Effects of UNIBUS traffic loading on the performance of VAX data acquisition systems

The author reports on an experimental investigation into the effects of UNIBUS traffic on the performance of VAX-MBD data acquisition systems used in the field of physics research. An analysis is presented for the main performance parameters influenced by the traffic loading, together with their significance in a data acquisition environment. An overview of the mechanisms influencing each performance parameter is given. Time spectrum measurements of the dead time duration under various levels of simulated bus loading are presented and the effects on both the percentage dead time and the percentage CPU time usage are discussed. Implications of the problem are outlined and recommendations made for possible solutions.