Description

The ABB TB851 is a specialized Terminal Unit designed for the ABB S800 I/O system, specifically serving as the CEX-Bus termination point. The CEX-Bus (Controller Extension Bus) is the high-speed serial communication link that connects the AC 800M controller (via a CI854. CI855. or CI867 communication interface) to the distributed S800 I/O modules (DI, DO, AI, AO, etc.).

The primary function of the TB851 is to provide a physical connection point for the CEX-Bus cables at the end of a segment and to house the mandatory terminal resistors (typically 120 Ohms) required to prevent signal reflections that can corrupt data transmission. Without a properly installed TB851 (or equivalent termination) at both ends of the bus segment, communication errors, intermittent I/O faults, and system instability are highly likely. The unit features a robust design with clear labeling for Bus In/Out and termination settings, often including DIP switches or jumpers to enable/disable the internal termination resistors. The “hole type” refers to its wiring terminals, which accept stripped wires secured by screws or spring clips, ensuring a gas-tight, vibration-resistant connection suitable for harsh industrial environments.

Application Scenarios

In a large offshore oil platform, the CEX-Bus network connecting the central AC 800M controller to remote I/O stations on the drilling deck experienced frequent “Communication Loss” alarms. Investigation revealed that the bus segment was missing a proper termination at the far end, causing signal reflections due to the long cable run and marine vibration. The engineering team installed ABB TB851 units at both ends of the CEX-Bus segment. They configured the DIP switches on the TB851 to activate the internal 120-ohm terminators. Immediately after installation, the communication errors ceased, and the I/O update rate stabilized. The TB851’s robust screw terminals maintained connectivity despite the constant vibration, ensuring reliable control of critical blowout preventer (BOP) systems and mud pumps.