Issue with +5v pins on USB-6501

[MKBNIS]

Hello, I am using a USB-6501 (advertised as new) in a simple Digital I/O application.  It is recognized by the PC when plugged in, and I can control it with a LV 32 bit UI with no problem.  My issue is the +5v pins on the unit are not steady, they appear to go between 0 and +5v, appearing to be in sync with the status LED.  My experience is the +5v should be steady on unless it is put in Suspend mode.  Is the intermittent +5v normal, and is there a way to stop it?  Or do I have a defective 6501?  Thanks.

[santo_13]

+5V pin is directly off the USB's 5V supply and tend to fluctuate based on the load which is dependent on the 6501's functionality, as the 6501 circuit and +5V pin share the same rail over the USB cable.

As observed from the snippet, the +5V rail can be between 4V and 5.25V

[crossrulz · Accepted solution]

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@santo_13 wrote:

+5V pin is directly off the USB's 5V supply...

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One of many reasons I highly recommend using an externally powered USB hub when dealing with DAQ devices. I have had many issues with USB hubs on motherboards, especially laptops, not being able to supply the current despite the USB specification.

[MKBNIS]

Thanks for the replies; two things though.  The +5v is fluctuating from 0v to 5v, with none of the I\O's connected to anything, just the USB cable to a USB3 compatible port on a Dell Precision.  However I will try a powered hub, as I have run into issues with unpowered hubs before.  I also have a USB-6009, and it does not seem to have this problem with its +5v out.

[MKBNIS]

I tried a test today with a powered hub (supply is rated to over 4 amps), and still have the problem with the USB-6501 (+5v pin to GND at the connector outputs pins 31 and 32 fluctuate from 0 to +5v, synced to the flashing of its connection LED).  With the same setup I swapped a USB-6009, and measured the voltage at pins 31 and 32 as a steady 5.18V.

To make the problem a bit more confusing, I purchased two of the USB-6501's at the same time, and both have the same +5v behavior.  Ugh...

[santo_13]

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@MKBNIS wrote:

I tried a test today with a powered hub (supply is rated to over 4 amps), and still have the problem with the USB-6501 (+5v pin to GND at the connector outputs pins 31 and 32 fluctuate from 0 to +5v, synced to the flashing of its connection LED).  With the same setup I swapped a USB-6009, and measured the voltage at pins 31 and 32 as a steady 5.18V.

To make the problem a bit more confusing, I purchased two of the USB-6501's at the same time, and both have the same +5v behavior.  Ugh...

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How much current are you loading on +5V pin?

[MKBNIS]

Hello Santhosh, thanks for the reply.  There is nothing connected to the 6501 other than the USB cable to the USB hub.  All 32 IO pins are connected.

But after a bit more testing, I found one of the two 6501's has a steady +5v on its power pins, so I believe it is safe to assume that the original 6501 has some sort of defect.  It seems strange that its 0-5v power pin transition is in sync with the flashing LED.  But I plan to return the 6501 in question for a replacement.

Thanks to all of you for the replies!