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print this documentDocument ID:1996112511055
Last Modified:01/16/2007

Resolving IRQ/COM port conflicts

Situation:This document describes how to identify and resolve IRQ and COM port conflicts.


Two devices sharing the same Interrupt Request (IRQ), I/O port, or base address cause a port conflict. With modems, duplicate Interrupt Requests or I/O ports usually cause conflicts. For example, installing an internal modem configured for COM2, IRQ3 into a computer which also has a physically-installed COM port will result in a conflict.

Some terminal-oriented programs such as ProComm, Windows Terminal, and faxing programs may not appear to have hardware conflicts. However, pcANYWHERE is a full-featured remote control program and is more sensitive to hardware conflicts. pcANYWHERE's full usage of communications hardware is more susceptible to adverse effects.

IRQ and other hardware conflicts can manifest in many different ways. Here are a few examples:

  • Modem not recognized or initialized.
  • Slow connections or loss of connection.
  • Black screen after connect or system rebooting.
  • Computer lockup.
  • pcANYWHERE for Windows does not work reliably with any IRQ conflicts in host mode.
  • pcANYWHERE for DOS does not work reliably with any IRQ conflicts in host mode while starting Windows. It may work in DOS.

Avoiding IRQ/COM port conflicts

There are several steps you can take to ensure there are no conflicts with other devices or ports. First, determine which serial ports are active on the computer before connecting or installing the modem. Most computers have the following communication ports:

I/O address Title  IRQ   Description
(3F8) COM1   IRQ4 The first serial communications port on the PC.
(2F8)  COM2   IRQ3 The second serial communications port on the PC.

Because the original design of the PC did not take into account the use of more than two COM ports, newer computers started duplicating IRQ numbers for additional COM ports. For example:

(3E8)  COM3 IRQ4 Conflicts with COM1
(2E8) COM4 IRQ3 Conflicts with COM2

This is a duplication of the IRQ and therefore a conflict. Because of this, it is important to be careful when adding new hardware devices (like modem cards, network cards, sound cards, emulation cards). Some of these hardware devices have jumpers to configure the IRQ or COM port to another setting. Others are reconfigured via the computer's BIOS. How the BIOS is accessed varies from machine to machine. Seek the correct information in the documentation that came with the computer, or ask a technician for assistance.

Here are some common mistakes:

  • Adding an internal modem configured the same as an existing COM port.
  • Adding an internal network card configured the same as a device using an existing IRQ.
  • Using an external modem on COM2, IRQ3 when any other internal device is using IRQ3.
  • Setting an internal modem at COM3, IRQ4 (conflicts with COM1, IRQ4).
  • Setting a sound card to IRQ5 and an internal modem at IRQ5.

Some popular scenarios

This scenario is an ideal setting for basic systems with an internal modem, and without other additional internal devices, present (like a sound card, CD-ROM, or network card). Note that no IRQ numbers are duplicated:

COM1 IRQ4 Standard COM1 port
COM2 IRQ3 Standard COM2 port
COM3 IRQ5 Internal modem

This scenario works well if you need to add an internal modem and a network card:

COM1 IRQ4 Standard COM1 port
COM2 IRQ3 Internal modem *
Network Card IRQ5 Network card

NOTE Note In this case, disable the original COM2 port before adding the internal modem. Some computers have BIOS SETUP programs that allow you to disable COM ports; others may require changing switches or jumper settings on the I/O card or motherboard.

IRQ availability

Below are some common hardware uses of Interrupt Requests

  • Sound cards often use IRQ 5 or 7.
  • Network cards often use IRQ 2, 3, 4, 5, 7, 10, 11, 12 or 15.

Below are some additional Interrupt Requests that may be available:

IRQ5 Reserved for LPT2 Most systems do not have an LPT2; generally IRQ5 is not in use.
IRQ7 Reserved for LPT1 Many sound cards do not have a problems using this.
IRQ10 Free
IRQ11 Free
IRQ12 Free Often used for a PS/2 mouse.
IRQ15 Free

Tools for finding out information

The most accurate way to check hardware settings is to visually look at the jumper settings, then look up this information in the product's manual. Some software utilities, like MSD (Microsoft Diagnostics), can help. Be aware that these utility programs may not always give accurate information.

Microsoft Diagnostics, or MSD, comes with Microsoft Windows, as well as MS-DOS 6.0 and above. Run MSD, and press C to display COM port's information. Use the I/O address number when determining the COM Ports. Some older BIOSs may not accurately detect COM ports.

Port Address 03F8H 03E8H   N/A N/A
Baud Rate   2400    2400   
Parity  None None
Data Bits 8 8
Stop Bits 1 1
Carrier Detect (CD) No No
Ring Indicator (RI) No No
Data Set Ready (DSR) No Yes
Clear To Send (CTS) No Yes
UART Chip Used 8250 16550AF

The above MSD screen reveals that you have a COM1 (3F8) port active and a COM3 (3E8) port active. Notice that the COM3 is in the COM2 column. This is incorrect. This is due to a DOS feature called Address Packing. Address Packing fills any gaps in the COM port addresses by moving the displayed location of a higher-numbered COM port over to fill the unused COM port space. See the diagram below. For accuracy, always look at the reported port address.

