Introducing PXI instrumentation into an existing VXI based tester

With the large number of PXI form factor instruments that are now available, many users are looking to introduce these instruments into existing VXI based systems. Adding PXI instruments to existing systems is much more cost effective versus building a new tester since various assets, such as user power supplies and the system controller can be shared by both instrument types. But introducing this new instrumentation presents the System Engineer with many design issues that need to be considered. One major consideration is whether to use the existing VXI user interface or to expand to a second interface. It is not uncommon that when a PXI chassis is added to an existing system that only a few instruments are introduced. When a small number of instruments are added it may make sense to just provide cabling that allows the user to connect to the existing interface, connect a wire harness directly to the ITA, or simply cable from a PXI instrument directly to an I/O port of the UUT. However, if the design of the cabling becomes too complex these approaches may become impractical. In this case the user's only practical choice is to use a second interface. Another factor when determining the best interface choice is the technology used by the new instrumentation. If the signals are differential then the cable lengths are less of an issue, although shorter cables are always preferred. But, single ended technology such as LVTTL signals, may only support cabling up to a foot or two and this can force the System Designer to institute a second user interface option in front of the PXI chassis. If it is determined that a second interface is required there are other decisions that have to be made regarding the sharing or addition of system, assets. This paper will discuss all of these issues using real life examples of systems which have combined both VXI and PXI instrumentation. The examples will show various instrumentation and the cabling solutions employed.