Base de Dados de Conhecimento

Artigo: 92447 - Última revisão: quarta-feira, 4 de Agosto de 2004 - Revisão: 3.3



This document provides information about serial communications
in the Microsoft Windows version 3.1 operating system environment. The
first section, "What's New in Windows Version 3.1," gives an overview
of improvements in Windows 3.1. The section "Conceptual Overview"
explains basic and advanced concepts in serial communications,
including port addresses, device contention, and so forth. The section
"Why Conflicts Occur" discusses the most common reasons why you may
experience difficulties with serial communications, and the section
"How to Resolve Conflicts" explains what to do once you have
identified the problem.

If you need help with a specific problem, you may want to start by
reading the "Troubleshooting" section.


Here are some highlights about improvements in how Windows 3.1 handles
serial communications.

Performance Improvements

 - Windows 3.1 supports the high-speed 16550AFN UART buffer; Windows
   3.0 does not. (The 16550AFN buffer is supported for Windows-based
   applications only.)

 - Windows 3.1 supports baud rates of up to 57.6K. (You may need to
   update your application to take advantage of these higher baud
   rates.) Some users experienced lockups during high-speed, full-
   duplex transfers while using Windows 3.0. These problems are
   resolved in Windows 3.1.

 - Windows 3.1 has a new serial driver interface that allows data to
   be passed in blocks, instead of one character at a time. (Each
   block is treated as a single character.) This means faster data
   throughput and fewer transmission errors. Windows 3.0 passes data
   one character at a time.

 - Windows 3.0 uses a "constant polling" method to determine when
   serial ports are occupied. Windows 3.1 uses the
   EnableCommNotification API; this enables applications to directly
   notify Windows of serial events, thereby increasing overall system

Better Support for Your Ports

 - Windows 3.1 provides better support for COM ports 3 and 4. You can
   now configure these ports by using Windows Control Panel.

 - Windows 3.1 fully supports IRQ sharing; Windows 3.0 does not. (To
   take advantage of this feature, your PC must have Extended Industry
   Standard Architecture (EISA) or Micro Channel(R) Architecture
   (MCA), or you must have a serial I/O card that supports IRQ

                          CONCEPTUAL OVERVIEW

This section provides a general introduction to the subject of serial
communications and explains many of the concepts and issues that are
discussed throughout the "Windows 3.1 and Serial Communications"
Application Note.


Input/output (I/O) ports provide the means for your PC to send or
receive data through external devices such as modems, printers, fax
machines, and so forth. Virtually all PCs have both parallel and
serial ports. There are several distinctions between serial and
parallel I/O ports.

Parallel ports are generally used only for printers, which is where
the name LPT, for line printer, originates. Parallel ports transmit
data in complete bytes, and in most cases they handle outgoing data
only; they cannot receive information. (The IBM[R] PS/2[R] parallel
ports are bidirectional; in addition, you can use certain peripherals,
such as network card adapters, to enable your ports to transmit data
bidirectionally.) You do not configure parallel ports; they are
preconfigured for your machine.

Serial ports are generally used for communications, or COM devices,
which include serial printers, modems, fax machines, and so forth.
Serial ports are capable of both sending and receiving data. You can
configure settings--such as parity, stop bits, and data length--for
your serial ports. (In Windows, you can specify these settings by
using Control Panel.) Serial ports transmit data one bit at a time,
sequentially (or serially).


Every time you start your PC, your system BIOS checks your COM ports
to see what serial devices are installed on your PC and posts this
information to the system BIOS data area (BDA). Windows "reads" the
BDA, and uses the default interrupt request line (IRQ) for each device
that is registered there. Usually, each COM port requires a unique IRQ
to communicate with your PC. Certain PCs and some I/O cards support
IRQ sharing.

  NOTE: Some earlier BIOS versions may not check for devices on COM
  ports 3 and 4. If you have devices installed on COM port 3 or 4 and
  your system BIOS does not recognize these ports, you need to
  register the addresses for COM3 and COM4 by using Windows Control
  Panel. This is explained in the procedure "To register a serial
  device" later in this Application Note.

How Should My COM Ports Be Configured?

