VXIbus--A Modular Standard for Test & Measurement

Extending the VMEbus for Instrumentation

A goal of the VXIbus Consortium was to allow for the broadest possible range of instrument performance and cost. Another goal was to make as much use of existing standards as possible. The A and B card sizes of VMEbus were included as part of the VXIbus standard. In fact, VXIbus retains the Pl connector and the center row of the P2 connector exactly as defined by the VMEbus. This includes the 5V and + 1 2V power pins on Pl.

A major objective of VXIbus is to standardize instrumentation. However, the majority of high performance instruments will not physically fit in the smaller VMEbus areas. In order to overcome this. the Consortium added two additional card sizes for VXIbus. These include C-size, which is approximately 13 inches (33cm) deep and nine inches (23cm) high. and D-size, which is approximately 13 inches (33cm) deep and 14 inches (35.5cm) high. The C-size card has the same connectors as B-size VMEbus modules, however all pins on P2 are fully defined. The D-size card may have an additional connector, known as P3 which adds extra resources necessary for higher instrumentation performance.

A VXIbus module may be a printed circuit board (PCB) or an enclosed assembly that contains several PCBs. If an instrument needs more than 1.2 inches (3cm), it may take up multiple slots in a VXIbus mainframe.

The VXIbus specification requires manufacturers to publish module specifications relating to VXIbus, as well as those relating to the instrument's performance characteristics. These include cooling requirements (Le. minimum airflow for maximum allowable temperature rise) and maximum power requirements. All modules must also meet EMC radiation and susceptibility criteria. These EMC requirements ensure that high performance instruments do not interfere with each other

A VXIbus system may have up to 256 devices, including one or more central timing and arbitration modules. These are referred to as the Resource Manager or Slot 0. Thirteen single-slot instrument modules conveniently fill a standard 19-inch cabinet when mounted vertically on 1.2 inch centers. Although there is a maximum of 13 single-slot modules in a VXIbus subsystem, there is no minimum number. For example, a subsystem may contain just a resource manager with two or three modules. A number of different size mainframe are available to suit the different card sizes. However, the most popular is C-size as this can handle the standard A and B-size VMEbus cards as well as the many available C-size instruments while maintaining a reasonable size.

The VXIbus specification precisely defines the cooling, power and EMC tolerances of a VXIbus mainframe. Selection of chassis and modules is an interactive process as the chassis must be able to meet the total power and cooling requirements of the modules.

The VXIbus specification expands upon the VME specification by defining ail pins on the P2 backplane and adding P3. The VXIbus P2 adds a 1OMHz clock. ECL and analog supply voltages, ECL and TTL trigger lires, an analog sumbus. a module identification line, and a daisy chain structure called the Local Bus. P3 provides more of the above and adds a 10OMHz clock and a star bus.

Module Sizes

Single Width Module

Example of a D-size System

Pins Definition on P1, P2 & P3