package compatibility with through-hole and surface-mount designs, and guaranteed operation across 0°C to 70°C ambient temperature range.

Technical Context

The SN74LS155ADR implements two identical 2:4 decoders sharing one active-low enable (G̅) input. Each decoder accepts two binary inputs (A₀/A₁ and A₂/A₃) and produces four mutually exclusive active-low outputs (Y₀–Y₃ per section). No internal latching or clocking is present - it is purely combinational logic.

Input thresholds are defined for standard LS-TTL: V_IL ≤ 0.8 V, V_IH ≥ 2.0 V; output drive capability is I_OL = 8 mA (low), I_OH = –0.4 mA (high) at V_CC = 5 V. Power dissipation is 24 mW typical per section under static conditions.

Key Specifications

Pinout & Package

SN74LS155ADR uses a 16-pin SOIC (D) package with 1.27 mm pitch, 10.4 mm × 7.5 mm body, and 2.00 mm max height. Pin 1 is located at the top-left corner with notch or beveled edge marking.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar dual 2:4 decoder applications.

Compared with SN74LS139DR and SN74HC139PWR, the SN74LS155ADR uniquely provides a single shared enable for synchronized dual decoding - critical for address mirroring and bus arbitration where coordinated activation is mandatory.

Availability

SN74LS155ADR is available at Aetrix Electronics and suitable for memory address decoding, industrial PLC backplane logic, and legacy test equipment signal routing requiring stable component supply and long-term obsolescence support.

Supply support for SN74LS155ADR 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, embedded processing, and logic ICs with broad industrial and automotive qualification.

The SN74LS155ADR belongs to TI's legacy 74LS logic product line, designed specifically for reliable, pin-compatible replacement in discontinued TTL-based systems and maintenance of backward-compatible digital control architectures.

FAQ

What is the function of the G̅ pin on the SN74LS155ADR?

The G̅ (Gate Enable) pin on the SN74LS155ADR is an active-low common enable input that controls both internal 2:4 decoder sections simultaneously. When G̅ is high, all eight outputs (Y₀–Y₇) are forced high (inactive); when G̅ is low, each decoder responds to its respective address inputs (A₀/A₁ or A₂/A₃). This shared enable simplifies system-level control in applications like memory banking where synchronized selection is required. The SN74LS155ADR datasheet specifies V_IL ≤ 0.8 V and V_IH ≥ 2.0 V for reliable G̅ recognition.

Can SN74LS155ADR replace SN74155 in existing designs?

Yes, the SN74LS155ADR is a direct functional and pin-compatible replacement for the original SN74155, offering improved speed (22 ns vs. 30 ns typical propagation delay) and reduced power consumption (24 mW vs. 40 mW per section). Both share identical pinout, logic function, and SOIC-16 packaging. The SN74LS155ADR maintains full backward compatibility while delivering tighter timing margins and better noise immunity due to LS-TTL input thresholds. No PCB or firmware changes are needed when substituting SN74LS155ADR for SN74155.

Is SN74LS155ADR compatible with 3.3 V logic systems?

No, the SN74LS155ADR is specified only for 5 V ±5% V_CC operation and is not 3.3 V tolerant. Its input thresholds (V_IL ≤ 0.8 V, V_IH ≥ 2.0 V) and output drive levels are optimized for LS-TTL, and applying 3.3 V signals may result in marginal or incorrect switching behavior. For 3.3 V systems, consider CMOS alternatives such as SN74HC139. The SN74LS155ADR must be used with 5 V supply and interfaced to other 5 V logic families to ensure correct operation.

What does the "DR" suffix mean in SN74LS155ADR?

The "DR" suffix in SN74LS155ADR indicates a SOIC-16 package supplied in large tape-and-reel format (2500 units per reel), with RoHS-compliant NiPdAu lead finish and moisture sensitivity level (MSL) 1 - meaning it can be reflowed at 260°C without baking. This distinguishes it from the "D" variant (obsolete, no reel info) and "AN" (PDIP, tube-packaged). The SN74LS155ADR is actively manufactured and qualified for commercial temperature range (0°C to 70°C), making it the preferred choice for high-volume automated assembly.

Does SN74LS155ADR have internal pull-up or pull-down resistors on inputs?

No, the SN74LS155ADR has no internal pull-up or pull-down resistors on any input pins (A₀–A₃, G̅). All inputs require externally defined logic states; floating inputs will cause unpredictable output behavior and increased power consumption due to input stage contention. TI recommends tying unused inputs to V_CC or GND via 1–10 kΩ resistors. This requirement applies equally to all variants including SN74LS155ADR, and is documented in the SDLS057 datasheet Section 6.1.