Generally, PCs come with built-in ports COM1 and COM2 preset to the
following values:

  Port       Address       IRQ
  COM1        03F8          4
  COM2        02F8          3

In most cases, the default values for COM ports 1 and 2 should work
for your system. You can view your COM port settings by using Windows
Control Panel.

Most PCs do not have built-in ports for COM3 and COM4. The default
settings for these additional ports are listed here:

  Port              Address        IRQ
  COM3               03E8           4
  COM3 (PS/2)        3220           3
  COM4               02E8           3

Because COM1 and COM3 both use IRQ4 as the default, and COM2 and COM4
both use IRQ3, you may need to reassign the IRQ if you use serial
devices (such as a fax card or modem) on COM ports 3 or 4. You can
reassign IRQs by using Control Panel. For more information, see the
procedure "To reassign the IRQ" later in this Application Note. (You
do not need to reassign the IRQ if you have an MCA or EISA machine, or
a serial I/O card that supports IRQ sharing.)


Every serial port uses a Universal Asynchronous Receiver-Transmitter
(UART) chip to send and receive data. Inside your PC, data travels in
parallel form. When you send or receive data by means of a serial
device, the UART chip converts the data to a serial format so that
your port can read it.

Many machines still use either the 8520 or the 16450 UART chip. The
main advantage of the 16550AFN UART chip is its first in, first out
(FIFO) buffering scheme, which can dramatically improve performance
for modem transfer speeds of 9600 baud or higher. If you usually set
your modem to 2400 or 1200 baud, chances are you don't need the
16550AFN UART.

  NOTE: The 16550, an earlier version of the chip, was used in IBM
  PS/2 computers, models 50, 60, and 80, and replaced by the 16550AFN
  in the PS/2 model 70. You may want to check with your manufacturer
  to determine which version of the 16550 chip is installed in your
  machine. The 16550 chip does not support FIFO buffering.


Windows-based applications share a single time slice; each MS-DOS-
based application running under Windows receives its own time slice. A
time slice is the amount of processor time allocated to an
application, usually measured in milliseconds. The smaller the time
slice, the more efficiently Windows can run multiple tasks; therefore,
in general, you will notice better performance with Windows-based
applications running under Windows. You can also use "Windows-aware"
software to emulate timeslice sharing for MS-DOS-based applications
and thereby improve performance.

You run MS-DOS-based applications under Windows by writing PIF files
that specify how you want Windows to schedule processor time for these
applications. (For more information about PIF files, refer to Chapter
8, "PIF Editor," in the version 3.1 "Microsoft Windows User's Guide.")

                          WHY CONFLICTS OCCUR

For serial communications to be successful, three settings must match:
the port addresses in the BIOS data area, the COM port entries in the
SYSTEM.INI file, and the switches on your serial hardware (if you have
any installed). This section discusses the situations that can occur
if these settings do not match, as well as other situations that can
cause conflicts.

Determining whether these settings match, and changing them if they do
not, is discussed in the section "How to Resolve Conflicts" later in
this Application Note.


Many serial communications problems result from two devices attempting
to use the same IRQ. For example, if you install a serial mouse on
COM1 and a modem on COM3, both devices could attempt to use IRQ4 (the
default setting) at the same time. Unless your PC supports IRQ sharing
(the ability to have multiple ports using the same IRQ), this could
cause your mouse or modem to lose functionality.

MCA and EISA machines support IRQ sharing. In addition, some
serial I/O cards support IRQ sharing even if your machine is not
specifically configured to do so. Windows 3.1 fully supports such
serial I/O cards.

For more information, see the "IRQ Sharing" section under "How to
Resolve Conflicts" later in this Application Note.


There are four sequential spaces in the BDA for COM port addresses 1
through 4. The BIOS lists port addresses so that no blank spaces occur
between addresses. Windows reads the BDA sequentially and actually
expects a blank address where there is no COM port. This can cause
conflicts when Windows assigns IRQs.

