Details of DAQ modules

TDC400.

The heart of our data acquisition system is a custom designed, deadtime-free TDC module, the TDC400, built by the PSI electronics shop in the VME64 format. Each module effectively stores the state of 64 input channels once per clock cycle (currently 5 MHz). This module was designed for reading out any arbitrary tracks occurring in our TPC in an unbiased fashion. This continuous readout allows us to disentangle multiple crossed tracks - an ability necessary to reduce time distribution distortions. Data is recorded only if one or more of the input channels where hit. The data word consists a 16 bit time stamp (bit 16 flags a rollover) and 48 data bits. Every single signal is recorded and can be expanded to the correct absolute time. Two separate data buffers of 32k depth allow for data collection over a contiguous time region with 6ms minimum time. Within that region we can define smaller pile-up protected regions as far as we need before and after a track of interest. The second memory allows the VME computer board to read out one event-block while the next event-block is being taken simultaneously. 10 TDC400 modules were tested in parallel in our test runs giving 480 time-stamped TDC channels.

CAEN-V767.

The CAEN multihit TDC modules read in 128 channels each with a time resolution of 1.6ns and a pulse pair resolution of about 10ns. These units can store 32K hits of an arbitrary pattern before needing to be read out. In the case of a rare sub-buffer overflow, an error bit is set allowing the event to be discarded. Unlike the TDC400 modules, there is only one memory, but it can be read out simultaneouly with the data collection and the beginning of the next time region is flagged in this buffer. Therefore the time-regions of interest of the TDC400's and the CAENs can easilly be synchronized and assembled into an agregate event buffer. The data is made up of paired channel numbers and times. A pulser driven by an accurate time source will be fed into a channel of each CAEN and one of the TDC400's to assure perfect synchronization.

FADC.

We connect some of the central anode wires of the TPC to FADCs to sample the analog signals during the run. Using simple logic on the first and last wires of this region we can determine if the muon has stopped in the region of interest. Further, by watching for high double pulses on these wires we can efficiently trigger on impurity capture events.This analog data is used monitor several important things: Sub threshold events, drifts of thresholds and gas gains, detailed study of a sample of typical events and special events like capture on impurites. These units have been running in their own VME with a dedicated DAQ. The FADC DAQ and the TDC DAQ are synchronized by inputing the event number of the FADC DAQ into the TDC400's along with the normal TDC400 time stamp. We have included the FADC's in the main DAQ VME crates here for simplicity. In the case that the old DAQ running these FADCs fails, we will bring that FADC data into the main data stream.

MVME2604.

The MVME2604 is a 333Mhz PowerPC based VME processor card with 100BaseT ethernet and a PCI Mezzanine plug (for the PVIC card). The card has DMA controllers both for the VME bus and the PCI bus so it can simultaneously do some data processing while also controlling readout of the TDCs and transfer of the data to the desktop processors or the other crate. We have two of these, one for each VME crate. We have been able to program these cards very conveniently with the VxWorks real-time operating system.