We have designed, built and tested an on-line pre-analysis and data-storage unit for the CHIMERA data acquisition system. The unit uses special algorithms to allow for particle identification and automatic peak detection. It tags in charge each event and stores the results on tape, together with the raw-data without any additional dead-time. The computational unit consists of two commercial 2 GHz PCs connected together through 1 Gb/s Ethernet boards and to the 100 Mb/s Ethernet of the data acquisition system.