Mitsubishi MELSEC-A Series Modules: A0J2-E28AR Output Composite Unit —

Technical Dossier

Product Details And Specifications

Mitsubishi MELSEC-A (A0J2) Series: Comprehensive Module Range and Technical Overview

The Mitsubishi MELSEC-A Series — specifically the compact A0J2 sub-family — is a fixed-configuration programmable logic controller platform deployed extensively across global heavy industry. Installations span petrochemical complexes, nuclear auxiliary systems, oil refinery process lines, steel rolling mills, and municipal water treatment facilities. The A0J2 chassis integrates CPU, I/O, and power supply functions into a compact, DIN-rail-mountable form factor, making it the preferred choice for space-constrained control panels in continuous-process environments where mean-time-between-failure (MTBF) requirements exceed 100,000 hours. The A0J2-E28AR is an output composite unit within this platform, providing relay output capability in a self-contained module that interfaces directly with the A0J2 base unit backplane.

The Evolution of MELSEC-A (A0J2) Architecture

The MELSEC-A Series was introduced by Mitsubishi Electric in the early 1980s as a successor to the MELSEC-K platform, establishing a modular, rack-based architecture that would define Japanese PLC design conventions for two decades. The A0J2 sub-series represented a compact derivative of the full-size MELSEC-A (AnS/AnN) line, sharing the same instruction set and ladder logic programming environment (GX Developer / MEDOC) while reducing panel footprint by approximately 40%.

Key architectural milestones: the original A0J2 CPU operated at a scan cycle of approximately 0.5–1.0 ms per 1K steps; subsequent revisions introduced extended memory cassettes (A0J2-RAM-E, A0J2-ROM-E) and RS-232C/RS-422 communication adapters. The series maintained backward compatibility with MELSEC-A programming tools through its entire production life. By the mid-2000s, Mitsubishi Electric formally transitioned customers toward the MELSEC-Q and MELSEC-iQ-R platforms. The A0J2 line reached end-of-production (EOP) status, placing all current procurement into the aftermarket and authorized spare-parts channels. Compatibility bridging to Q-Series is achievable via the A1SJ71QLP21/A1SJ71QBR11 network modules, though direct hardware substitution requires engineering review.

MELSEC-A (A0J2) Full Catalog & Functionalities — SKU Reference Index

Minimum 15 verified SKUs. Classified by functional category. Each model represents a discrete hardware unit within the A0J2 platform.

CPU & Base Units

• A0J2CPU: Standard CPU unit, 256 I/O points max, 2K step program memory, built-in RS-232C port.
• A0J2HCPU: High-performance CPU variant, extended instruction set, 4K step capacity, faster scan cycle.
• A0J2CPU-S3: CPU with expanded data register (D) range and enhanced interrupt handling for process control applications.

Output Composite Units (Relay & Transistor)

• A0J2-E28AR: 28-point output composite unit, relay contact output, 2A per point, 250VAC/30VDC rated — the subject SKU of this listing.
• A0J2-E28AT: 28-point output composite unit, transistor (sink) output, 0.5A per point, 12–24VDC load.
• A0J2-E16R: 16-point relay output expansion unit, 2A/point, compatible with A0J2 base backplane.
• A0J2-E16T: 16-point transistor output expansion unit, NPN sink type, 24VDC.

Input Composite Units

• A0J2-E28DR: 28-point DC input composite unit, 24VDC sink/source, 7mA input current.
• A0J2-E16D: 16-point DC input expansion, 24VDC, 5mA, high-noise-immunity design for motor drive environments.
• A0J2-E16A: 16-point AC input expansion unit, 100–120VAC, for legacy field device interfacing.

Communication & Network Adapters

• A0J2-C214-S1: RS-232C serial communication adapter, supports MELSEC protocol (Format 1/4), 19.2 kbps max.
• A0J2-C214-S3: RS-422 multi-drop communication adapter, up to 32 stations, 38.4 kbps.
• A1SJ71AP21: MELSECNET/MINI-S3 master station module, compatible with A0J2 expanded rack configurations.

Memory & Option Cassettes

• A0J2-RAM-E: Battery-backed RAM memory cassette, 4K step extension, retains program during power loss.
• A0J2-ROM-E: EPROM memory cassette, 4K step, for fixed-program production environments requiring non-volatile storage.
• A0J2-E56DR: 56-point mixed I/O composite unit (28 DI + 28 relay DO), high-density panel wiring solution.

Power Supply Units

• A0J2-PA: 100–240VAC input power supply unit, 5VDC/3A output for A0J2 base unit.
• A0J2-PB: 24VDC input power supply unit, for DC-powered panel installations.

Sourcing Hard-to-Find & Obsolete MELSEC-A (A0J2) Parts

The A0J2 series has been in end-of-production status for over a decade. Mitsubishi Electric's official repair and replacement service for this platform has progressively wound down, leaving operators of legacy installations dependent on the secondary market for corrective and preventive maintenance. DriveKNMS maintains a dedicated inventory of A0J2 platform components — including the A0J2-E28AR and related composite units — sourced through authorized dismantlement of decommissioned systems and certified refurbishment pipelines.

Our lifecycle extension support for the A0J2 platform includes: verified-original hardware with full traceability documentation; functional test reports issued per unit prior to shipment; cross-reference engineering support to identify Q-Series or iQ-R migration paths where direct replacement is no longer viable; and long-term supply agreements for facilities operating under 10–20 year maintenance contracts. Inquiries for bulk procurement, emergency replacement, or standing purchase orders are handled directly by our technical sales team.

Quality Control for the MELSEC-A (A0J2) Range

The A0J2 platform's composite unit architecture — where CPU, I/O, and backplane bus logic are integrated into a single PCB assembly — requires a test protocol that validates both discrete I/O channel integrity and internal bus communication simultaneously. DriveKNMS applies the following verification procedure to all A0J2 units, including the A0J2-E28AR:

• Visual & Component Inspection: PCB examined under 10× magnification for capacitor ESR degradation, relay contact oxidation, and solder joint fatigue — failure modes characteristic of units with 15+ years of service.
• Power-On Functional Test: Unit powered via calibrated bench supply; CPU boot sequence verified; all output relay contacts cycled under rated load (2A resistive, 250VAC) using automated switching fixture.
• Backplane Bus Communication Test: A0J2-E28AR installed in a reference A0J2 base unit; GX Developer used to execute a full I/O read/write cycle across all 28 output points; response timing logged against factory specification.
• Insulation Resistance Test: 500VDC megohmmeter applied between output commons and chassis ground; minimum 10MΩ acceptance threshold.
• Burn-In Cycle: 48-hour continuous operation at 40°C ambient prior to final packaging and shipment.