Texas Instruments SN74LS123NG4

Overview

SN74LS123NG4 from Texas Instruments is a dual retriggerable monostable multivibrator IC in 16-pin PDIP package, operating from 4.75 V to 5.25 V with typical propagation delay of 45 ns at V_CC = 5 V and C_EXT = 10 nF, used for pulse shaping, timing control, and debouncing in industrial logic systems.

For engineers reviewing the SN74LS123NG4 datasheet, SN74LS123NG4 pinout, SN74LS123NG4 application, or SN74LS123NG4 equivalent, this page delivers verified functional identity, confirmed TTL-compatible input thresholds, validated output drive capability (I_OH/I_OL = –0.4 mA / 8 mA), exact package dimensions, and two field-validated alternative parts for legacy TTL timing designs.

Technical Context

The SN74LS123NG4 integrates two independent retriggerable monostable multivibrators, each with separate A and B trigger inputs (active-low and active-high), clear input, and Q/Q̅ outputs. Each section supports external timing via R_EXT and C_EXT, with output pulse width determined by t_W ≈ 0.33 × R_EXT × C_EXT.

It uses bipolar TTL technology, features Schmitt-trigger inputs on B pins for noise immunity, and maintains guaranteed operation across 0°C to 70°C ambient temperature. Output stages are designed for direct interface with standard LS-TTL loads without external pull-ups.

Key Specifications

Pinout & Package

SN74LS123NG4 is supplied in a 16-pin plastic dual in-line package (PDIP-N), 7.62 mm pitch, 19.3 mm body length, 6.35 mm body width, and 3.94 mm max height per TI drawing N0016A.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS123N, SN74LS123NG4 offers identical timing and logic behavior with updated environmental compliance, while SN74LS221N provides non-retriggerable operation - making it unsuitable as a drop-in replacement but viable for fixed-interval timing where pulse overlap must be avoided.

Availability

SN74LS123NG4 is available at Aetrix Electronics and suitable for industrial control timing, keyboard debounce circuits, legacy bus arbitration, and pulse width modulation preconditioning requiring stable component supply across long-lifecycle embedded programs.

Supply support for SN74LS123NG4 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 company founded in 1930, specializing in analog and embedded processing technologies with broad industrial, automotive, and communications product portfolios.

The SN74LS123NG4 belongs to TI's legacy 74LS logic family, designed specifically for robust, low-power TTL-compatible timing functions in cost-sensitive, long-lifecycle industrial and instrumentation systems.

FAQ

What is the function of the B input on SN74LS123NG4?

The B input on SN74LS123NG4 is an active-high Schmitt-trigger input that initiates or re-triggers the monostable output pulse on a rising edge. Its built-in hysteresis (~0.5 V) rejects noise and allows reliable triggering from slow-rising or degraded waveforms - a key advantage over standard TTL inputs. This behavior is documented in the SDLS043 datasheet for SN74LS123NG4 and applies identically across all LS123 variants.

Can SN74LS123NG4 operate from a 3.3 V supply?

No, SN74LS123NG4 is specified only for 4.75 V to 5.25 V operation per its LS-TTL design. Applying 3.3 V will result in undefined logic thresholds, marginal output drive, and unreliable timing - potentially causing failure to trigger or premature timeout. For 3.3 V systems, consider modern CMOS alternatives like CD74HC221 or SN74LVC1G123, not SN74LS123NG4.

What is the minimum external timing capacitor value supported by SN74LS123NG4?

The minimum recommended C_EXT for SN74LS123NG4 is 10 nF, as validated in TI's SDLS043 characterization data. Using smaller values risks timing instability due to stray capacitance dominance and increased sensitivity to layout parasitics. At 10 nF with R_EXT = 1 kΩ, SN74LS123NG4 achieves ~3.3 µs pulse width - the practical lower limit for repeatable operation.

Is SN74LS123NG4 pin-compatible with SN74LS122N?

No, SN74LS123NG4 is not pin-compatible with SN74LS122N. The SN74LS122N is a 14-pin device with different pin count, assignment, and functionality (non-retriggerable dual monostable with different trigger configuration). Attempting PCB substitution would require rewiring and logic redesign - SN74LS123NG4 has 16 pins and distinct CLR, A/B, Q/Q̅, and R_EXT/C_EXT mapping.

Does SN74LS123NG4 require external pull-up resistors on its outputs?

No, SN74LS123NG4 does not require external pull-up resistors. Its totem-pole output stage actively drives both high (V_OH ≥ 2.7 V at I_OH = –0.4 mA) and low (V_OL ≤ 0.5 V at I_OL = 8 mA), meeting LS-TTL fanout requirements directly. Adding pull-ups may distort rise/fall times and violate output voltage specs - avoid unless interfacing with non-TTL loads requiring higher V_OH.