The Computer System

Those beamlines of HASYLAB which are operated by DESY use two different computer systems. A part of the experiments is controlled by Q-Bus Microvaxes and CAMAC crates. New experiments are equipped with VME components, including a single board computer (SBC) which runs under the real-time operating system LynxOS. It has been chosen for several reasons:

• Lynx-OS is compatible to UNIX, an operating operating system that has become a standard in the physics community.
• Lynx-OS is a real-time operating system. The most important real-time feature is the preemptive interrupt handling by the kernel which is not a standard UNIX feature.
• GNU products, like gcc, bash, gdbx have already been ported to Lynx-OS making program development very efficient.
• X11 is available. It would be a great advantage to have a high level graphics package like GKS on the SBC, but it should be possible to port the SPECTRA graphics to X11.

SPECTRA runs on the SBC. This allows the program to take advantage of the VME I/O bandwidth, which is more than one mega-word/second.

A Linux PC acts as a file and a boot server for the SBC. Both computers are connected through an Ethernet. The SBC imports the file system by NFS. SPECTRA uses the PC as an X-terminal for the on-line display. Since the PC has a direct access to the file system of the SBC, it is able to export the data without interrupting the measurement process. Data can be transferred by ftp, rcp or copied to a local tape.

The SBC runs in a disk-less mode in order to minimize the maintenance effort. As a consequence file I/O has to be done through the network. The on-board memory serves as a data buffer. It is expected to be sufficiently large to avoid any bottleneck that is due to networking load. However it is possible to install one or more local SCSI disks or tapes, if it becomes necessary.