Speaker
Description
Summary
The CMS (Compact Muon Solenoid) detector is one of the four large experiments that
will be installed in the new accelerator LHC (Large Hadron Collider) that is being
built at CERN. A key point of CMS is its ability to trigger on and reconstruct muon
tracks at high luminosities, as this will allow deeper exploring into matter, probing
the Higgs mechanisms and other aspects of the Standard Model [1]. This task is
performed by the various CMS sub detectors, among them, the Drift Tube chambers (DT),
that provide muon identification and precise momentum measurement.
The read-out electronics of the CMS DT chambers is designed to perform time
digitization of the signals generated by charged particle tracks and to do further
data merging in order to achieve a read-out of the full detector (172,200 channels)
at a Level 1 trigger rate of 100 kHz. The system is divided in three levels. First,
the 1500 ROB (Read-Out Boards), placed inside the so-called Minicrates [2] attached
to the DT chambers. Second, 60 ROS (Read-Out Server) boards located inside the CMS
cavern in the balconies on both sides of the CMS wheels. And finally, 5 DDU (Detector
Dependend Unit) boards which will operate in the underground counting room.
ROBs are built around an HPTDC (High Performance Time to Digital Converter) [3] ASIC
developed by the CERN/EP MIC group. Inside each ROB, four HPTDC´s are connected in a
token ring scheme for digitization of up to 128 chamber channels with a time
resolution of 781 ps and further data transmission to the ROS through a 240 Mbps
serial copper link.
ROS boards perform a data merging of 25 input ROB channels, reducing data overhead to
achieve ~ 8 kbytes per event in the whole detector. Data is serialized and
transmitted to the DDU through an optical link at 800 Mbps. ROS boards also perform
main tasks of data quality monitoring and event synchronization.
Each DDU merges data from 12 ROS in order to build an event fragment at the Level 1
trigger rate (up to 100 kHz) and send it to the global CMS DAQ through an S-LINK64
[4] output at 320 MB/s. One of the main tasks of the DDU is keeping synchronization
with the whole detector. In order to do this, the DDU receives synchronization
commands from the TTC system (Timing, Trigger and Control [5]), performs errors
detection on data and sends a fast feedback to the trigger system through the TTS
output (Trigger Throttling System [6]) to slow down the trigger rate or stop data
acquisition in case of failures. In total the 5 DDU boards can manage event sizes up
to ~15 kbytes, at the trigger rate of 100 kHz, about the double of requirements.
In summary, the chosen architecture, as well as the dimension of the memories, links
bandwidths, pooling mechanisms, failure detection circuitry, etc, fulfils the
experiment requirements of trigger and data rate, radiation tolerance, limited
supervision and power consumption.
The complete functionality of these electronics has been validated in laboratory and
in several test-beams [7]. Moreover, Magnet Test and Cosmic Challenge exercise, has
demonstrated full operation of the readout chain and integration within the final CMS
framework. Right now, the read-out system is being installed and commissioned for CMS
start up by fall 2007.
References
[1] “CMS. The Muon Project. Technical Design Report.” CERN/LHCC 97-32. December 1997.
[2] “Electronics for the CMS Muon Drift Tube Chambers: The Read-Out Minicrate.”. IEEE
Transactions on Nuclear Science. 10.1109/TNS.2005.852698. Vol. 52 NO. 4. August 1st,
2005.
[3] “High Performance Time to Digital Converter. Version 2.1”. J. Christiansen.
CERN/EP-MIC. July 2002.
[4] “The S-Link 64 bit extension specification: S-link64”. A. Racz, R. McLaren, E.
van der Bij.
EP Division, CERN. August 4th, 2003.
[5] “CMS L1 Trigger Control System”. J. Varela, CMS Trigger/DAQ Group, CERN. CMS NOTE
2002/033. November 7th, 2002.
[6] “Trigger Throttling System for CMS DAQ”. A. Racz. EP Division, CERN. September
21th, 2000.
[7] “Test Beam Analysis of the First CMS MB2 Drift Tube Muon Chamber.” M. Cerrada et
al. CMS NOTE 2003/007. January, 2003.
