by Michael Schwartz
More and more of the hardware we have to automate comes standard with a Universal Serial Bus (USB) communication interface. Manufacturers are replacing the older RS-232 with USB; with its low price and high speed we are seeing USB installed on more and more complex test equipment. We are at a point where USB measurement hardware is being engineered without any user interface.
Originally, USB was designed by several companies, with the overall goal to make it fundamentally easier to connect an external device to a computer. It was a revolutionary leap forward in technology, because it standardized communication while simultaneously replacing a multitude of connectors. Keyboard, mouse, floppy drives, printers, and network cards were all wrapped up into one connector. With speeds up to 12Mbits/s, it was sure to be the next big thing.
Test equipment manufacturers got right on board in 2003 creating the USBTMC-USB488 standard. This standard described the requirements for creating a USB message base measurement class. Covering both 488.1 and 488.2, instruments that were compliant to the USBTMC-USB488 standard all become plug and play. Plugging in your measurement hardware became as easy as plugging in a thumb drive.
But, one thing you have to keep in mind when programming on USB interfaces is that communication is a one-way street. This causes a lot of confusion for some people, but if you look, the connectors on each end of your cable are different. One connection is for the host controller (typically your computer) and another for the attached device. The host controller is at the center of a star topology and can connect up to 127 endpoint devices. Each device, when connected, identifies itself to the host, allowing the host to assign it an address, then direct traffic to that specific device.
Many of us have used the USB-GPIB adapter to control GPIB instruments from our computers. This is very common tool in today’s calibration lab… install the manufacturer’s software on our computer, plug it in, and voilà, we can control test equipment.
Many software packages support the USB to GPIB adaptors, but have a problem communicating directly with USB devices. At first we want to connect and adapt the USB-GPIB adapter to our USB only Unit Under Test and quickly find out the USB one-way street makes that impossible.
There is an alternative: Tektronix makes a GPIB to USB adaptor that works with more than just Tektronix hardware. The TEK-USB-488 bridges the USB to GPIB gap. It attaches to just about any USBTMC-USB488 compliant instrument, allowing it to have a GPIB address. With this adaptor, you can control USB only hardware using a GPIB Interface.
It is a pretty simple device to use as well. Apply power and connect the GPIB Cable to the TEK-USB-488 adaptor. Then power on your USB based test equipment. Then connect the USB cable from the host connector on the Tektronix adapter to the device connector on the test hardware. If the instrument is USBTMC-USB488 compliant, you will get a green status light within a few seconds.
Once you have a green light, it is easy to send commands to a USB instrument over the GPIB bus. It will come up at GPIB address 1 by default, but that can be changed if needed. The USB instrument will now function just line a GPIB 488 instrument. This allows the programmer to use the tried and true GPIB Interface to communicate with the device without having to install special software on the local work station. This can be a big time saver if you are working in an organization that does not allow users admin rights to their computers.
I really like this device and wanted to share it with the world. I have used it for years to support Tektronix scopes using Fluke MET/CAL®, but found it invaluable when I discovered it could be used to control other test equipment as well. So far I have used it on Agilent Technologies and Rohde & Schwarz hardware.