Valmet ABMB MT940 – Analogue Bus Mother Board PCB

The Valmet ABMB MT940 is a printed circuit board that functions as an Analogue Bus Mother Board (ABMB) for Valmet DNA and Metso I/O subracks. This backplane board provides the physical and electrical infrastructure for plug‑in modules, distributing power rails and analogue/digital communication buses across multiple slots. The MT940 is designed to support a mix of analogue and digital I/O cards, offering a reliable and low‑impedance interconnect solution for critical automation systems.

Slot Configuration and Bus Architecture

The MT940 provides 8 slots (though some variants offer 16), each equipped with a 96‑pin DIN 41612 connector that carries the system bus, including address, data, and control lines. The board implements a parallel bus architecture for high‑speed data transfer, along with a serial CAN‑based bus for configuration and diagnostics. The bus is actively terminated with onboard resistor networks that can be enabled or disabled via DIP switches depending on the rack position. The MT940 includes a dedicated synchronization bus for time‑critical applications.

Power Distribution and Filtering

The backplane distributes +5 V DC, +24 V DC, and ±15 V DC (for analogue modules) from rear power connectors to each slot. High‑current traces are reinforced with thick copper planes to minimize voltage drop. Each rail is decoupled with capacitor banks near each slot to suppress switching noise. The board includes a bank of LEDs that indicate the presence of each power rail, and a fuse on the main input for overcurrent protection. The MT940 also features a power‑good monitoring circuit that can generate a system alarm.

Signal Integrity and Impedance Control

The MT940 is designed with controlled impedance traces (50 Ω single‑ended, 100 Ω differential) and length‑matched critical lines to ensure reliable data transmission at speeds up to 25 MHz. The board uses a multi‑layer stackup with solid ground planes to minimize crosstalk and electromagnetic interference. All bus signals are protected by ESD suppression diodes to safeguard connected modules.

Mechanical Specifications and Mounting

The PCB is a 6U height (233 mm) by 160 mm depth, with a thickness of 2.4 mm for rigidity. It is designed to be mounted at the rear of a standard 19‑inch subrack using screws and standoffs. The board has cutouts for cable management and ventilation. It weighs approximately 1.5 kg.

Connectors and Interface Details

At the rear, the backplane provides a 50‑pin ribbon cable header for external expansion, a 20‑pin power input connector, and a 10‑pin diagnostic port. Each slot connector is keyed to prevent incorrect module insertion. Test points are provided for measuring power rails and critical signals without removing modules.

Environmental and Compliance

Operating temperature: 0 °C to +60 °C; storage: ‑40 °C to +85 °C; humidity: 10‑90 % non‑condensing. The PCB is made of FR‑4 material with a high TG (170 °C) and is conformal coated. It meets UL 94 V‑0 and EN 61000‑6‑2/‑4 standards.

Installation and Configuration

Install the backplane into the rack enclosure, securing it with screws. Attach the power supply wiring to the rear connectors. Insert the plug‑in modules into the front slots, aligning with the guides. The backplane requires no software configuration; however, the bus termination jumpers must be set correctly if the rack is at the end of a daisy‑chain.

Applications

The MT940 is used in all Valmet DNA racks, serving as the foundation for CPU, communication, and I/O modules. It is found in paper mill, chemical, and power plant control cabinets.

Maintenance

Check the power rail LEDs periodically. Cleaning with compressed air is recommended. The backplane is a passive component and rarely fails.

Conclusion

The Valmet ABMB MT940 provides a robust, high‑performance backplane solution for modular automation systems, ensuring reliable power and signal distribution.