A project recently got me through tough times. We have connected almost all instruments using USB and ended up using multilevel USB hubs. The hardware connection is something like below. We thought that everything will be fine because
- All of these USB ports are detected by Windows without any issues
- The payload of these instruments’ data are very low and
- We’re able to test all these instruments in modular level.
However we faced some subtle issues which is not lucid at the beginning.
The project in itself is about a multi-site product calibration. The golden values from master instrument looked perfect, however the calibrated product looked pretty off from the golden device values. On top of that, the parameter being calibrated is inconsistent over time, i.e., the timing of the calibration played important role here.
Being unknown about the root cause of this issue, we started to narrow down the issues in several ways. One of the narrow down method also included the verification of these USB instruments one by one. The method is to run each sites by literally disconnecting USB Instruments wire for other untested site. Then running 2 sites in parallel, then 4 sites, then 6 sites. All by literally wiring only those instruments required for the sites being tested. Until 6 sites the calibration results were perfect. Nevertheless, going beyond 6 sites (i.e., 7 sites or more) started to disturb all other sites’ result. On the other hand, the calibration results for all sites (7 and more sites) were perfect when we connected few instruments to PC directly (not through USB Hub) as shown below.
We couldn’t arrive at the source of this issue yet, but the strong assumption is that when we connect more instruments via a USB hub, then there could be subtle overheads even when the payload of these connections are very low that will disturb the results.
Ajay is a professional developer and architect of NI-LabVIEW applications with extreme interest in getting the hardware connected to LabVIEW and automating the stuff. Recently he is also putting his hands in NI-TestStand to get very dirty on it. He is also a good mentor for the various interns in his career. He is ready to help the people in techie roles.