BaBar Data Aquisition

The BaBar experiment at the Stanford Linear Accelerator Center is designed to perform a search for CP violation by analysing the decays of a very large sample of B and Bbar mesons produced at the high luminosity PEP-11 accelerator. The data acquisition system must cope with a sustained high event rate, while supporting real time feature extraction and data compression with minimal dead time. The BaBar data acquisition system is based around a common VME interface to the electronics read-out of the separate detector subsystems. Data from the front end electronics is read into commercial VME processors via a custom "personality card" and PCI interface. The commercial CPUs run the Tornado operating system to provide a platform for detector subsystem code to perform the necessary data processing. The data are read out via a non-blocking network switch to a farm of commercial UNIX processors. Careful design of the core data acquisition code has enabled us to sustain events rates in excess of 20 kHz while maintaining bulk data transfer rates into the VME processors close to the theoretical maximum of the PCI bus (130 MB/s).