CERN Accelerating science

Data acquisition

DAQ overviewThe readout electronics consists of three main subsystems: the part responsible for reading out the TPC Front-End Electronics, the part for reading out the FASTBUS based ToF system, and the part for reading out the CAMAC based beam detectors.

The TPC readout system comprises ~6000 FEE cards, each of those reading out ADCs of 32 TPC pads, sampled in 256 x 200ns time slices using an analog charge storage capacitor array for each channel. The readout sequence is steered by the MotherBoards, which receive a common trigger signal and a synchronization clock. The raw ADCs of the FEEs are read out by the MBs, are pedestal subtracted, noise suppressed, bit truncated and zero compressed. An MB can host 24 FEEs, thus ~250MBs are necessary for handling the full TPC system. The MBs forward their data to the ConcentratorBoxes via a ground independent LVDS connection for further serialization. Each CB serializes data of 32 MB onto a DDL optical line for long distance transmission towards the Central DAQ computer where final event building is performed. The TPC system is served by 8 CBs. The PSD calorimeter of the experiment is read out in a similar manner using 44 MBs and 2 CBs.

The FASTBUS based ToF and CAMAC based beam detector system is built in a way similar to each-other. The actual low level readout software runs on a FIC controller hosted in a VME crate. Upon a trigger received by the FIC, the ADCs and TDCs are accessed via VME-to-FASTBUS and VME-to-CAMAC bridges, and are stored in a ring buffer. A VME-to-USB bridge from the Central DAQ PC provides access to the subevents stored in these ring buffers. The Central DAQ polls the pertinent ring buffers and copies their content to the memory of the Central DAQ PC.

The data volume of a full raw event is order of ~50MByte. This, after zero compression stage, is reduced to order of ~2.5MByte. The maximal event rate is order of ~83Hz, which is determined by the deadtime of the TPC FEE readout (12ms).

Further reading with more details:

Nuclear Instruments and Methods in Physics Research A798 (2015) 1.