S-LINK: a Prototype of the ATLAS Read-out Link

Erik van der Bij¹, Robert McLaren¹, Owen Boyle^1,2, Zoltán Meggyesi¹

1EP-division CERN, CH-1211 Geneva 23

- Summary -

The ATLAS data acquisition system needs over 1500 read-out links between the underground read-out drivers (ROD) which interface to the detector electronics, and the surface-level read-out buffers (ROB) which store the data until a Level-2 trigger decision has been made. The ROD designs will be different for each detector; the ROBs and the read-out links will be common for all detectors.

Link technologies are changing rapidly and it is therefore too early to specify the physical layer of these read-out links. On the other hand designers of the RODs and ROBs have to build systems for testbeams in order to work towards the final system. To cope with this problem the S-LINK specification has been written. Instead of defining the physical layer of a link, it defines a media independent 32-bit synchronous FIFO-like interface for both the sender and receiver side of a link. S-LINKs can send control and data words, can detect link errors and have a self-test mode.

Today two types of link cards exist: one based on Fibre Channel technology, capable of moving data serially at 103 MByte/s over maximum 300 m fibre-optic or electrical media and another based on parallel electrical transmission over a SCSI cable. These are implemented as plug-in cards which hide the underlying link technology. For final applications the link electronics may be integrated into the ROD and ROB designs. PCI and PMC interfaces to S-LINK and several tools such as data generators and data sinks are all commercially available.

In ATLAS S-LINK will be used as the read-out link prototype: the DAQ prototype-1 project has written a LynxOS library and uses S-LINK to emulate ROD data. In 1997 the MDT detector used the test beam at H8 in which the data was read out over a fibre-optic S-LINK and an S-LINK to PMC interface into the RD-13 DAQ system. Furthermore different Level-2 demonstrators have used S-LINK to send and receive trigger and Region-of-Interest information. Different detectors such as the TRT, the TileCal and the Level-1 Calorimeter Trigger are making designs with S-LINK as the ROD outputs. The TileCal will also use S-LINK as the front-end link for the testbeam.

S-LINK is also used outside ATLAS. The COMPASS experiment will use 200 commercial fibre-optic S-LINKs and has designed an S-LINK to PCI interface with a large buffer memory that can hold the data from one spill. Applications outside high-energy physics such as the ASDEX Upgrade fusion experiment and the MegaPrime astronomy camera are also considering using S-LINK. The Olivetti and Oracle Research laboratories use S-LINK in combination with Linux drivers for moving video and keyboard data to and from remote terminals.

Currently, the S-LINK project focuses on supporting users (www.cern.ch/hsi/s-link) and on reducing the cost of the physical links so that the final users will have the lowest possible price for the many links needed.