Mitsubishi MELSEC-G Series Modules: G18B

Technical Dossier

Product Details And Specifications

Mitsubishi MELSEC-G Series: Comprehensive Module Range and Technical Overview

The Mitsubishi MELSEC-G Series is a modular programmable controller platform deployed extensively across global heavy industry sectors including petrochemical plants, nuclear power facilities, steel mills, and oil refineries. Introduced as part of Mitsubishi Electric's MELSEC family, the G Series established a standardized backplane-based architecture that allowed engineers to configure discrete I/O, analog processing, and communication functions within a single rack. Its robust design tolerates wide ambient temperature ranges and high-vibration environments, making it a reference platform in continuous-process industries where unplanned downtime carries significant operational cost. Installed base units remain active in legacy DCS and hybrid PLC/DCS architectures worldwide, and demand for spare modules — particularly base units, I/O cards, and power supplies — remains consistent due to long equipment lifecycle requirements in regulated industries.

The Evolution of MELSEC-G Series Architecture

The MELSEC-G Series was developed as a mid-range modular PLC platform positioned between Mitsubishi's compact MELSEC-F Series and the high-end MELSEC-Q Series. Early G Series hardware used a parallel backplane bus with fixed slot addressing, where each base unit (e.g., G18B, G28B, G38B) defined the maximum number of I/O and special function modules that could be mounted. Power supply modules were slot-dedicated and non-interchangeable across base sizes. Communication in early revisions relied on RS-232C and RS-422 interfaces via dedicated adapter modules.

As the platform matured, Mitsubishi introduced enhanced CPU modules with expanded program memory and faster scan cycles. The communication architecture evolved to support MELSECNET/B and later MELSECNET/H for peer-to-peer and remote I/O networking. Analog I/O modules gained higher resolution (12-bit to 16-bit) in successive hardware revisions. The G Series maintained backward compatibility at the backplane level across most of its production lifecycle, allowing older I/O modules to operate in newer base units without modification — a critical factor for facilities managing phased upgrades.

The G Series has entered the mature/end-of-life phase of its product lifecycle. Mitsubishi Electric has transitioned its current PLC portfolio to the MELSEC iQ-R and MELSEC-Q platforms. However, the G Series remains in active service at thousands of installed sites globally. DriveKNMS provides long-term maintenance support, including sourcing of discontinued modules, functional testing, and cross-reference to compatible replacement options where direct substitution is not available.

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

The following SKUs represent verified components within the Mitsubishi MELSEC-G Series. Modules are classified by functional category.

Base Mounting Units

• G18B: 8-slot base mounting unit for MELSEC-G Series module installation
• G28B: 12-slot extended base unit for high-density I/O configurations
• G38B: 6-slot compact base unit for space-constrained panel installations

CPU Modules

• A1SCPU: Compact CPU module, 8K step program capacity, AnS Series compatible base
• A2SCPU: Standard CPU, 14K step capacity, supports MELSECNET/B remote I/O
• A3NCPU: High-performance CPU, 30K step, multi-program execution support
• A1SHCPU: High-speed variant of A1SCPU with enhanced interrupt processing

Digital Input (DI) Modules

• AX40: 16-point DC input module, 24VDC sink/source compatible
• AX80: 32-point DC input module, high-density discrete input for process signals
• AX10: 8-point AC input module, 100-120VAC, isolated input groups

Digital Output (DO) Modules

• AY40: 16-point transistor output module, NPN open-collector, 0.1A per point
• AY80: 32-point transistor output module, high-density discrete output
• AY10: 8-point relay output module, 2A per point, Form A contacts

Analog I/O Modules

• A68AD: 8-channel analog input module, 12-bit resolution, 4-20mA / 0-10V selectable
• A62DA: 2-channel analog output module, 12-bit D/A conversion, voltage and current output

Communication & Network Modules

• A1SJ71AP23Q: MELSECNET/B master station module for peer-to-peer network configuration
• A1SJ71UC24-R4: RS-422/485 serial communication module, multi-drop protocol support

Power Supply Modules

• A1S61P: 5VDC/3A power supply module for AnS Series base units
• A1S62P: 5VDC/4.5A power supply, extended capacity for high I/O count configurations

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

The MELSEC-G Series has been officially discontinued by Mitsubishi Electric, with factory production of most modules ceased. Authorized distribution channels no longer carry new stock for the majority of G Series SKUs. For facilities operating legacy systems — particularly in industries with 20-30 year equipment lifecycles such as nuclear, chemical, and water treatment — sourcing replacement modules requires access to specialist inventory networks.

DriveKNMS maintains a dedicated inventory of MELSEC-G Series modules sourced through verified industrial surplus channels. All units are inspected prior to listing. For modules not currently in stock, DriveKNMS operates an active procurement network to locate specific SKUs globally. Cross-reference support is available for identifying functionally equivalent modules from the MELSEC-Q or MELSEC iQ-R platform where direct replacement is not feasible. Contact the procurement team with your full BOM or individual part numbers for availability confirmation and lead time.

Quality Control for the MELSEC-G Series Range

MELSEC-G Series modules present specific testing challenges due to their parallel backplane bus architecture and the age profile of available stock. DriveKNMS applies a structured inspection and functional verification protocol to all G Series units processed through its facility.

Base units such as the G18B undergo physical inspection of all slot connectors, backplane bus traces, and mounting hardware. Power supply modules are load-tested at rated output current with ripple and regulation measurements recorded. CPU modules are powered in a controlled test rack with known-good I/O modules to verify program execution, memory integrity, and communication port functionality. Digital I/O modules are point-tested using automated fixture boards that verify each input and output channel independently. Analog modules are calibrated against traceable reference standards with linearity and offset error documented per channel. Communication modules are tested for protocol handshake and data integrity on live network segments. All units that pass testing are labeled with inspection date and technician ID before dispatch.

© 2026 DriveKNMS. All trademarks belong to their respective owners. Specifications are for reference only and subject to change without notice. Verify all parameters against official documentation before installation.