onsemi DM74ALS646WM

Overview

DM74ALS646WM from Fairchild Semiconductor is an octal 3-STATE bus transceiver and register IC with independent A/B registers, dual clock inputs (CAB/CBA), and real-time/stored data multiplexing. It operates at 5V, supports 40 MHz clock frequency, delivers ±24 mA output drive, and features totem-pole 3-STATE outputs for direct bus driving in industrial control and memory interface applications.

For engineers reviewing the DM74ALS646WM datasheet, pinout, applications, or equivalent options, key selection criteria include bidirectional bus timing (tPLH/tPHL ≤30 ns), register edge-triggering on positive clock transitions, independent SAB/SBA select logic, and SOIC-24 package compatibility with JEDEC MS-013 footprint.

Technical Context

The DM74ALS646WM integrates two synchronized octal D-type flip-flop registers-A register triggered by CAB and B register by CBA-with separate store/real-time selection via SAB and SBA pins using make-before-break switching to prevent glitches. Its 3-STATE outputs are TTL-compatible and designed for high-capacitance bus loading without pull-ups.

Operation modes are controlled by G (enable) and DIR (direction): G=LOW enables bidirectional real-time or stored data transfer between A/B buses; G=HIGH disables outputs while preserving input functionality. Clock-to-output propagation delays range from 5–30 ns under 50 pF load at VCC = 4.5–5.5 V.

Key Specifications

Pinout & Package

DM74ALS646WM uses a 24-lead SOIC package (JEDEC MS-013, 0.300" wide, Package Number M24B) with standard thermal characteristics (θJA = 80.5°C/W).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74ALS646N and 74F646PC, the DM74ALS646WM offers SOIC-24 surface-mount compatibility with moderate speed and lower static current, making it optimal for space-constrained industrial boards requiring proven reliability and JEDEC-standard footprint.

Availability

DM74ALS646WM is available at Aetrix Electronics and suitable for industrial PLC backplanes, legacy memory expansion modules, test equipment data capture buffers, and embedded diagnostic subsystems requiring stable component supply across long-lifecycle production programs.

Supply support for DM74ALS646WM 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 mixed-signal ICs, acquired by ON Semiconductor in 2016. Known for robust TTL and ALS logic families.

The DM74ALS646WM belongs to Fairchild's advanced low-power Schottky (ALS) logic product line, engineered for reliable bidirectional bus interfacing in commercial-grade industrial and instrumentation systems.

FAQ

What is the operating voltage range for DM74ALS646WM?

The DM74ALS646WM operates within a recommended VCC range of 5.0 V to 5.5 V, with absolute maximum rating up to 7 V. Its VIH and VIL thresholds (2.0 V min and 0.8 V max) ensure compatibility with standard TTL logic families. The device draws 47–88 mA depending on output state, and its SOIC-24 package has a thermal resistance of 80.5°C/W, requiring attention to board-level copper pour in sustained high-current operation. DM74ALS646WM must not be operated outside its specified 0°C to +70°C free-air temperature range.

How does the DM74ALS646WM handle real-time versus stored data selection?

The DM74ALS646WM uses dedicated SAB and SBA control pins to select between real-time and stored data paths: a LOW level on SAB routes live A-bus data to B-bus outputs, while HIGH selects data previously latched into the A register on the rising edge of CAB. Similarly, SBA controls B-bus data routing. Both select inputs feature make-before-break switching to eliminate output glitches during transitions. This dual-path architecture allows seamless switching without bus contention, and is fully documented in the Function Table on page 2 of the DM74ALS646WM datasheet.

What are the key timing parameters that define DM74ALS646WM performance in bus arbitration?

Key timing parameters for DM74ALS646WM in bus arbitration include tPLH/tPHL propagation delays (5–30 ns), tSU setup time (10 ns), and tH hold time (0 ns) relative to CAB/CBA clock edges. Output enable/disable times (tPZH/tPLZ, tPHZ/tPLZ) range from 1–20 ns, enabling rapid bus release and acquisition. All switching specs are guaranteed at 50 pF load and full temperature range. These values directly determine minimum bus cycle time and synchronization margin in multi-master systems using DM74ALS646WM as a shared resource gate.

Can DM74ALS646WM be used in place of standard 74LS646 devices?

DM74ALS646WM is not a direct drop-in replacement for 74LS646 due to differences in input thresholds, output drive strength, and propagation delay. While both share identical pinout and basic function, DM74ALS646WM offers improved noise immunity (VIH = 2.0 V vs. 74LS646's 2.0 V but tighter tolerances), higher sink current (24 mA vs. 16 mA), and faster typical propagation (10–20 ns vs. 15–25 ns). System validation is required before substitution, especially in timing-critical paths or marginal noise environments where DM74ALS646WM's ALS process advantages may alter signal integrity behavior.

What package type and footprint does DM74ALS646WM use?

DM74ALS646WM uses a 24-lead Small Outline Integrated Circuit (SOIC) package conforming to JEDEC MS-013 standard, 0.300" wide, with Package Number M24B. Its body dimensions are 0.390" × 0.154" (9.91 mm × 3.91 mm), lead pitch 0.025", and thermal pad not included. This surface-mount footprint is compatible with automated pick-and-place and reflow processes, and differs from the PDIP-24 variant (DM74ALS646NT) which uses Package Number N24C. Board layout must observe 80.5°C/W θJA and recommended solder mask clearance per Fairchild's physical dimensions drawing.