ntained within the module, eliminating exposed circuitry.The use of multi-slot “carrier” modules in VXI also lends flexibility to instrument design. For example, Racal’s two-slot Adapt-a-Switch module accepts up to six switching and digital I/O PCB’s, permitting the system integrator to create a custom configuration using COTS products.SpeedThe VXIbus backplane has a theoretical maximum of 40 Mbytes/s. This speed is more than adequate for functional test applications, whose throughput is limited by the laws of physics. For example, commercially available DMMs, operating in a low-resolution 4.5 digit mode, can only take about 1000 readings per second. Armature relays used in switch matrices have de-bounce times on the order of 15ms.PXI has a higher speed backplane than VXI, with a theoretical maximum of 132Mbits/s. This makes it ideal for repetitive, low-speed, high-resolution measurements as are commonly found in data acquisition systems. Such systems typically measure physical characteristics such as position, temperature, strain, etc.It should be noted that PXI systems using more that seven instruments, or built using the 6U form factor chassis, will require a PCI bridge, significantly reducing system throughput.ConclusionFred Bode, editor of the VXIbus and PXI Newsletter interviewed Ron Wolf, Director of Strategic Marketing at National Instruments. Fred reported that Ron saw PXI replacing rack and stack instrumentation more than replacing VXI at the present time.PXI is not a new low-cost replacement for VXI. The two architectures are complementary, and each is suited to a different range of applications. VXI is generally more robust and suited to high performance functional test systems. PXI is generally suited to small systems requiring lower measurement accuracy and high data transfer rates such as data acquisition systems. Neither VXI nor PXI implementations are inherently less expensive. Costs are more clo...
