Why do comm port numbers change?

[teslac]

    I have a problem with Windows 7 that is an annoyance but I have to periodically find all my instrument comm ports.  It seems that Windows is periodically reordering them which makes the opperator of my application hunt them down again in MAX.  My systems have an extended NI 232 card with an additional 8 ports and I also have a 485 card with 8 ports that also changes periodically but not as often.  Has anyone witnessed this behavior before?

[mikeporter]

I have never seen Windows renumber com ports on its own. That would be a behavior that would not be justifiable in any sense.

What I have seen is that when plugging in a Serial dongle on different machines it doesn't always show up in the same place. I have also seen some cheap, knock off dongles do strange things.

What seems to trigger the renumbering?

Mike...

[teslac]

It sometimes happens after the systems have not beem used for a while and have been re-booted.  Do you think it is possible it may have somthing to do with IT mucking about?

[mikeporter]

It is certainly a possibility, though I don't know what they would do to cause it.

Mike...

[tst]

Are these USB cards? If you disconnect a USB device and connect it to another port, Windows sees it as a new device (although I never understood why, since I believe they are supposed to have a UUID) and in the case of virtual COM ports, that will often cause the driver to assign it a different COM port, presumably to avoid collisions.

[teslac]

I don't have USBs on my system only 232 and 485 but that would have been a good explaination if it were true.

[teslac]

I know Im replying to myself but I wanted to put this out there that I found the issue.  The problem is with the BIOS and the numeric reassignment of ports happens on re-boot and is a know problem.  NI has a tool for port configuration.exe or simply changing the port assignments in MAX to a specific name will also fix this problem.

[Hooovahh]

I've never heard of this type of issue before, do you have a white paper, or knowledge base to link to for more information?  Where did you get the utility?Assigning aliases in MAX is probably the best solution, assuming they are based on something like the hardware serial number.

[teslac]

Actually I was on the phone with support on a different MAX issue and posed the question to him and gave me the explaination and said it happens a lot and so much infact they created this app. 

Also, before I forget about it, here is some more information on the COM port utility I mentioned on the phone. You can find it at “C:\Program Files (x86)\National Instruments\NI-Serial”. This is a really simple utility, so there isn’t much documentation on it, but the document below explains how to use it. http://digital.ni.com/public.nsf/allkb/AEF778866CA41A458625763F0062A805

[mikeporter]

The really troubling part of this is that according to the page you referenced this problem was discovered in Sept of 2009! I wonder what it takes to get NI to fix this problem? They must not sell many of their serial boards: and perhaps for good reason. I know I'm going to be remembering this.

Any idea whether this is happening with later releases of Windows?

Mike...