The new implementation of the event building protocol for the ALEPH data acquisition system

The flow of data through the ALEPH readout system is effected under the control of a special protocol between the various readout stages. The software library used to implement this protocol has been re-engineered using the finite state machine modelling technique. The state of each system component can be observed at any given time and this has greatly simplified the detection of, and recovery from, protocol errors. The model and its implementation are described together with their integration with the applications which are used by the operator to control data taking. Operational experience gained with this protocol implementation during the whole 1992 running period will also be given to show the advantages of this design. >