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USB connection drops after Frequency Drives run


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I have an issue where the high voltage appears to be inducing noise on my 24vdc to power the JAZZ 2. I am going to scope it next week to see how bad the noise is. The noise is very apparent when a frequency drive starts because I lose the USB connection and can not re-establish it. Today, I tried adding a ferrite ring to the incoming power and it appears to be night a day. When its not installed I have an issue when it is there, no problem. Has anyone else added a ring to their setup to cut down on noise or found connection issues with the USB port? Also, a wild question could too much noise cause a glitch in the PLC program? Every so often the PLC misses a math function from a positive transition. Any thoughts?

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On Question 1 yes VFDs can cause the USB connection to stop between a laptop and the PLC.  What happens is the VFD is imparting noise into the localised earth ground and it gets picked up by the cable.  A ferite core as you have seen can sometimes (not always) correct the issue.  At his point I do field reprogramming with an SD card unless I am needing to look at operations in a Visilogic program live to make changes. 


I have found that older serial connections (if your laptop has a serial port) and the cable that came in the boxes before USB cables can work better in noise environments.


On your second question.  I have never had noise interupt calculation in the PLC.  (I have had noise stop a PLC all together but not creat a hiccup in operation. 


Using positive or negative transitions can have wierd effects.  Usually if you look into it far enough you find that there is something that created a delay in the program and the transition (for lack of a better way to describe it) gets missed.  Another way to get to the same place is to use positive transition to increment an integer to then use the integer not equal to 0 to make the math run and at the end of the math line store a 0 back into the integer. 


Finding little gremlins like this can be difficult, so sometimes working around the issue becomes easier.  If you find that noise is the issue it would be nice to hear back from you on this.


With VFDs the ability to have an excellent ground makes all the difference int he world.  The better the ground (ground wire needs to be same gauge size as the 3 phase wires) the faster the stray voltage the VFD imparts into the transmission cables can be drained away.  You will be able to see the voltage with a scope.



Good Luck


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  • MVP 2023

Most people don't realize that the USB port on their computer is connected to the motherboard ground.  On a laptop this is also connected to the common of the battery circuit.


As soon as you connect your computer via USB to the PLC you have provided a current path for what is usually an difference in ground potential.  If you're using a laptop unplug the AC adapter and the problem magically vanishes.


I have solved this problem by using an optically isolated USB to serial adapter.  A Google search reveals several sources and the average cost is about $100.00.


Joe T.

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