The CMS GCT project - Design

The CMS GCT is modular in order to simplify, as much as possible, the design, construction, testing and maintenance of the system.

The design consists of 5 modules;

• Source Cards

  The Source Cards recieve data from the RCT and serialises it for transmission, via optical fibre, to the rest of the GCT system.
  (Click here for more info).

• Optical Patch Panel

  An Optical Patch Panel is used to route the optical fibres so that, rather than being organised regionally (as they emerge from the Source Cards), the data is organised by type.
  (Click here for more info).

• Leaf Cards

  Each Leaf Card accepts data from the optical fibres and performs analysis on the data.
  (Click here for more info).

• Wheel Cards and Concentrator Cards

  The Wheel Cards and Concentrator Cards route, compress and perform final analyses on the data before passing the data to the GMT and GT.
  (Click here for more info).

Source Card

The RCT 'divides' the CMS calorimeters into 18 geometric regions and each region is analysed by its own electronics crate. Each crate transmits its data on 6x68pin differential ECL SCSI cables running at 80MHz. The data consists of information about isolated & non-isolated electron candidates, muon candidates and regional energy sums, but many of the data sets are split over multiple cables.

In order to perform analyis, the data carried by all 108 cables (18 crates x 6 cables/crate) must be brought together in a coherent fashion. The solution chosen was to use a Source Card which takes 2x68pin cables, serialises the data and retransmits it on 4 optical fibres. As such 3 source cards are required per RCT crate. The Source Card organises the data so that, for each RCT crate,

• all muon data is on one fibre.
• the isolated/non-isolated electron candidate data is spread over three fibres.
• the regional energy sums are spread over eight fibres.

The Source Card utilises a Xilinx Spartan-3 FPGA to route the data, provide data integrity features such as CRC/FEC functionality and a cyclic memory element which can be used for analysis in case of upstream failure as well as USB interface capability again for testing.

In order that the design be fully testable, the optical links can be looped back for self-testing and a test card is also being developed to drive the ECL links.

The Source Card and its associated test card is being designed by John Jones and Andrew Rose at Imperial College London.

Optical Patch Panel

The Optical Patch Panel routes the fibres so that those carrying similar data types are bundled together. The fibres are mapped as

• 18x1 muon fibres map to 1 bundle of 18 fibres
• 18x3 electron fibres map to 2 bundles of 27 fibres
• 18x8 regional energy sum fibres map to 6 bundles of 24 fibres

Leaf Card

Each Leaf Card accepts up to 36 optical fibres and performs analysis on the data using two Xilinx Virtex-2 Pro FPGAs.

There are three different types of Leaf Card; Electron, Muon and Jet, the nature of the card being determined solely by the firmware.

• The Muon card collates the data from the muon fibres and passes the data to a dedicated concentrator card, where it is compressed and passed to the Global Muon Trigger.
• Each Electron card receives isolated/non-isolated electron data from 9 RCT crates (1 bundle of 27 fibres), corresponding to 1 half of the CMS detector in the z-direction. The card sorts the electron candidates according to rank and selects the four highest rank electrons to be sent to the Global Trigger. The Electron cards are mounted directly on a Concentrator card.
• Each Jet card receives energy sum data from 3 RCT crates (1 bundle of 24 fibres) and perform the 3x3 region sliding window algorithm and transmit the jet lists (central, tau and forward) to the Wheel card for sorting. The jet cards are structured to reflect the geometry of the detector. Each jet card handles one half of the detector in the z-direction and one third of the detector in the f-direction. The cards pass data between themselves in the 'f-direction' to ensure that the edges between regions are handled properly. The Jet Cards also execute the rest of non-electron algorithms and pass all results up-stream to a Wheel card and ultimately to the same Concentrator card as the electron data.

The Leaf Card design is a modified version of a card which has been show work. The jet finder algorithm has been implemented on this card and has also been shown to work.

The Leaf Card is being developed by Magnus Hansen and Matthew Stettler in CERN.

Wheel and Concentrator Cards

Each Wheel Card uses two Xilinx Virtex-4 FPGAs to sort and compress the data from a group of 3 Leaf Cards before the data is passed to the concentrator card, where it is merged with the data from the other 3 Leaf Cards. In addition, the Wheel card performs the transverse energy trigger sums and the jet count trigger sums.

The Wheel card is being developed by Matthew Stettler and Magnus Hanson at CERN.

The Concentrator Cards also use two Xilinx Virtex-4 FPGAs connect to either the two electron and two wheel cards or to the muon leaf card. The electron-jet concentrator card performs the final sorting of electrons, completes the jet finding on the boundary between groups of Leaf cards, sorts all the jets found, completes the calculation of the transverse energy quantities and sends the final results for all quantities to the Global Trigger. The muon concentrator card compresses the muon data and sends it to the Global Muon Trigger.

The Concentrator card is being developed by Matthew Stettler, Magnus Hanson and Greg Iles at CERN.