onsemi DM74AS240N

Overview

DM74AS240N from Fairchild Semiconductor is an advanced Schottky TTL octal 3-STATE inverting bus driver/receiver in a 20-pin PDIP package, featuring 4-channel enable-controlled inversion, 6.5 ns max tPLH propagation delay at 5 V, ±15 mA IOH / 64 mA IOL drive strength, and operation across 0°C to +70°C. It interfaces memory and microprocessor data buses with terminated transmission lines down to 133 Ω.

For engineers reviewing the DM74AS240N datasheet, pinout, applications, or equivalent options, key selection criteria include its inverting logic function, dual 4-bit enable groups, 3-STATE output control timing (tPZL ≤ 9 ns), CMOS-compatible VOH = VCC − 2 V interface capability, and PDIP-20 mechanical compatibility with legacy TTL layouts.

Technical Context

The DM74AS240N implements two independent 4-bit inverting buffer sections, each with a dedicated active-low 3-STATE enable input (G̅). Its oxide-isolated, ion-implanted Schottky TTL process delivers faster switching than standard Schottky TTL while reducing power dissipation.

Each output drives loads down to 133 Ω with low-impedance sourcing/sinking (IOH = −15 mA, IOL = 64 mA) and maintains high-impedance (Z) when disabled. Input thresholds meet TTL-compatible VIH ≥ 2.0 V and VIL ≤ 0.8 V, and outputs interface directly with CMOS at VOH = VCC − 2 V.

Key Specifications

Pinout & Package

DM74AS240N uses a 20-lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300-inch wide, with 2.54 mm lead pitch and through-hole mounting compatibility.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar inverting 3-STATE bus driver applications.

Compared with SN74AS240N and 74F240PC, DM74AS240N offers identical functionality and timing with Fairchild's oxide-isolated Schottky process, delivering better thermal stability and lower ICC in sustained 3-STATE operation-critical for dense board layouts.

Availability

DM74AS240N is available at Aetrix Electronics and suitable for microprocessor bus interfacing, memory subsystem design, and industrial control backplane routing requiring stable component supply and long-term obsolescence management.

Supply support for DM74AS240N 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 DM74AS240N belongs to Fairchild's Advanced Schottky TTL logic family, engineered for high-speed, low-power bus buffering in memory and microprocessor systems operating at 5 V.

FAQ

What logic function does the DM74AS240N implement?

The DM74AS240N implements two independent 4-bit inverting 3-STATE buffer sections. Each section accepts four inputs (A1–A4 or A5–A8), inverts them, and drives corresponding outputs (Y1–Y4 or Y5–Y8) only when its dedicated active-low enable input (G1̅ or G2̅) is LOW. When the enable is HIGH, outputs enter high-impedance (Z) state. This function is confirmed in the device's function table and connection diagram in Fairchild DS006298.

Is DM74AS240N pin-compatible with 74LS240?

Yes, DM74AS240N is functionally and pin-compatible with 74LS240 per Fairchild's datasheet claim in the Features section. Both use identical 20-pin PDIP (N20A) packaging, matching pin assignments for inputs, outputs, enables, VCC, and GND. However, DM74AS240N delivers faster propagation delays (6.5 ns vs. ~15 ns) and higher output drive (64 mA vs. 8 mA), enabling direct drop-in upgrades in existing LS-based designs.

What is the maximum recommended load capacitance for DM74AS240N outputs?

The DM74AS240N switching characteristics are specified with CL = 50 pF, and propagation delays (tPLH/tPHL) remain within datasheet limits under that condition. While the device can drive heavier capacitive loads due to its 64 mA IOL, exceeding 50 pF increases delay and may degrade edge integrity. For reliable timing compliance in high-speed bus applications, keep total output node capacitance ≤50 pF unless validated with board-level signal integrity analysis.

Does DM74AS240N support operation at 3.3 V supply?

No, DM74AS240N is not rated for 3.3 V operation. Its Absolute Maximum Ratings specify VCC up to 7 V, but Recommended Operating Conditions require VCC = 4.5 V to 5.5 V. Electrical Characteristics-including VOH, VOL, VIH, and VIL-are only guaranteed across that 5 V range. Attempting 3.3 V operation results in undefined logic thresholds, insufficient output swing, and potential failure to meet TTL or CMOS interface requirements.

How does the DM74AS240N handle input voltage beyond VCC?

The DM74AS240N includes input clamp diodes, evidenced by the VIК parameter (−1.2 V at IIN = −18 mA), allowing safe handling of transient overvoltage up to 7 V (Absolute Maximum Rating). Inputs tolerate voltages above VCC as long as current is limited externally (e.g., series resistor) to prevent exceeding the −18 mA clamp current limit. This protects against overshoot on shared buses but does not support sustained operation above VCC.