Setting up and using a multi-computer VXIbus system

VXI has matured into a flexible bus architecture with which to develop various types of instrumentation systems. More and more systems are being developed that require multiple chassis with dozens of instruments, each of which may perform a distinct yet important function to the system. This can place a tremendous burden on the system controller, especially if this same controller has to perform intensive graphic display updates, writes to the hard disk, printing functions, or other I/O Tasks. Modern computers in general, and PC-based computers in particular, have advanced to the point where some of this burden can be overcome by the sheer power of the processor. However, there are still many systems that would benefit from using a multi-computer approach. This paper details the multi-computer approach used in the VXIbased system that CACI developed for the Air Force. The system, the Engine Test/Trim Automated System II (ETTAS II) is designed to test all Air Force jet engines. The paper discusses how to integrate multiple computers in a VXI-based system, including discussions on: setting up the computers, selecting register-based versus message-based computers, setting up and using shared-memory, defining and separating tasks for each computer. The shared memory discussion talks about different ways to structure the shared memory, including setting up a system-level "Current-Value Table" (CVT) for all instruments, as well as how other devices, including another computer can access the shared memory space. The paper shows how Commercial Off-the-Shelf(COTS) software products NI-VXI LabVIEW; and NI-VISA (National Instruments) can be used to satisfy all these requirements. The paper also shows how the multi-computer approach can be cost-effective in many cases.