VISA device detected but VISA Read doesn't work properly

[DavideBrusamolino02]

Hi everyone,

I'm currently working on a project where i communicate to a board sending TCL scripts via CMD, and since here everything works fine.

Then i have to read the answers of this board through USB serial port, but when i power on the PC and the board for the first time Labview detects the COM port correctly, but the visa Read doesn't work for some reason. To make it work i have to switch off the board, unplug the usb cable, refresh the VISA control and plug again everything SEVERAL TIMES, then after some tries it starts communicatijg with no problems.

Do you have any idea on what could be the problem? Is not the first time i use VISA communication and i never had this issue.

Thanks.

[MaSta]

I don't think this is a VISA or LabVIEW problem, rather one of the driver. I work with USB COM ports every day and never have this problem.

Something that quite a few user will have, but don't necessarily realize: if the USB device is also USB powered, the voltage on the USB port has a big influence on the stability. If there is current demand at the limit plus a long, thin USB cable then the voltage drop could lead to temporarily malfunction. It means, the controller inside bounces between "I can't anymore" and "I'm ready".  

[crossrulz · Accepted solution]

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@MaSta wrote:

Something that quite a few user will have, but don't necessarily realize: if the USB device is also USB powered, the voltage on the USB port has a big influence on the stability. If there is current demand at the limit plus a long, thin USB cable then the voltage drop could lead to temporarily malfunction. It means, the controller inside bounces between "I can't anymore" and "I'm ready".  

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Plus many USB hubs on motherboards cannot handle the full current draw reliably.  I like to use commercial grade externally powered USB hubs for these kinds of situations.

[DavideBrusamolino02]

No, the board is powered externally by a 20V power supply, so the USB is only for data, plus i don't think is a driver issue because there are no drivers for this board (is custom) and after a fer tries as i explained, it starts working fine.

[BertMcMahan]

Make sure everything is grounded right. If you're using a laptop, they are frequently NOT earth-grounded and are insulated via their power supplies.

[rolfk]

So it’s a custom design! Has there been a serious design review by an experienced PCB designer? EMC, emission and suspicibilty measurements been done?

Even the smallest CPUs nowadays operate at frequencies that were considered major trouble makers 25 years ago. Electronics design looks easy as long as you consider traces, Op-Amps and logic gates as ideal but the reality is always quite a bit different!

And a 20V external power supply means you have at least 2 or more on board power supply generations. Usually with switched power supplies. But even linear power supplies can cause trouble if not proper filters are used and the layout is not correctly done. Switched power supplies are generally only performing well if designed by experienced PCB designers, even if a reference design is used as starting point.

[GRCK5000]

Hi Masta,

So what would be your advice. Is there another alternative to use for powering these USB devices?

[AnalogKid2DigitalMan]

Ground loops between the PC USB and the device?

Perhaps try an opto-isolator between the two?

https://www.commfront.com/products/industrial-usb-2-0-isolator?variant=39268677222497

-AK2DM