Digital Electronics: Principles, Devices and Applications

Troubleshooting Digital Circuits and Test Equipment 70116.19.2.3 Interface Bus StructureCommonly used interface bus structures for a computer–instrument interface are the IEEE-488,described in Section 16.18.1, the PC-bus and the VXI-bus. The other two are described here.In a PC-bus virtual instrument set-up, the instrument function available on a printed circuit card(Fig. 16.41) is inserted directly into a vacant slot in the personal computer. Since these cards areplugged directly into the computer back plane and contain no embedded command interpreter as foundin IEEE-488 instruments, these cards are invariably delivered with driver software so that they can beoperated from the computer. PC-bus instruments offer a low-cost solution to building a data acquisitionsystem. Owing to the limited printed circuit space and close proximity to sources of electromagneticinterference, PC-bus instruments offer a lower performance level than their IEEE-488 counterparts.VXI-bus instruments are plug-in instruments that are inserted into specially designed card cagescalled mainframes (Fig. 16.42). The mainframe contains power supplies, air cooling, etc., that arecommon to all the modules. VXI-instruments combine the advantages of computer back-plane busesand IEEE-488. A VXI-bus instrument has high-speed communication as offered by computer backplanebuses (such as the VME-bus) and a high-quality EMC environment that allows high-performanceinstrumentation similar to that found in IEEE-488 instruments.One of the methods to communicate with VXI instruments is via IEEE-488, as shown inFig. 16.43. In this case, an IEEE-488 to VXI-bus converter module is plugged into the VXIbusmainframe. The mainframe then interfaces with the IEEE-488 interface card in the computerusing the standard interface cable. The set-up is easy to program, but the overall speed islimited by the IEEE-488 data transfer rate.Another technique is to use a higher-speed interface bus between the hardware mainframe and thecomputer. One such bus is the MXI-bus, which is basically an implementation of the VXI-bus on aflexible cable. In this case, the VXI-MXI converter is plugged into the mainframe and an MXI-interfaceFigure 16.41Instrument function on a PC card. Photo courtesy of National Instruments Corporation.