The performance of DAQ system is determined by the trigger rate and event size. For BESIII detector, the level-1 trigger rate is estimated to be 4 KHz and average event size is 12 Kbytes. The BESIII DAQ system is designed to meet requirements of event data readout and processing with such a high event rate. Compared with BESII, it has a much higher design target both in scale and in performance.

         Multi-level buffering, parallel processing, high-performance computing, high-speed VME readout and network transmission techniques are introduced in BESIII DAQ design. It should also have high reliability, maintainability, stability, scalability and portability. The architecture of BESIII DAQ system is shown in Fig.1 .

*** More details about the event building

A multi-step event building technique is adopted by BESIII DAQ system, as shown in Fig.2. A sub-event in one branch can be sent to one node of computer farm, at the same time, sub-events of other events in other branches can be sent to other nodes of computer farm. So a high speed and parallel event builder can be achieved with advantages of scalability, high reliability and best price performance ratio.

*** More details about the dataflow of BESIII DAQ

The dataflow of DAQ system is shown in Fig.3. The farm supervisor should be a highly reliable workstation. There are two queues in the supervisor. One is the queue of event number to be built, which comes from level-1 trigger; the other one is the queue of free nodes of computer farm. According to these two queues, the supervisor informs one of free nodes to read out the proper sub-events from readout PCs. All the data processing tasks are done on this assigned node.