For example, if you have a mouse on COM1, a printer on COM2, and a
modem on COM4, Windows expects the BDA to look something like this:

                (COM1)        (COM2)        (COM3)        (COM4)
  0040:0000     F8  03        F8  02        00  00        E8  02

Because the BIOS does not use zeros as placeholders, however, the BDA
actually looks something like this:

                (COM1)        (COM2)        (COM3)        (COM4)
  0040:0000     F8  03        F8  02        E8  02        00  00

This behavior is known as address packing, or address shifting.
Windows assigns IRQs in the order that it finds a COM port address. In
this situation, Windows would assign IRQ4 (the default for COM3) to
the device that is actually on COM4. This could cause loss of
functionality on COM4 because IRQ4 is already being used by COM1.

BIOS packing can affect COM ports 1 and 2 as well. For instance if you
have no device on COM1, your COM2 address may register in the COM1
BIOS area.

Whether or not you experience problems as a result of address packing
may depend on what COM ports you are using, what type of hardware you
have, and whether you are using MS-DOS-based communications software.

To correct this type of situation, you write a Debug script. A Debug
script records the correct port addresses in your system memory. This
procedure is described in the section "How to Resolve Conflicts" under
'"Address Packing."


MS-DOS-based applications and Windows-based applications handle serial
communications in different ways. This section discusses guidelines
for using your MS-DOS-based communications programs under Windows 3.1.
For more information, see chapters 7, "Non-Windows Applications," and
8, "PIF Editor," in the version 3.1 "Microsoft Windows User's Guide."

General Guidelines

You use 386 enhanced mode Windows, and PIF files, to give Windows
control of your MS-DOS-based communications applications. We don't
recommend running MS-DOS-based communications applications in standard
mode Windows, as this may cause data loss or other problems.

For better performance, run your MS-DOS-based applications in full-
screen mode rather than in a window. If you run an MS-DOS-based
communications application in the background during a data transfer,
run it minimized as an icon rather than in a window.

PIF File Guidelines

 - Select the Background check box (under Execution in the main PIF
   Editor dialog box), or your MS-DOS-based application will stop
   running when you switch away from it.

 - Select the Lock Application Memory check box (in the PIF Editor
   Advanced Options dialog box), or you may receive an error message.
   (This setting applies only if you have a permanent swap file and 32-
   bit disk access is enabled.)

 - If you encounter errors during data transfers, specify a larger
   Background Priority or Foreground Priority setting (in the
   Multitasking Options area of the Advanced Options dialog box).

Device Contention

In the Windows multitasking environment, two applications sometimes
simultaneously request the use of a device (such as a COM port or
modem). This is known as device contention. When you are running MS-
DOS-based communications applications under Windows in 386 enhanced
mode, you need to set device contention options in Control Panel.
Doing so specifies how you want Windows to handle device requests from
your MS-DOS-based applications.

Problems that can occur as a result of serial device contention
include the inability to access a COM port. For more information, see
the procedure "To set device contention options" later in this
Application Note.

UART Support for MS-DOS-Based Applications

Windows 3.1 provides application support for the new 16550AFN UART
buffer (see the section on the UART earlier in this Application Note).
However, MS-DOS-based applications may not recognize the 16550AFN
UART, treating it instead like the earlier 8250 version. This, in
turn, may result in data loss. Whether you experience these problems
also depends to a certain extent on your hardware manufacturer.

Pacific Commware's TurboCom is an advanced serial port communications
driver for Windows that provides UART support for MS-DOS-based
applications running under Windows. TurboCom also supports IRQ sharing
and baud rate speeds of up to 57.6K. If you regularly use MS-DOS-based
communications software or software that requires a baud rate of 9600
bits per second (bps) or higher, you may want to use the TurboCom
driver. For additional information, contact your local dealer, or call
Pacific Commware at (510) 540-8080.

  NOTE: If your communications software runs at 2400 baud or lower,
  using the 16550AFN UART probably won't noticeably improve your
  system's performance.
Address Packing and MS-DOS-Based Applications

If Windows assigns an IRQ that is being used by an MS-DOS-based
program, you may receive the message "The COM port is currently
assigned to a DOS application. Do you want to reassign the port to

The problem occurs when your machine BIOS packs addresses before
posting them to the BDA. Windows reads the BDA sequentially, assigning
the default IRQ for COM1 to the first entry it finds, the IRQ for COM2
to the second entry, and so forth. For example, if you have a device
on COM2 but not on COM1, and your system BIOS packs addresses, the
COM2 address "shifts" into the BDA slot for COM1. Consequently,
Windows assigns IRQ4 (the default for COM1) to the device that is
actually on COM2.

