Texas Instruments SN74LS123N

Overview

SN74LS123N from Texas Instruments is a dual retriggerable monostable multivibrator IC in 16-pin PDIP packaging, operating over 0°C to 70°C, with typical propagation delay of 25 ns at V_CC = 5 V and output drive capability of ±8 mA. It provides precise, edge-triggered pulse generation for timing control in legacy TTL systems.

For engineers reviewing the SN74LS123N datasheet, SN74LS123N pinout, SN74LS123N application, or SN74LS123N equivalent, this page delivers verified functional identity, confirmed package mapping (PDIP-16), validated timing parameters, and real-world use context for industrial control, instrumentation, and retro-computing design.

Technical Context

The SN74LS123N contains two independent retriggerable monostable multivibrators, each with separate A and B trigger inputs (active-low and active-high), reset input, and Q/Q̅ outputs. Its retriggerable architecture allows pulse width extension during an active output period without requiring external gating logic.

Timing is set externally via R_EXT and C_EXT, supporting pulse widths from 30 ns to 30 s. Output stage uses standard TTL-compatible push-pull drivers with guaranteed high-level output voltage ≥2.7 V at I_OH = –0.4 mA and low-level output voltage ≤0.5 V at I_OL = 8 mA.

Key Specifications

Pinout & Package

SN74LS123N is housed in a 16-pin plastic dual in-line package (PDIP-N), 0.3-inch body width, with standard 0.1-inch pin pitch and through-hole mounting. Pin 1 is located at the left end of the top row, identified by a notch or dot marking.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar monostable multivibrator applications.

Compared with SN74LS123DR and SN74LS221N, the SN74LS123N uniquely combines retriggerable operation, through-hole mountability, and commercial-temperature LS-TTL compatibility - making it the sole choice for legacy system repair and hand-soldered prototyping where pulse extension under burst triggering is essential.

Availability

SN74LS123N is available at Aetrix Electronics and suitable for industrial timing control, legacy system maintenance, and retrocomputing hardware development requiring stable component supply and long-term obsolescence management.

Supply support for SN74LS123N 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 logic ICs, microcontrollers, and educational technology.

The SN74LS123N belongs to TI's classic 74LS logic family, designed specifically for robust, low-power TTL-compatible timing and sequencing in industrial, test, and computing equipment from the 1970s–1990s.

FAQ

What is the maximum pulse width achievable with SN74LS123N?

The SN74LS123N supports pulse widths up to 30 seconds using external R_EXT and C_EXT components. This is achieved with R_EXT ≤ 10 MΩ and C_EXT ≤ 100 µF, per the SDLS043 datasheet. The upper limit is constrained by leakage current and capacitor stability-not by the SN74LS123N itself. For reliable sub-microsecond timing, values below 1 kΩ and 100 pF are recommended.

Can SN74LS123N operate with a 3.3 V supply?

No, the SN74LS123N is specified only for 4.75 V to 5.25 V operation. It is an LS-TTL device and does not guarantee correct function or timing at 3.3 V. Attempting 3.3 V operation may result in marginal noise margins, increased propagation delay, or failure to recognize input logic levels. For 3.3 V systems, consider modern CMOS alternatives like the 74LVC123A.

Is SN74LS123N pin-compatible with SN74LS221N?

No-while both are dual monostables in PDIP-16 packages, SN74LS123N and SN74LS221N have different pin assignments. Specifically, SN74LS221N places its reset inputs on Pins 1 and 15, whereas SN74LS123N uses Pins 3 and 13. Direct substitution without PCB modification will cause functional failure. Always verify pinout diagrams before replacement.

Does SN74LS123N require pull-up resistors on its inputs?

No-inputs 1A, 1B, 2A, 2B, 1R̅, and 2R̅ are TTL-compatible and internally biased. They accept clean logic-level signals directly. Pull-ups are unnecessary unless driving from open-collector sources or long traces susceptible to noise. In such cases, 4.7 kΩ pull-ups to V_CC are sufficient and consistent with LS-TTL design practice.

What is the purpose of the dual trigger inputs (A and B) on each channel of SN74LS123N?

Each channel of the SN74LS123N provides two independent trigger inputs: A (active-low) and B (active-high). This allows flexible edge detection-either a falling edge on 1A or a rising edge on 1B initiates the monostable output. Designers can choose the input that best matches upstream signal polarity, eliminating inverters and reducing component count in timing-critical paths.