Mitsubishi MELSEC-A Series Modules: A1SCPU

Technical Dossier

Product Details And Specifications

Mitsubishi MELSEC-A Series Technical Notes

The Mitsubishi MELSEC-A Series is a foundational programmable logic controller (PLC) platform developed by Mitsubishi Electric and deployed across global heavy industry sectors including petrochemical refineries, nuclear power auxiliary systems, steel rolling mills, and continuous process chemical plants. First introduced in the early 1980s, the MELSEC-A architecture established the baseline for Mitsubishi's ladder-logic execution model and parallel I/O bus topology that persisted across subsequent generations. Installed base counts in the hundreds of thousands of units globally, with a significant proportion still operating in brownfield facilities where replacement of the control layer is cost-prohibitive. The A1SCPU, as the compact CPU variant of this series, is one of the most widely referenced spare parts in the MELSEC-A ecosystem.

The Evolution of MELSEC-A Series Architecture

The MELSEC-A Series was launched as Mitsubishi Electric's response to the industrial automation standardization wave of the 1980s. The original A-Series (large-format) used a parallel backplane bus operating at TTL logic levels, with CPU modules executing relay-ladder diagrams at scan rates measured in tens of milliseconds. The compact A1S sub-family (including the A1SCPU) was introduced to address panel space constraints while maintaining full instruction-set compatibility with the full-size A-Series.

Key architectural milestones: the transition from mask ROM to EEPROM-based program storage (enabling field reprogramming without module extraction), the introduction of the AnS-compatible I/O bus (allowing mixed-slot configurations), and the later addition of MELSECNET/B and MELSECNET/10 communication adapters for network-layer integration. Compatibility between A-Series and Q-Series (the successor platform) is not native — migration requires Mitsubishi's A-to-Q conversion modules or full rack replacement. This architectural discontinuity is the primary driver of sustained demand for original A-Series spare parts.

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

CPU Modules

• A1SCPU: Compact A1S-format CPU, 2K step program capacity, basic instruction set
• A1SCPUC24-R2: A1S CPU with dual RS-232C/RS-422 built-in serial ports
• A2SCPU: Enhanced compact CPU, 8K step capacity, extended data register range
• A2SCPU-S1: A2SCPU with expanded I/O points (256 points) and SFC support
• A3NCPU: Large-format CPU, 30K step, multi-program execution support
• A3HCPU: High-speed large CPU, 60K step, 0.2µs basic instruction execution
• A4UCPU: Ultra-large CPU, 120K step, redundant memory architecture

Digital Input (DI) Modules

• A1SX10: 16-point DC input module, 24VDC sink/source, 10ms response
• A1SX20: 16-point DC input, 24VDC, 1ms high-speed response
• A1SX30: 32-point DC input module, 24VDC, standard response
• A1SX40: 16-point AC input module, 100–120VAC, isolated commons

Digital Output (DO) Modules

• A1SY10: 16-point relay output, 2A per point, 240VAC/24VDC load
• A1SY40: 16-point triac output, 0.6A per point, 100–240VAC
• A1SY80: 32-point transistor output, 0.1A per point, 12–24VDC sink

Analog I/O Modules

• A1S62DA: 2-channel analog output, 12-bit resolution, 0–10V / 4–20mA selectable
• A1S64AD: 4-channel analog input, 12-bit resolution, voltage/current input

Communication & Network Modules

• A1SJ71UC24-R2: Serial communication module, RS-232C/RS-422, computer link protocol
• A1SJ71BR11: MELSECNET/B master/local station module, coaxial bus
• A1SJ71QLP21: MELSECNET/10 optical loop module, 10Mbps

Power Supply Modules

• A1S61PN: 5VDC/3A power supply for A1S base units, 100–240VAC input
• A1S62PN: 5VDC/5A extended power supply, supports larger I/O configurations

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

Mitsubishi Electric officially discontinued active production of the MELSEC-A Series CPU and I/O modules. Repair parts, replacement boards, and tested surplus units are no longer available through standard distribution channels. DriveKNMS maintains a dedicated inventory of MELSEC-A Series modules sourced from decommissioned plant equipment, verified OEM overstock, and controlled disassembly of functional systems. All units are catalogued by firmware revision where applicable. For CPU modules such as the A1SCPU and A2SCPU, program memory battery condition is assessed and documented prior to dispatch. Cross-reference support is available for customers seeking equivalent functionality under the MELSEC-Q or MELSEC-iQ-R migration path.

Quality Control for the MELSEC-A Series Range

MELSEC-A Series modules present specific test challenges due to their parallel backplane bus architecture and age-related capacitor degradation. DriveKNMS applies the following test protocol to all A-Series units: (1) Visual inspection for PCB corrosion, burnt traces, and capacitor bulge on the main logic board and power regulation section. (2) Backplane bus continuity verification using a dedicated A1S/A2S test rack with known-good power supply. (3) CPU modules are powered through a full cold-start cycle; program memory read/write integrity is confirmed via GX Developer or GX Works2 in A-Series compatibility mode. (4) I/O modules are tested under load using a calibrated signal generator (for input modules) and resistive load bank (for output modules). (5) Communication modules are verified for protocol handshake completion against a reference PLC. All units that pass are issued a DriveKNMS test certificate with date stamp and technician ID.