Re Assigning Com Port Numbers Using The Windows Registry

Summary

Application Note
AN_132
Re-Assigning COM Port Numbers Using
the Windows Registry
Version 1.1
Issue Date: 2016-04-27
This application note illustrates how to re-assign automatically assigned FTDI COM port numbers using
the Window OS registry

Future Technology Devices International Limited (FTDI)
Unit 1,2 Seaward Place, Glasgow G41 1HH, United Kingdom
Tel.: +44 (0) 141 429 2777 Fax: + 44 (0) 141 429 2758
E-Mail (Support): [email protected] Web: http://www.ftdichip.com
Copyright © Future Technology Devices International Limited
AN_132 Re-Assigning COM Port Numbers Using the Windows Registry
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D oc ument Reference N o.: FT _000196 C learance N o.: FT DI# 1 2 1
Table of Contents
1 Introduction ................................................................. 2
2 System COM Port Assignment ........................................ 3
3 How to Re-Assign COM port numbers by registry
modification ...................................................................... 4
3.1 Open the OS registry .............................................................. 4
3.1.1 Windows XP ...........................................................................................4
3.1.2 Windows 7 Onwards ................................................................................4
3.2 Re-assign the COM Name Arbiter in ComDB............................. 7
3.3 Import the modifed FTDIUSB registry file ............................... 9
4 Contact Information .................................................... 13
Appendix A - References .................................................. 14
Acronyms and Abbreviations .........................................................14
Appendix B – List of Tables & Figures ............................... 15
List of Figures ...............................................................................15
Appendix C - Revision History .......................................... 16
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AN_132 Re-Assigning COM Port Numbers Using the Windows Registry
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1 Introduction
This application note describes how to re-assign the COM port numbers allocated during installation on
Windows operating systems by modifying registry entries. The application note provides a walkthrough
of manually editing the registry, but the same technique could be employed by a program

WARNING: Any edits made to a PC registry may be harmful to the operation of the PC. As such
we recommend you create a backup of the registry before making any changes. FTDI cannot
be held responsible for any damage done as a result of incorrect manual editing of the
registry

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AN_132 Re-Assigning COM Port Numbers Using the Windows Registry
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2 System COM Port Assignment
COM ports are assigned to FTDI devices during installation, based on the next free port as indicated by
the PC registry. In most systems this would imply starting at COM3 as COM 1 and COM2 are usually
reserved for legacy COM ports. If COM3 is already in use, the next available COM port index will be
claimed. In the case of multiple interface chips, such as the FT2232H or FT4232H, a co -installer is used
to claim consecutive COM ports for the interfaces of the device. If three FTDI FT4232H devices are
connected to a PC, then 12 COM ports will appear to the operating system. In the example below , the PC
has assigned these COM ports with com port numbers from COM3 to COM14

Figure 1 - System COM Port Assignment
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AN_132 Re-Assigning COM Port Numbers Using the Windows Registry
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3 How to Re-Assign COM port numbers by registry
modification
In the example above, the operating system automatically assigns the 12 COM ports from COM3 to
COM14. If it is required to re-assign the COM port numbers without setting each port individually, then
the FTDIUSB file in the registry can be used to do this. Follow the procedures below to re-assign COM
Port numbers using the operating system registry

3.1 Open the OS registry
3.1.1 Windows XP
To open the OS registry, select the “Start” button and press the “Run”. In the pop-up box that appears,
type “regedt32” and press enter to access the registry. This is shown in the figure below

Figure 2 - Accessing the Registry (Windows XP)
3.1.2 Windows 7 Onwards
To open the OS registry, select the “Start” button and in the search box type “regedt32”. This should
display the registry executable as shown below in the image to the left. Right click on this to open the
context menu where you should select the “Run as administrator” option, as shown in the image to the
right. If you do not select “Run as administrator” the remaining steps will not work

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AN_132 Re-Assigning COM Port Numbers Using the Windows Registry
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Figure 3 - Accessing the Registry (Windows 7)
Within the registry, navigate to the following area to access all FTDI devices:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\FTDIBUS

Select FTDIBUS and then under the “file” pull-down menu select “export” – as shown in the figure below

This will export the FTDIBUS registry key, sub-keys and values to a file

Figure 4 - Export the FTDIBUS Registry to a File
Right click on this saved file and select “Edit” to open the file, as shown in the figure below

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AN_132 Re-Assigning COM Port Numbers Using the Windows Registry
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Figure 5 - Edit the Registry File
In the FTDIUSB.reg file, modify the “FriendlyName” and “PortName” in the file for each port which needs
to re-assigned

