A RISC based SCSI interface for a protein crystallography detector

In the past, some X-ray detectors were interfaced to VAX workstations using a parallel input/output board supplied by DEC. Work was carried out on a new detector interface based on the SCSI bus. Hence, any modern general purpose workstation can be connected via the SCSI bus to the X-ray detector system and act as a data acquisition controller. We will present the new interface board, a data generator used far tests and the software for the interface. The interface board is organized around an IDT 3051 family processor connected to a FAS201 SCSI bus controller and detector data input FIFOs. The latter provide the necessary buffering between the detector electronics and the processor. The FAS201 is a specialized controller which implements the SCSI bus protocol and therefore, considerably off-loads the main processor. A data generator has been built to simulate the data stream coming from the detector. Programmable devices are used to generate known data sequences. Thus one can easily verify the proper functioning of the interface and the communication with the host. The software for the interface was developed in C language using the GNU tools for the MIPS processors. The main functions are the communication with the host over the SCSI bus and the handling of the input data stream. Controlled acquisition by a SCSI bus based host computer is possible through a well defined set of commands. The interface also provides on-the-fly data corrections using look-up tables which can be dynamically downloaded by the host.