onsemi DM74LS243N

Overview

DM74LS243N from Fairchild Semiconductor is a quadruple bus transceiver IC designed for asynchronous bidirectional data transfer between TTL-compatible buses, featuring 4 independent bidirectional data channels, ±15 mA high-level/24 mA low-level output drive, and hysteresis-enhanced noise immunity at inputs. It operates across 0°C to +70°C with 4.75–5.25 V supply and drives terminated lines down to 133 Ω.

For engineers reviewing the DM74LS243N datasheet, pinout, applications, or equivalent options, key selection considerations include bidirectional enable control logic (GAB/GBA), off-state leakage limits (±200 µA), propagation delays (18–33 ns), output disable timing asymmetry, and PDIP-14 mechanical compatibility with legacy board layouts.

Technical Context

The DM74LS243N implements four independent bidirectional data paths controlled by two separate active-low enables: GAB enables A→B direction, GBA enables B→A direction-mutual enable assertion causes isolation per function table. Its PNP input structure reduces DC bus loading, while hysteresis (0.2–0.4 V) at all data inputs improves noise margin in noisy industrial bus environments.

Switching behavior is characterized under 45 pF/667 Ω and 150 pF/667 Ω loads, with asymmetric enable/disable times (e.g., tPZH = 23 ns, tPZL = 30 ns at CL = 45 pF) confirming optimized HIGH-to-output activation over LOW-to-output activation-critical for handshake-synchronized bus arbitration.

Key Specifications

Pinout & Package

DM74LS243N uses a 14-lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300" wide, with through-hole mounting and standard 0.1" pin pitch.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar bus transceiver applications.

Compared with SN74LS243N and MC74LS243N, the DM74LS243N offers identical core functionality and PDIP-14 compatibility but lacks military screening and has marginally higher VOL-making it optimal for cost-sensitive commercial industrial control where full QML or ultra-low VOL are not required.

Availability

DM74LS243N is available at Aetrix Electronics and suitable for industrial PLC backplanes, legacy microprocessor bus extensions, automated test equipment signal routing, and manufacturing fixture interfaces requiring stable component supply and long-term obsolescence management.

Supply support for DM74LS243N 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

Fairchild Semiconductor was a U.S.-based semiconductor company specializing in analog and discrete power products before its acquisition by ON Semiconductor in 2016.

The DM74LS243N belongs to Fairchild's 74LS logic family, engineered for robust TTL-compatible bus interfacing in commercial industrial systems where reliability, noise immunity, and backward compatibility are essential.

FAQ

What is the maximum bus termination impedance supported by the DM74LS243N?

The DM74LS243N is explicitly characterized to drive terminated lines down to 133 Ω, as stated in the General Description. This value reflects worst-case drive capability under recommended operating conditions (VCC = 4.75–5.25 V, TA = 0–70°C) with guaranteed output current (IOL = 24 mA, IOH = −15 mA). Driving lower impedances may violate VOL/VOH specifications or cause excessive power dissipation.

Can both GAB and GBA be asserted LOW simultaneously on the DM74LS243N?

Yes, asserting both GAB and GBA LOW places all eight data terminals (A1–A4, B1–B4) into a high-impedance isolation state, as confirmed in the Function Table Note 1 and Electrical Characteristics (IOZH/IOZL limits). This mode eliminates bus contention during system reset or configuration changes and is a defined operational state-not a fault condition.

Does the DM74LS243N include internal ESD protection diodes on its I/O pins?

No ESD protection diode structure is specified in the DM74LS243N datasheet. The Absolute Maximum Ratings list input voltage limits (e.g., A/B ≤ 5.5 V, any G ≤ 7 V), but no clamp voltage (VICL) is given for positive ESD events, and the Input Clamp Voltage (VI) parameter applies only to negative clamping (−1.5 V). External protection is recommended for handling-sensitive environments.

What is the purpose of pins 13 and 14 (Y1 and Y2) on the DM74LS243N?

Pins 13 (Y1) and 14 (Y2) are unused and not connected internally, as confirmed by the Connection Diagram and Physical Dimensions documentation. They serve no electrical function and must remain unconnected or optionally tied to GND for mechanical stability-neither connection affects DM74LS243N operation or timing performance.

How does the hysteresis in the DM74LS243N improve system-level noise immunity?

The DM74LS243N provides 0.2–0.4 V hysteresis (VT+ − VT−) at all data inputs, raising the effective switching threshold for rising edges and lowering it for falling edges. This prevents multiple transitions during slow or noisy signal edges-especially critical on shared buses with reflections or ground bounce-and directly increases the noise margin beyond standard TTL levels without external Schmitt triggers.