Honeywell MLM Series Modules

Technical Dossier

Product Details And Specifications

Honeywell MLM Series: Comprehensive Module Range and Technical Overview

The Honeywell MLM (Modular Logic Module) Series represents a foundational control platform deployed across global heavy industry verticals, including petrochemical complexes, nuclear power generation facilities, offshore oil and gas platforms, and large-scale refinery operations. Engineered under Honeywell's industrial automation division, the MLM Series was designed to meet the deterministic scan-cycle requirements of continuous process environments where unplanned downtime carries direct safety and financial consequences. Installed base figures place the MLM Series among the most widely maintained legacy PLC platforms in North America, Europe, and the Middle East, with active installations documented at facilities operating under IEC 61511 and ISA-84 functional safety frameworks.

The Evolution of MLM Architecture

The MLM Series was introduced as a modular successor to Honeywell's earlier fixed-I/O controller platforms, addressing the scalability limitations of monolithic rack designs. Early-generation MLM hardware (circa late 1990s) utilized a parallel backplane bus architecture with a proprietary inter-module communication protocol, supporting scan rates in the 5–50 ms range depending on I/O density. Mid-generation revisions introduced enhanced CPU modules with expanded memory maps and support for Modbus RTU and HART pass-through communication, enabling integration with field instrumentation without gateway hardware.

Later MLM variants incorporated dual-port Ethernet options for supervisory SCADA connectivity, bridging the gap between legacy serial fieldbus environments and modern OPC-DA/OPC-UA data architectures. As of 2026, the MLM Series is classified as a mature/end-of-life platform by Honeywell's lifecycle management policy. Honeywell has formally transitioned customers toward the Experion PKS and Safety Manager SC platforms. However, the installed base remains substantial, and direct hardware replacement is not economically viable for many facilities operating under long-term maintenance contracts. Compatibility between MLM generations requires careful attention to backplane revision levels, firmware versions, and I/O bus addressing schemes — cross-generation substitution without engineering validation is not recommended.

MLM Full Catalog & Functionalities (SKU List)

CPU & Controller Modules

• MLM-DR16S: 16-point discrete relay output controller module, DIN-rail mount, 24 VDC logic supply
• MLM-CPU01: Base CPU module, 256 KB user program memory, RS-232/RS-485 serial ports
• MLM-CPU02: Enhanced CPU module, 512 KB memory, dual serial + Ethernet 10/100 port
• MLM-CPU03: High-performance CPU, 1 MB memory, redundancy-capable, hot-swap support

Digital Input (DI) Modules

• MLM-DI16A: 16-channel 24 VDC digital input, sink/source configurable, LED status per channel
• MLM-DI32A: 32-channel 24 VDC digital input, high-density, 2-wire field wiring
• MLM-DI16B: 16-channel 120 VAC digital input, optically isolated, DIN-rail mount
• MLM-DI08H: 8-channel HART-enabled digital input with diagnostics pass-through

Digital Output (DO) Modules

• MLM-DO16A: 16-channel 24 VDC transistor output, 0.5 A per channel, short-circuit protected
• MLM-DO16R: 16-channel relay output, Form C contacts, 2 A @ 240 VAC per channel
• MLM-DO08H: 8-channel high-current relay output, 5 A per channel, surge suppression

Analog Input (AI) Modules

• MLM-AI08A: 8-channel analog input, 4–20 mA / 0–10 V selectable, 12-bit resolution
• MLM-AI08H: 8-channel HART analog input, supports HART 5/6/7, field device diagnostics
• MLM-AI16A: 16-channel analog input, 4–20 mA, 13-bit resolution, channel-to-channel isolation

Analog Output (AO) Modules

• MLM-AO04A: 4-channel analog output, 4–20 mA, 12-bit resolution, loop-powered option
• MLM-AO08A: 8-channel analog output, 4–20 mA / 0–10 V, 13-bit, short-circuit protected

Communication & Adapter Modules

• MLM-COMM01: Modbus RTU/ASCII master/slave communication adapter, RS-485, 115.2 kbps
• MLM-COMM02: PROFIBUS DP slave adapter, 12 Mbps, GSD file configurable
• MLM-ETH01: Ethernet TCP/IP communication module, 10/100 Mbps, Modbus TCP support

Power Supply Modules

• MLM-PSU24: 24 VDC rack power supply, 10 A output, 85–264 VAC input, redundancy-capable
• MLM-PSU48: 48 VDC power supply module, 5 A, wide-range AC input, DIN-rail mount

Sourcing Hard-to-Find & Obsolete MLM Parts

With the MLM Series formally classified as end-of-life by Honeywell, OEM factory supply for many modules — particularly first- and second-generation CPU boards, legacy communication adapters, and high-density I/O cards — has been discontinued. DriveKNMS maintains a dedicated inventory program for MLM Series lifecycle extension, sourcing verified surplus, refurbished, and new-old-stock (NOS) units through a global network of industrial equipment distributors, decommissioned plant auctions, and authorized secondary market channels.

DriveKNMS stocks or can source the following categories of MLM parts: CPU modules across all three hardware revisions, full-range I/O modules (DI, DO, AI, AO), communication adapters including legacy PROFIBUS and Modbus variants, rack backplane assemblies, and power supply modules. All units are catalogued with firmware revision data where available, enabling compatibility verification prior to shipment. For facilities operating under long-term service agreements or turnaround maintenance schedules, DriveKNMS offers reserved inventory allocation and multi-year supply contracts to ensure continuity of spare parts availability beyond standard market lead times.

Quality Control for the MLM Range

The MLM Series presents specific quality control challenges due to its backplane bus architecture and the age profile of available surplus inventory. DriveKNMS applies a multi-stage inspection and functional test protocol to all MLM modules prior to dispatch. Visual inspection covers PCB condition, capacitor health, connector pin integrity, and conformal coating status. Functional testing is performed on dedicated MLM rack test benches replicating the original backplane bus electrical environment, verifying correct module enumeration, I/O channel response, and communication protocol handshaking.

For analog modules, calibration verification is performed against NIST-traceable reference standards, with channel-by-channel accuracy documentation provided on request. Communication modules are tested for protocol compliance using hardware protocol analyzers. All tested units are issued a DriveKNMS inspection certificate with test date, firmware version recorded, and technician identification. Units that fail any test stage are quarantined and not returned to inventory.