Figure 6 - Modifying the FTDIUSB.reg Registry File
For example to re-assign COM7 to COM17 modify the FTDIUSB.reg file as follows:
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Replace FriendlyName from “USB Serial Port (COM7)” to “USB Serial Port (COM17)”, and modify
PortName from “COM7” to “COM17” at the same time. This will re-assign COM7 to COM17 after the
registry file has been uploaded (illustrated later)

The same method can be used to modify each port

Note: Care should be taken that the same COM port number is not repeated

3.2 Re-assign the COM Name Arbiter in ComDB
If COM3 to COM14 are re-assigned to COM13 to COM24, then it is necessary to free up the unused com
ports (COM3~COM12) and allocate the new additional ports COM15~COM24. To do this it is necessary to
modify the ComDB database by using the following procedure

Open the registry and navigate to the following area of the registry
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\COM Name Arbiter
Open the “ComDB” as shown in the figure below. The example shows that the original value of “ComDB”
is as follows:
ComDB original value = FF 3F 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00
The ComDB value is a bit mask of COM ports already allocated. This value is equivalent to COM3~COM14

The first byte “FF” equates to the com ports COM8~COM1 and indicates that these COM ports are
occupied. The second byte “3F” equates to the com ports COM14~COM9 and this indicates that these
COM ports are occupied. (Note that COM1 and COM2 are the COM port of motherboard)

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AN_132 Re-Assigning COM Port Numbers Using the Windows Registry
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Figure 7 - Editing the ComDB Database
To allocate the COM port numbers COM13~COM24, it is necessary to modify the ComDB value as follows:
ComDB new value = 03 F0 FF 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00
This is shown in the following diagram:
Figure 8 - Change the Value 3
The first byte “03” is used for COM2 and COM1 of motherboard. COM8~COM3 are not used

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The second byte “F0” is equates to COM12~COM9 that are not used and COM16~COM13 that are
occupied

The third byte “FF” equates to COM24~COM17 which are occupied

Note: The first byte controls COM8~1, the second byte controls COM16~9, the third byte
controls COM24~COM17 and so on. A value=0 means that the port is not used. A value=1
means that the port is occupied

3.3 Import the modifed FTDIUSB registry file
When the FTDIUSB registry file has been modified it is necessary to import the file back into the registry

To be able to do this a user must have full access control rights to modify it in the registry

To modify the permissions, right click on the FTDIUSB folder in the registry and select the Permissions to
enable the permission for modification – as shown in the figure below -
Figure 9 - Modifying Registry Permissions for FTDIUSB Folder
The following window will appear. Select “full control” then press OK

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AN_132 Re-Assigning COM Port Numbers Using the Windows Registry
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Figure 10 - Allow Full Control Access Rights to the FTDIUSB Registry
Note: You may encounter an “Access is denied” error as per the below diagram. If so follow the below
steps

Figure 11 - "Access Denied" Message Window
Open the Permissions window for FTDIBUS again and click on the advanced button instead. In the
“Advanced Security Settings” window click on the “Owner” tab. If the current owner is not you then
change to you by highlighting your name in the “Change owner to:” window

Click Apply > OK

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Repeat the steps at the beginning of section 3.3

Note: It is always a good practice reversing the changes back to their default settings, once you have
made the necessary changes

Next import the modified FTDIUSB file. Select Import and choose the modified registry file to import the
registry file. This is shown in the following figure:
Figure 12 - Importing the FTDIUSB File to the Registry
To check that the procedure has worked, open the device manager and press “Scan for hardware
changes”. This should show the new COM port assignments as illustrated in the following figure:
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AN_132 Re-Assigning COM Port Numbers Using the Windows Registry Version 1.1 Document Reference No.: FT_000196 Clearance No.: FTDI# 121 13 Product Page

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Frequently Asked Questions

Where can i find the assigned com port numbers in windows?

The HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows NTCurrentVersionPorts registry key contains the list of all assigned COM port numbers. You can remove all unnecessary port reservations. In our example, we’ll leave only COM3 and delete the rest ports;

How to manually reassign the com port number?

First of all, try to reassign the COM port number for a specific COM device manually. Suppose that the necessary COM port is already busy and we would like to free it. Select View->Show Hidden Devices in the console menu. Expand Ports (COM & LPT) branch and find the device, which is assigned the necessary COM port in the list.

What are the registry settings for a legacy com port?

The registry settings for a legacy COM port are under a corresponding legacy COM port subkey that is under the ..ServicesSerialParameters key. The following entry values are the same as described for a Plug and Play serial device: The following additional entry values are used with legacy COM ports:

How to assign a free com port to another device?

The freed up COM port can be assigned to another device. Find your device in the same branch, and open the Properties window. Then go to the Port Settings tab and click Advanced. In the dropdown list, select the free COM port.