Port Address 03F8H N/A 03E8H   N/A
Baud Rate   2400      2400   
Parity  None None
Data Bits 8 8
Stop Bits 1 1
Carrier Detect (CD) No No
Ring Indicator (RI) No No
Data Set Ready (DSR) No Yes
Clear To Send (CTS) No Yes
UART Chip Used 8250 16550AF

Microsoft Windows also uses the current settings MSD reports. Hardware added after the installation of Windows may require you to manually change the COM Port settings in Windows Control Panel. They must match the hardware settings described later in this document.

Determine which IRQ a COM Port is using in Windows 95:

Click Start > Settings > Control Panel.
Double-click the System icon.
Click the Device Manager tab.
Double-click Ports (COM and LPT).
Select the COM Port the modem is installed on, and click on Properties.
Click the Resources tab.
The IRQ is listed as Interrupt Request.

Check these areas to find the IRQ setting on network cards:

  • NVER: May display IRQ number
  • NET.CFG: A configuration file that may display your IRQ number
  • IPX: If using, try IPX /i
  • Install Program: For software configurable cards, the original install program may display the IRQ
  • Network Card: Check the card itself for jumper settings. This requires the network card reference manual
  • MSD: Might display a "Network Driver Name" in the "Handled By" section of IRQ status
  • NDIAGS: Diagnostic utility shipped with Norton Utilities. This can give a detailed description of COM port and IRQ usage

Check these areas to find the IRQ setting on sound cards:

    This is a DOS environment setting that can be found in the AUTOEXEC.BAT. Many popular sound cards use this setting. It would appears as: SET BLASTER=A220 I7 D1. The I7 suggests the card is at IRQ7
  • Setup Program
    Might display the current IRQ settings
  • Control Panel
    This Windows utility may have setup information in the Drivers Section

As mentioned earlier, any software means of determining the IRQ setting of any piece of hardware with software may not be 100% reliable.

PCMCIA modems

Users of PCMCIA modems have a few additional issues when resolving hardware problems. Computers with these slots have Card Service, Card ID, and Socket driver configurations, which the computer manufacturer provides. Some PCMCIA modem cards come with their own socket drivers. If you experience problems with installing the modem, try using the drivers from the modem manufacturer instead. Typically, the configuration for the COM port and the IRQ for the PCMCIA slot are through these software drivers. These drivers usually load in the CONFIG.SYS. Here is a sample CONFIG.SYS:

DEVICE=C:\MHZ\CARDID.EXE (Determines card ID, available IRQs)
DEVICE=C:\MHZ\SS365SL.EXE /ADA=0 /SKT=1 (Socket services driver)
DEVICE=C:\MHZ\CS.EXE /IRQ=B (Card services driver)

Software configuration

After you configure the hardware device, you must configure the communications software as well (and Windows if applicable) so it can find the modem.

Configure custom COM ports in pcANYWHERE for DOS

In pcANYWHERE for DOS, use the Custom Ports settings for a non-standard COM port (for example, COM3 using IRQ5). In the Hardware Configuration screen, change the Device/Port to Serial:Custom, then set the port address and IRQ to match that of the modem's settings.


MS-DOS only recognizes IRQ 0 through 7.

Configure custom COM ports in pcANYWHERE for Windows

Set the Port and IRQ properly from the Windows Control Panel, and select the communication port in pcANYWHERE for Windows (see below). This applies to Call a Host PC, Call an Online Service, and Be a Host PC. You must deactivate the DOS TSR prior to changing these settings.

Configure the Windows 3.1x Control Panel ports:

In Program Manager, open the Main Group.
Open the Windows Control Panel.
Double-click Ports. The Ports configuration screen appears, displaying four communications ports, COM1-COM4.
Double-click the COM port for the communications device. The Settings for COM#: dialog box appears.
Click Advanced.
Click the IRQ button, and select the hardware's IRQ, for example IRQ5.
Click the Base I/O Port Address button, and then click Default.

Configure Windows 95 ports:

Open the Windows 95 Control Panel.
Double-click the System icon.
Click the Device Manager tab.
Click Ports (COM & LPT). The device ports available for configuration appear.
Select the COM port for the communications device.
Click Properties.
Click the Resources tab. The current port address (Input-Output Range) and IRQ (Interrupt Request Line) settings for that port appear.
Uncheck the Use automatic settings option.
Double-click the Interrupt Request Line setting.
Choose the appropriate IRQ to match the modem's settings.

Set the COM port in pcANYWHERE for Windows:

Click the Configure menu, and then click System Hardware.
Highlight your Hardware Configuration, for example "Modem."
Click Edit. The Hardware Setup Entry dialog box appears.
Click Device, and select the communications port you edited in the Control Panel.
If you edited COM Port 3 in the Control Panel, select COM3 in pcANYWHERE. (Version 1.0 users should not select Custom.)
Continue configuring the information in the Modem group.
Click OK when the configuration is complete.

NOTE Note When using pcANYWHERE for Windows Version 1.0, run the Host Install icon after you have configured the communications ports. It is in the pcANYWHERE group in Windows. When the Host Install screen appears, uncheck the "Run DOS applications full-screen" option. Follow the instructions for rebooting or restarting Windows. If you wish to enable the DOS host TSR (after determining that the modem is functioning correctly), run the pcANYWHERE Host Install icon again and re-check that option.

print this documentDocument ID:1996112511055
Last Modified:01/16/2007

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Product(s): pcAnywhere 9.0
Operating Systems(s): All Supported
Date Created: 11/25/1996

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