In the architecture of a high-integrity Distributed Control System (DCS), every component must be engineered for reliability—not just the processors and I/O modules, but also the points where field wiring connects to the system. The ABB TU846 is a critical component in this chain: a high-reliability termination unit designed for ABB's S800 I/O system. This article provides a comprehensive technical examination of the TU846, detailing its function, key features, and why it is a vital element for ensuring signal integrity and maintenance efficiency in mission-critical applications.
The ABB TU846 is not a programmable controller or a signal-processing module. It is a termination unit or terminal unit. Its primary function is to serve as the physical and electrical interface between the field wiring from sensors and actuators and the electronic S800 I/O modules (such as the AI810 analog input or DI810 digital input modules).
Think of it as a sophisticated and highly organized junction box. Instead of wiring field devices directly to the I/O module, which is often hardwired to the backplane, you wire them to the TU846. The TU846 then connects to the I/O module via a short, reliable, pre-manufactured cable. This creates a clear separation between the field environment and the sensitive electronics of the I/O system.
The operation of the TU846 is fundamental to system reliability and serviceability:
Field Wiring Termination: Field cables from devices in the plant (e.g., pressure transmitters, thermocouples, solenoid valves) are landed on the robust screw terminals of the TU846.
Internal Connection: A standardized, shielded cable (often called a "pluggable connection") then connects the TU846 terminal unit to the corresponding S800 I/O module, which is plugged into the baseplate.
Signal Pass-Through: The TU846 typically provides a direct electrical pass-through for the signals. However, its design incorporates features that enhance the overall system's robustness.
Understanding the design of the TU846 reveals its value in a high-reliability system.
The unit features modular, removable terminal blocks. This allows for:
Easy Wiring: Terminals are designed for easy access and secure connections.
Module Replacement without Rewiring: This is the most significant advantage. If an I/O module fails, a technician can simply unplug the cable from the faulty module and plug it into a new one. The field wiring remains untouched on the TU846, drastically reducing mean time to repair (MTTR).
While basic TU units are pass-through, they can be designed to support additional functionality or work with specific signals.
Intrinsic Safety (IS) Barriers: In hazardous areas, the TU846 can be used in conjunction with intrinsic safety barriers that are mounted on a separate rail. These barriers limit the energy available to the field, preventing the possibility of ignition.
Fusing and Protection: Some configurations may include fuses or surge protection devices on the terminal unit to protect the I/O modules from overcurrent or voltage transients from the field.
The unit is designed for clarity, with ample space for wire markers and channel identification. This is crucial for correct installation and for troubleshooting under pressure, ensuring technicians can quickly identify and test specific loops.
Built to withstand the vibration, temperature variations, and humidity of an industrial cabinet, the TU846 is constructed from high-quality materials that ensure long-term reliability of the electrical connections.
The TU846 is part of a structured system architecture:
S800 Baseplate: Mounted on a DIN rail in the control cabinet.
S800 I/O Module: e.g., an AI830 analog input module, is inserted into the baseplate.
TU846 Termination Unit: Mounted adjacent to the I/O module.
Interconnecting Cable: A short, shielded cable plugs into the front of the I/O module and the TU846, bridging the connection.
This modular approach is a hallmark of the S800 system's flexibility and maintainability.
The ABB TU846 termination unit is far more than a simple set of terminals. It is a key enabler of high system availability and ease of maintenance. By providing a robust, organized, and service-friendly interface between the harsh field environment and the sensitive control system electronics, it plays an indispensable role in minimizing downtime and ensuring the long-term reliability of critical automation infrastructure in industries like power generation, oil and gas, and chemical processing.