MS-DOS-based communications programs access the COM ports directly,
without posting their port address to the BDA. Using the same example,
if you have no serial device on COM1, and your MS-DOS-based
communication program is using COM2, the MS-DOS-based program still
uses the default IRQ for COM2.

You can remedy this situation by writing a Debug script. For more
information, see the procedure "To write a Debug script" later in this
Application Note.


Some Windows-based third-party programs install their own drivers to
replace the drivers provided by Windows. If these third-party drivers
are installed in your system when you upgrade from Windows 3.0 to
Windows 3.1, Setup does not install the Windows communications driver
(COMM.DRV). In most instances, it is best to use the Windows 3.1
driver, rather than a driver designed for Windows 3.0.

If you are experiencing driver problems (such as an inability to use
higher baud rates, or problems accessing the modem), you can reinstall
COMM.DRV or contact the manufacturer of your third-party driver for an

You can tell whether you have the Windows communications driver, or
whether it has been replaced by the driver(s) that came with your
communications software, by looking in your SYSTEM.INI file. See the
procedure "To check for third-party drivers" later in this application

                       HOW TO RESOLVE CONFLICTS

You should now be familiar with the most common reasons for serial
communications problems in Windows 3.1. This section describes a few
simple procedures you can follow to correct these problems.


If you have serial devices on both COM1 and COM3, or COM2 and COM4,
and your machine architecture does not support IRQ sharing, you need
to reassign the IRQ for COM3 or COM4. (If you are using a serial card
that supports IRQ sharing, read the section that follows this

To properly reassign the IRQ:

1. Determine the valid IRQ settings for the serial device by referring
   to the manufacturer's documentation.

   Most serial devices such as I/O cards or internal modems can use
   either of several valid IRQs. For example, a modem on COM3 may be
   able to use IRQ5 if IRQ4 is not available.

2. Determine which IRQs are available on your PC.

   You can do this by referring to the table provided below, or you
   can use the Microsoft Diagnostic (MSD) utility. (Refer to the
   procedure, "To use MSD to determine available IRQs" later in this
   Application Note.) MSD is included with Windows version 3.1.

3. Point both your serial device and Windows to the IRQ that you have
   determined is valid and available.

   Set your serial device to the alternate IRQ by following the
   directions in the manufacturer's documentation. Point Windows to
   the alternate IRQ by using Control Panel to place an entry in your
   SYSTEM.INI file.

    NOTE: Your serial device setting MUST match the COMxIRQ setting in
    your SYSTEM.INI file. If these two settings do not match, the COM
    port will not function properly.

Standard IRQ Settings (Table)

The following table shows the most common IRQ settings. You can use
Control Panel to specify an IRQ from this table if it matches your
hardware requirements. Or, if you'd like to view a table similar to
this that lists the current settings in your PC, you can use MSD.

       IRQ Number       Description
            0           Timer
            1           Keyboard
            2           Link to IRQs 9-15
            3           COM2, COM4
            4           COM1, COM3
            5           LPT2, or Reserved
            6           Floppy disk controller
            7           LPT1, LPT3
            8           Real time clock
            9           Redirected IRQ2
           10           Reserved
           11           Reserved
           12           PS/2 mouse
           13           Math coprocessor
           14           Hard disk controller
           15           Reserved

The IRQs with "Reserved" entries in the corresponding Description
column are often available. (This does not guarantee they are
currently available on your PC.) You can use Control Panel to choose
one of the "Reserved" IRQs, such as IRQ10, and then restart your
system to see whether the COM port now works.

  NOTE: You must also change the hardware setting to the same IRQ. To
  do this, follow the manufacturer's instructions.

If the first IRQ you choose does not work, you can try the next IRQ
from this table, and so forth. If you do not have a PS/2 mouse port,
you may be able to use IRQ12. Or, you can use MSD to view the IRQs
that are actually available on your PC.

