Most current diagnostics are limited to recording data either at fixed times and data rates, or in response to certain predefined events - such as the injection of a pellet. The previous Soft X-Ray trigger system at Joint European Torus Joint Undertaking (JET)(A.W. Edwards et al., Rev Sci Instrum. 57(8), p2142, 1986) improved upon this by using Analogue Signal Processors to monitor the analogue data in real time and to provide `triggers` to the data acquisition system in response to an event such as a sawtooth collapse. This system was however limited in the type of events that could be detected. It was also incapable of being rapidly re-configured. Advances in digital electronics caused a study to be undertaken to see if this situation could be improved. The system described below is the result of this study and has successfully run at JET since the summer of 1990, providing a greatly increased quality of data as well as recording some new phenomena such as the spontaneous snake. This note has been produced to describe the function and operation of the trigger system. (author).