Accessing Serial Port


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Article ID: Q500002

The information in this article applies to:

  • Direct I/O, version 4.0, 4.1, 4.2

For Direct I/O, version 4.3 and above see article Q500004.


Normally the serial port (COM) is controlled by Windows NT. Software needing direct access to the serial ports registers does not run properly.
Direct I/O can be configured to grab the serial port resources and allow those programs to run successfully.


Follow the steps given below to configure your system properly:

  1. Open the Direct I/O Control Panel
  2. Identify the serial port you want to use and enter the resources in the tab I/O Ports
    COM1:   3F8-3FF
    COM2:   2F8-2FF
  3. Enter the appropriate pathname of your process in the tab Security
  4. Check if your application utilizes the serial port IRQ. If you are unsure skip this step. You can enter an IRQ later if your application failed to run because of a missing interrupt.
    COM1:   4
    COM2:   3
  5. Close the Direct I/O Control Panel and open the Devices Control Panel
  6. Select the Direct I/O device and press the Startup button
  7. Change the startup type to System and close the dialogs
  8. Reboot your system

Depending on your configuration you may notice some error messages at startup stating that the device serial.sys cannot be started. This occurs in cases when you have grabbed the only serial port built into this PC. You can get rid of these messages by changing the startup mode of these devices to Disabled.


You cannot utilize a serial port grabbed by Direct I/O from within Windows NT. Only one device has control over a given hardware resource, either serial.sys or Direct I/O.

On some Windows 2000 systems you might fail in controlling the serial port due to ACPI conflicts. Please read Q700005 for further information.

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Last modified: 2018-04-24.
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