Frequent Test Cafe readers are familiar with the speed advantages of modular instruments. But just to recap, modular standards such as PXI or AXIe gain their speed advantage in functional test applications by avoiding the ASCII interpretation bottleneck of traditional “box” instruments. Recall that traditional instruments are typically programmed by SCPI commands, Standard Commands for Programmable Instruments. These natural language commands take milliseconds to interpret, while driver execution is measured in microseconds. I’ve seen PXI DMMs (digital multimeters) perform a complete measurement in 50 microseconds, including the command time. A traditional LXI instrument wouldn’t have traversed the TCP/IP stack in order to send the first character in that time. The bottom line is that SCPI is optimized for portability, not speed. For top speed, you need pre-compiled high-speed drivers that manipulate an instrument’s native memory map directly over a low latency interface. PCIe-based systems like PXI and AXIe do exactly that.
But are there ways around this bottleneck to make traditional box instruments competitive? Yes- there are some tricks that users or manufacturers can employ to narrow the difference or even equalize the playing field. Let’s review some here.