Description

The TU921S (Part Number: 3KDE175111L9210) is a specialized Redundant Termination Unit (MTU) manufactured by ABB. Designed for the AC 800M and System 800xA platforms, this unit serves as the physical foundation for high-integrity automation setups. Unlike standard bases that hold a single module, the TU921S is engineered to mount two I/O modules side-by-side (e.g., two AI810 or DO820 modules). It provides the necessary wiring interface and internal bus connections to support 2oo2 (Two out of Two) logic, ensuring that if one module fails, the second takes over instantly without disrupting the process.

Application Scenarios

Consider an offshore oil rig where the emergency shutdown system (ESD) must remain active at all times. A single point of failure in the control system could lead to catastrophic consequences. In this scenario, the TU921S is installed in the main control cabinet, holding two DO820 digital output modules responsible for actuating the safety valves. Both modules receive the same shutdown signal simultaneously. If the primary module suffers a hardware failure, the system seamlessly switches to the secondary module connected via the TU921S. This redundancy ensures that the safety system remains active, preventing a potential explosion. The TU921S effectively acts as the “insurance policy” for the physical wiring and module connection in critical safety applications.