Figure 1: VXIbus Module Sizes
Three module sizes are defined by the VXIbus specification. The relative sizes are illustrated in Figure 1. The B-size is the same size as the VMEbus B-size. The C-size which is by far the most popular and is the basis of Bustec products. It is deeper and wider than B-size VME. The extra depth provides adequate board space to implement most sophisticated instrumentation and physical isolation for low-level analog. D-size is quite large and has not found wide application. The focus of this paper will be directed to C-size VXI.
It is possible in some instances to mix both VXIbus and VMEbus modules, however this should only be done with great care. Some mainframes permit direct insertion of VME modules, however this only works if the VME module does not use any of the uncommitted VME pins. Inserting VME modules in a VXI backplane that use the uncommitted pins can have serious consequences due to the fact that VXI uses some pins for analog power. VME-to-VXI bus adapters do exist, but there can be side-effects due to stubs created when extending the bus due to the shorter VME card.