Mitsubishi MELSEC-A Series Modules – AD61 High-Speed Counter Local Unit & Full Catalog

Technical Dossier

Product Details And Specifications

Mitsubishi MELSEC-A Series Technical Notes

The Mitsubishi MELSEC-A Series (also referenced as the A-Series or AnS/AnU family) represents one of the most widely deployed programmable logic controller platforms in global heavy industry. Installed across petrochemical complexes, nuclear power auxiliary systems, crude oil refineries, steel rolling mills, and large-scale water treatment infrastructure, the MELSEC-A architecture established the foundational I/O bus topology that Mitsubishi Electric carried forward into the Q Series. Facilities commissioned between the mid-1980s and early 2000s frequently retain A-Series backplanes as the primary control layer, making spare part availability a critical operational concern for maintenance engineers worldwide. The AD61 high-speed counter local unit module is a representative component of this platform, designed for pulse-train input counting applications requiring hardware-level speed independent of CPU scan cycle.

The Evolution of MELSEC-A Series Architecture

The MELSEC-A Series was introduced in the early 1980s as Mitsubishi Electric's response to the demand for modular, rack-based industrial control in discrete and process manufacturing. The original A-Series used a parallel I/O bus with fixed-slot addressing, supporting CPU modules such as the A3CPU and A2CPU alongside dedicated special function modules. The AnS sub-family introduced a compact form factor for space-constrained panels, while the AnU line extended memory capacity and instruction throughput for more complex ladder logic programs.

By the mid-1990s, the A2USCPU and A3UCPU variants added floating-point arithmetic and expanded program capacity, bridging the gap toward the Q Series transition. Compatibility between sub-families is constrained by backplane bus voltage and slot pitch: AnS modules are not physically interchangeable with standard A-Series racks without adapter hardware. The MELSEC-A Series reached end-of-production announcement in the early 2010s, with Mitsubishi Electric designating the Q Series and later the iQ-R Series as the recommended migration path. However, the installed base remains extensive, and direct replacement modules continue to be sourced through specialist distributors.

MELSEC-A Series Full Catalog & Functionalities (SKU List)

The following SKUs represent verified components within the MELSEC-A and AnS/AnU product families, classified by functional category. Each entry reflects a distinct hardware module with a defined role within the A-Series system architecture.

CPU Modules

• A2CPU: Standard CPU module, 8K step program memory, basic instruction set for discrete control.
• A3CPU: Extended CPU, 30K step capacity, supports index register and interrupt processing.
• A2USCPU: High-performance CPU with floating-point, 60K step, USB programming port.
• A3UCPU: Large-scale CPU, 120K step, multi-program execution, enhanced I/O refresh.
• A1SCPU: Compact AnS-family CPU, 8K step, suited for small-panel installations.
• A2SCPU: AnS CPU with 28K step capacity and built-in RS-232C port.

Digital Input / Output Modules

• AX40: 16-point DC input module, 24VDC sink/source, standard A-Series rack mount.
• AX80: 32-point DC input module, high-density I/O for process monitoring applications.
• AY40: 16-point transistor output module, 0.5A per point, NPN open-collector.
• AY80: 32-point transistor output module, compact wiring terminal block.
• AY10: 8-point relay output module, 2A per point, for inductive load switching.

Analog Modules

• A68AD: 8-channel analog input module, 12-bit resolution, voltage/current selectable per channel.
• A62DA: 2-channel analog output module, 12-bit D/A conversion, ±10V or 4–20mA output.
• A616AD: 16-channel analog input, high-channel-density process signal acquisition.

High-Speed Counter & Positioning Modules

• AD61: High-speed counter local unit module. Accepts pulse-train inputs up to 200kHz (single-phase) or 100kHz (two-phase). Supports ring counter, linear counter, and coincidence output modes. Operates independently of CPU scan cycle, providing hardware-level counting accuracy for encoder feedback and flow metering applications. Compatible with A-Series main base units.
• AD72: 2-axis pulse output positioning module, open-loop stepper/servo control, trapezoidal acceleration profile.
• AD75P1: Single-axis positioning module with linear/circular interpolation support, compatible with servo amplifiers via pulse interface.

Communication & Network Adapter Modules

• A1SJ71UC24-R2: RS-232C/RS-422 serial communication module for AnS racks, supports MELSEC protocol and non-procedure mode.
• AJ71UC24: Dual-port serial communication module for standard A-Series racks.
• AJ71E71: Ethernet interface module, 10BASE-T, MELSEC communication protocol over TCP/IP.

Power Supply Modules

• A1S61P: AnS-family power supply, 100–240VAC input, 5VDC/3A output for compact base units.
• A62P: Standard A-Series power supply, 100–240VAC, 5VDC/6A, supports up to 8-slot main base.

Sourcing Hard-to-Find & Obsolete MELSEC-A Series Parts

For facilities operating A-Series systems beyond the OEM support window, DriveKNMS provides: direct module replacement without system migration, cross-reference identification for AnS-to-A-Series compatibility queries, and long-lead procurement for low-volume specialty modules such as positioning and high-speed counter units. Customers requiring multiple line items from a single plant shutdown or maintenance overhaul are encouraged to submit a consolidated parts list for batch quotation.

Quality Control for the MELSEC-A Series Range

MELSEC-A Series modules present specific verification challenges due to their age profile and the complexity of their backplane bus communication. DriveKNMS applies a structured test protocol to all A-Series units prior to dispatch. CPU modules are bench-tested under live program execution with verified ladder logic, confirming instruction processing, memory integrity, and I/O refresh timing. Special function modules such as the AD61 are tested under simulated pulse-train input at rated frequency to confirm counter register accuracy, coincidence output response, and parameter retention across power cycles.