Custom busses for large scale data acquisition systems

A data acquisition system for a real time large scale application (more than 30,000 acquisition channels) has been developed. This required the design of two custom busses (AUXbus and Cbus), a readout controller (ROCK) and a read-out controller manager (ROCK manager). A crate containing a VME CPU, a ROCK, (1-16) front end slave boards and an Auxbus backplane comprise one DAQ sub-system. The ROCK performs data acquisition in a event-driven fashion on the AUXbus, a custom high speed parallel bus. The AUXbus´s main features are data cycles labelled by event number, sparse data scan operations and an asynchronous protocol optimised to achieve random length block transfers at data rates up to 15 MHz. Up to 8 of the above described DAQ sub-system crates can be linked together via the Cbus in a daisy-chain fashion to the ROCK manager, to form a complex DAQ system. The ROCK manager performs event driven data acquisition in a token passing fashion on the Cbus. The Cbus´s main features are data cycles labelled by event number and random length block transfers at 10 MHz. A preliminary version of the DAQ system is presented and test results are briefly discussed