Data compression and readout of multiplexed drift chamber data in the CLAS detector at CEBAF

A unique system for multiplexing drift chamber wires is used in the CLAS detector at CEBAF. Discriminator outputs from two non-adjacent wires have different pulse widths and are XOR'd before being sent into the TDC. Additional circuitry integrates the charge on the wires, then discharges and uses the same TDC channel as a Wilkinson rundown ADC. This multiplexing, while reducing the cost of the electronics, causes the information read out of each TDC to be scrambled, and roughly three times larger than required. The event rate of 1-2 KHz and raw event size of 20-40 KBytes make data extraction and compression mandatory. This will be done in the Fastbus readout controllers, in special VME compression/transmission crates, and in the on-line farm. The authors briefly detail the multiplexing scheme, describe the extraction/compression algorithm, and present results of running this code on several Fastbus readout controllers, VME boards, and DSP's