Rijeka refinery has built in year 2010. new hidrocracker complex unit as a part of refinery modernization project. This was time when complete new approach in term modern communication protocols was introduced in a field of instrumentation equipment communication. Protocol name is FOUNDATION Fieldbus which by very common definition represent an all-digital, serial, two-way communications system that serves as the base-level network in a plant or factory automation environment. It is an open architecture, developed and administered by the Fieldbus Foundation. Most beneficial cost advantage is reflected in the amount of cabling required which is much lower in Fieldbus than in 4-20 mA installations, several parameters can be communicated per device in a Fieldbus network and it provides a good foundation for the creation of a predictive and proactive maintenance strategy. First experiences show that plant-wiring concepts, shielding, training and the right tools are essential to gain the benefits of fieldbus technology. This paper describes online troubleshooting possibilities and diagnostic tools which can be used to address issues with devices before they become critical problems.