Synchronization in a PLC/VAX based control and data acquisition system of a nuclear fusion experiment

This paper describes the concept and implementation details of the synchronization mechanisms used in the control and data-acquisitionsystem of the RFX (Reversed-Field Experiment) nuclear-fusion experimental device, at present under construction in Padova, Italy, within the framework of the co-ordinated nuclear-fusion research programme of the European Communities. The system employs industrial PLCs for the "slow" control and monitoring functions, and aVAX-based CAMAC for the "fast" functions of trigger-signal generation and dataacquisition during the experiment pulses. All subsystems communicative via Ethernet, using compatible software protocols. The operational sequence of the complete system is governed by a single state machine implemented on aPLC-based supervisor system. Equivalent "slave" state machines are implemented on all other subsystems (PLC-and VAX-based). These state machines are synchronized by means of the exchange of messages via Ethernet. This paper deals in detail with the following components which are involved in systemsynchronization: - the Message Exchange System which implements the system-wide exchange of short messages; - the Scheduler programs which implement the state machine on the various computing nodes, and which make use of the Message Exchange System.