Introduction

VXIbus is a powerful open standard designed for data acquisition and Automated Test and Measurement (ATE) applications. It was originally envisioned as a replacement for rack and stack instrumentation such as GPIB. VXIbus is an acronym for VMEbus eXtensions for Instrumentation. The goal of the standard is to define a technically sound instrumentation standard based on the VMEbus that is open to all manufacturers.

The basic building blocks of a VXIbus system is a powered 13-slot mainframe or chassis and a wide selection of modules that implement various I/O and instrumentation related functions. The standard implements the VME backplane bus and defines various functions to the VME uncommitted backplane pins. Crucial to Data Acquisition and ATE instrumentation applications are the extensions that provide chassis wide clocks, timing and control.

The following features illustrate the basic relationship between VME and VXI standards:

• VXI implements the VMEbus protocol for data transfers between modules.
• The VXI backplane connector is identical to VME with the same pinout for the P1 (top) connector and the center row of the P2 (bottom) connector.
• The two outer rows of the P2 connector are undefined in VME and are assigned specific functions in VXI.

VXIbus Features for Data Acquisition and ATE

VXIbus Features for Data Acquisition and ATE

The VXIbus specification provides a number of significant enhancements over VMEbus for data acquisition and ATE applications. These include:

• The VXIbus specification provides a larger (deeper) card format that increases the board real estate available to implement sophisticated analog and signal conditioning options. The deeper card provides better isolation between low-level analog signals and typically noisy digital circuitry and buses.
• The VXIbus specification includes mandatory analog power supply voltages and specifications for power supply noise which eliminate the need for on-board dc to dc converters to power analog circuitry as well as establishing a worst case power supply noise levels.
• The VXIbus specification provides shields on C- and D-size modules to help minimize noise pickup from adjacent modules.
• The VXIbus specification includes chassis-wide clocks and trigger lines for common clocking and triggering across modules an important consideration when analyzing data from different modules.
• Geographic addressing and dynamic address allocation is provided in the VXIbus specification. These features can be useful when modules are replaced or the system upgraded. Wiring is typically to physical slots, geographic addressing coupled with dynamic address allocation allows one to simply pull out an old module and replace it without having to set address switches. Not all VXIbus vendors provide dynamic addressing capability.
• The VXIbus specification includes cooling specifications that are crucial particularly in low-level analog applications and applications that use modules that require high power levels such as digital signal processors.
• The VXIbus specification requires that each module perform a self-test on power-up. A bit in the module status register indicates whether the module passed self-test or not.
• The VXIbus specification provides a number of standard registers that contain information relating to the module. These include:

  Manufacturers ID:

  Each VXIbus manufacturer is required to obtain a unique manufacturer ID that is available in this register.

  Module ID:

  An module type identifier assigned by the manufacturer ... typically the manufacturer model number.

  Module Serial Number:

  A manufacturer assigned serial number.

  Module Hardware/Firmware Revision Level:

  A manufacturer assigned revision level of the hardware and firmware.