We describe a modular architecture for process information systems. Each process unit is represented by a separate module that is connected to its neighbours in the process flow sheet through connections that represent the process streams. Each module uses only information that is available locally, either from local instruments or supplied through the connections. The resulting system is easy to configure and to maintain, and modules are easily portable to other processes. Within this modular architecture we perform state estimation and a versatile and robust topology-based diagnosis of process disturbances. The scheme forms the basis for a successful operator support system that has been in operation in a fertiliser plant since November 1992.