Changing COM port Number

[Ashish Sureka]

Hello,

I have a display device connected to the USB port of my
computer. The COM port number of this device is 10. I want
to change the COM port number of this device to 3 or 4.
I am unable to change the COM port number of this device
as the port settings tab for this device is not available.
I can go to the device manager and change the COM port
numbers of all the devices using the port settings tab but
I am unable to do so for this display device as the port
settings tab is not available.

I tried to change the port number by modifying the
registry but when I re-boot the machine the COM port is
again changed to the old value.

I have been trying to search about it on the web but not
able to find it. Please let me know if anyone came across
the same problem and what is the solution.

Thanks in advance,
Ashish

 

[Walter Clayton]

Start from the beginning.

Com port is an abbreviation that refers to communication port and is
associated with serial modems. Communication port numbers start at 1 and go
from there, but generally only exceed 2 when an addtional modem controller
is added. Onboard serial ports, typically 2 in the past are assigned comport
1 and comport 2 addresses. Display devices are not modems and do not have
com port addresses. It is possible for a USB device to emulate a com port,
but in general this will be 3 or higher since ports 1 and 2 are reserved,
typically, for motherboard "hardwired" com ports.

Now I'm going to make a massively gross assumption and like all assumptions
it's possible that it's wrong, but in this instance I doubt it. Do you mean
IRQ? IRQ is not the same as com port. IRQ refers to hardware interrupt
request line and is completely different from com ports. In fact com ports
sit on top of IRQs. All the hardware sits on top of IRQs for that matter. If
in fact you're referring to IRQs then you must reinstall the OS and select a
standard HAL so that you can manually control the IRQs. As long as an ACPI
HAL is in use the OS will control hardware resources. Since this has it's
own set of issues, I have to ask, exactly what do you think you're
attempting and what problem do you think you're solving?


--
Walter Clayton - MS MVP(WinXP)
Associate Expert
http://www.microsoft.com/windowsxp/expertzone
Any technology distinguishable from magic is insufficiently advanced.
http://www.dts-l.org

 

[Ashish Sureka]

First, I really appreciate for your time to answer my
question. I have spent many hours searching on the web
but I am unable to find the solution.

I have a display device that emulates the COM port.

Here's a link to the FAQ
(http://www.logiccontrols.com/web/prodCustDtech.htm#q1b)
of the display devices which suggests to change the COM
port number. I am pasting the question from the FAQ.

Q. The USB virtual COM port driver assigned a COM port
number higher than COM4 but the POS (Point of Sale)
software only works with COM ports COM1 to COM4. Is there
a solution?

A. The COM port number is assigned by Windows system and
it is the next available COM port number in the system.
If there were other COM port devices installed before,
even if they are physically removed, Windows registry
will still consider those ports to be occupied such that
when those devices are plugged in again, they will use
the same port numbers as before. As it's usually not
possible to uninstall those Windows registry information
on previously installed devices, when the USB pole
display is installed, they may then be assigned a higher
COM port number.

To change the port number back to COM3 or COM4, the
solution is to modify the Windows registry manually. As
the registry is vital to Windows operation, only
experienced users should work with the modifications. It
is also recommended that a back up of the registry is
performed before the modifications. Please also make sure
that the removed device that had occupied the lower COM
port number before should never be installed back to the
system. Otherwise there will be a hardware conflict.

To modify the COM port number assigned for the pole
display, search through the registry for the assigned
number (for example, search for "COM5" if the assigned
port is COM5). When search stops at a certain key, check
if it is related to the Logic Controls Line Display. If
so, modify the key value to COM4. Continue search through
the registry until all matched items are checked and
modified. Note that the "Friendly Name" key cannot be
modified. After modifications are done, restart the
computer and test pole display under COM4 to verify it's
operation.





I am trying to change the COM port through device manager
port settings tab but the port settings tab for
this device is unavailable. I also tried to change it
directly in the registry but that change is also not
taking place.

Thanks,
Ashish

 

[Walter Clayton]

Ahhhh. One of *those* devices. Multifunction devices can be rather
confusing.

The instructions are incorrect. I'm having to rack my brains since it's been
a long time since I had a USB emulated com port. The problem with the
instructions as sated is there is no provision for duplication. I assume
you're using XP else you wouldn't be posting in an XP group.

Launch a command prompt and enter

set devmgr_show_nonpresent_devices=1 [enter]

Close the command prompt.

Launch device manager (start->run->devmgmt.msc [enter])

From the view pulldown enable "show hidden devices". Delete all devices that
refer to com ports other than com1 and com2. Reboot the machine to do a
clean device redetection.

--
Walter Clayton - MS MVP(WinXP)
Associate Expert
http://www.microsoft.com/windowsxp/expertzone
Any technology distinguishable from magic is insufficiently advanced.
http://www.dts-l.org