The ABB TB840A is a ModuleBus Terminator designed for the S800 I/O system, a comprehensive, distributed, and modular process I/O system that communicates with parent controllers and PLCs over industry-standard field buses. The TB840A serves as an optical media converter, allowing the extension of the ModuleBus over long distances using fiber optic cables. This enhances signal integrity and provides excellent immunity to electrical noise, making it ideal for large-scale or challenging industrial environments. It connects to the CEX-Bus via a ModuleBus Modem and is typically used in pairs to establish a reliable optical link for seamless data communication between I/O clusters.