Appendix A Types of Instruments

Hardware Specifications

The GPIB is a digital, 24-conductor parallel bus. It consists of eight data lines (DIO 1-8), five bus management lines (EOI, IFC, SRQ, ATN, REN), three handshake lines (DAV, NRFD, NDAC), and eight ground lines. The GPIB uses an eight-bit parallel, byte-serial, asynchronous data transfer scheme. This means that whole bytes are sequentially handshaked across the bus at a speed that the slowest participant in the transfer determines. Because the unit of data on the GPIB is a byte (eight bits), the messages transferred are frequently encoded as ASCII character strings.

Additional electrical specifications allow data to be transferred across the GPIB at the maximum rate of 1 MB/sec because the GPIB is a transmission line system. These specifications are:

•A maximum separation of 4 m between any two devices and an average separation of 2 m over the entire bus.

•A maximum cable length of 20 m.

•A maximum of 15 devices connected to each bus with at least two-thirds of the devices powered on.

If you exceed any of these limits, you can use additional hardware to extend the bus cable lengths or expand the number of devices allowed.

Faster data rates can be obtained with HS488 devices and controllers.

HS488 is an extension to GPIB and is supported by most National

Instruments controllers.

VXI (VME eXtensions for Instrumentation)

VXI defines a standard communication protocol to certain devices. Through this interface, you can use common ASCII commands to control the instruments, just as with GPIB.

The VXIbus specification is an extension of the VMEbus (IEEE 1014) specification. As an electromechanical superset of the VMEbus, the VXIbus uses the same backplane connectors as VME, the same board sizes, and the same signals defined in the VMEbus specification. The VXIbus adds two board sizes, changes module width, and defines additional signals on the backplane.

Appendix A Types of Instruments

VXI Hardware Components

A VXI system consists of a mainframe, a controller, instruments, and cables. The VXI mainframe is the chassis, cage, or crate that contains the power supply, cooling system, backplane connections, and physical mounting for VXIbus modules. Mainframes come in four sizes (A, B, C, and D) which correspond to the largest-size board you can plug into the mainframe.

VXI Configurations

You can use VXI in a variety of ways. You can integrate VXI into a system alongside other GPIB instruments, or you can build a system using only VXI instruments. Each system configuration has the following unique benefits:

•Embedded Controllers

–Highest performance, smallest size

–Direct access to VXIbus/fast interrupt response

•MXI, Multisystem eXtension Interface

–Embedded performance with desktop computers

–Use remote PCs to control VXI systems

–MITE/DMA—23 Mbytes/s block transfers

•GPIB-to-VXI Translators

–Control VXI mainframe with IEEE 488

The first configuration embeds a custom VXI computer directly inside the mainframe. Using this configuration, you can take full advantage of the high-performance capabilities of VXI because your computer can communicate directly with the VXI backplane.

The second configuration combines the performance benefits of a custom embedded computer with the flexibility of general-purpose desktop computers. With this configuration, you use a high-speed MXIbus link to connect an external computer directly to the VXI backplane.

The third configuration consists of one or more VXI mainframes linked to an external computer through GPIB. You can use this configuration to integrate VXI gradually into existing GPIB systems and to program VXI instruments using existing GPIB software.