The VXIbus hardware specifications cover the following:
Mainframe Chassis
Modules
DIN Connectors
Local Bus
EMC Limits
Cooling and Power Specifications
The VXI mainframe chassis can hold up to 13 boards or instruments. While Slots 1 through 12 on the VXI mainframe chassis can hold modules or plug- in boards in a variety of sizes, Slot 0, which is a specific location in each VXI mainframe, is reserved for backplane management. The device installed in slot 0 provides data on clock sources and manages the flow of data across the backplane. The Slot 0 device also controls bus communications and the interface to an external GPIB controller.
The VXIbus standard specifies that plug-in modules may be in any of four sizes, defined as A, B, C, and D. These module sizes are as follows:
A—10 x 16 cm.
B—23.3 x 16 cm.
C—23.3 x 34 cm.
D—36.7 x 34 cm.
Modules A and B are part of the original VMEbus specification, which effectively means that A and B modules are VMEbus modules. The larger C and D modules were added as part of the VXIbus specification. Modules C and D are typically used for high-performance instrumentation, and they provide increased slot spacing.
The VXIbus standard includes three 96-pin DIN connectors, P1, P2, and P3.
P1—The P1 connector is part of the VMEbus standard and is a mandatory part of any VXIbus system. It transfers data at up to 24 bits addressing and 16 bits data.
P2—The P2 connector is optional, and can be used on B-size, C-size, and D-size modules. With the P2 connector, the data transfer bus becomes 32-bit and a number of other enhancements are available, such as additional power supply voltages, module identification capability, analog summing, and TTL and ECL trigger buses.
P3—The P3 connector, also optional, can be used only with D-size modules. It provides enhanced capabilities beyond the P2, such as 24 additional local bus lines and a 100 MHz clock.
VXIbus includes a local bus that insulates the operation of modules so that each module can communicate privately without disrupting the operation of adjacent boards. The local bus uses the following signal classes:
TTL
ECL
Analog (Low, Medium, and HI)
Local bus modules use hardware keys that interlock them with adjacent modules. The purpose of this keying is to allow only compatible modules to be placed next to one another.
VXIbus provides EMC limits for a module's generation of, and susceptibility to, radiation and conductivity. This element of the standard protects instruments with delicate or sensitive circuitry, allowing them to perform without interference from other modules.
Manufacturers of VXI mainframes are required to provide precise user documentation on their systems’ cooling capabilities in terms of airflow and pressure across a given module. With this information, a system integrator can compare a given instrument’s cooling requirements to the mainframe data, making sure that the two are compatible before installation.
Similarly, the VXIbus standard requires that mainframe manufacturers provide precise data on their systems’ power capabilities. Before installing a module, a system integrator can ensure that its voltage levels and current requirements are compatible with the mainframe.