To Use MSD to Determine Available IRQs

1. Quit Windows.

2. At the MS-DOS command prompt, type "msd"(without the quotation

   An initial screen appears, providing information about MSD. Choose
   the OK button to view the MSD main menu.

3. From the MSD main menu screen, choose the IRQ Status button.

   The IRQ Status screen shows which IRQs are currently assigned (not

4. Make a note of each IRQ number where the entry in the corresponding
   Detected column reads No.

   A No entry in the Detected column indicates that the corresponding
   IRQ is available. If you do not find a "No" entry in the Detected
   column, there are no available IRQs, and you should not attempt to
   reassign IRQs.

5. Use Control Panel to assign one of the available IRQs to the COM
   port whose IRQ you want to change.

   See the next procedure. You must also change the hardware setting
   to the same IRQ. Follow the manufacturer's instructions.

  NOTE: Windows 3.1 does not support nonstandard IRQ settings. If your
  serial communications device requires an IRQ that is different than
  what is described in the MSD listing, we cannot guarantee the

To Reassign the IRQ from Control Panel

1. In the Main group, choose the Control Panel icon.

2. In the Control Panel window, choose the Ports icon.

3. In the Ports dialog box, select the COM port for the device whose
   IRQ you want to change, and then choose the Settings Button.

4. In the Settings dialog box, choose the Advanced button.

5. Open the Interrupt Request Line (IRQ) list box, and select a

   This number must match your hardware setting.

6. Choose the OK button twice to exit the settings dialog boxes, and
   then choose the Close button to exit the Ports dialog box.

IRQ-Sharing Serial I/O Cards

If you are using an IRQ-sharing I/O card on a machine that does not
have MCA or EISA architecture, you need to add the following line to
the [386Enh] section of the SYSTEM.INI file:


  NOTE: This setting does not, by itself, provide your system with IRQ-
  sharing support. You must have the correct hardware configuration,
  and the hardware must be installed according to the manufacturer's


If you are using an MS-DOS-based communications program, Windows may
assign an IRQ to a Windows serial device that is already in use by
your MS-DOS-based program. If this happens, you may receive the error
message "The COMx port is currently assigned to an MS-DOS application.
Do you want to reassign the port to Windows?"

To remedy this situation, you need to familiarize yourself with the
BDA. (This error message may also occur if you have a BIOS that does
not search for serial devices on COM ports 3 or 4. If this is the
case, you can use Control Panel to register the devices.)

To View the BDA by Using Debug

1. Quit Windows.

2. At the MS-DOS command prompt, type "debug" (without the quotation
   marks), then press ENTER.

   A hyphen (-) prompt appears.

3. Type "d40:0" (without the quotation marks), then press ENTER.

   This returns a listing of the BIOS data area (BDA). The first row
   of the BDA shows the COM port and LPT port addresses, divided by a
   center hyphen. COM ports are listed to the left of the hyphen; LPT
   ports are listed to the right.

4. Once you have interpreted the BDA (see the following procedure),
   type "q" (without the quotation marks) to quit Debug.

To Interpret the BDA COM Address Entries

This table shows the four standard COM addresses as they should be
listed in the BDA. (The COMx headings do not appear in the BDA.)

                (COM1)        (COM2)        (COM3)        (COM4)
  0040:000      F8  03        F8  02        E8  03        E8  02

If you have a serial device on COM port 3 or 4, and the BDA does not
show an entry for the device, you can use Control Panel to register
the device in the BDA. However, if the BDA shows any of the standard
addresses in a location other than what is shown here (for example,
address F8  02 in the space for COM1, or address E8  02 in the space
for COM3), then you need to write a Debug script.

  NOTE: If you see an address in your BDA that is not shown here, your
  hardware may use nonstandard serial port addresses. In this case you
  need to register the nonstandard address by using Control Panel. See
  the procedure "To register a serial device."

To Write a Debug Script

1. Open any standard text editor, such as Windows Notepad.

2. Type the following three lines:

      F8 03 F8 02 E8 03 E8 02

   You must press ENTER after typing "q" (without the quotation
   marks). If you do not, the Debug script will cause your system to
   remain indefinitely in Debug mode.

3. Save the file as FIXCOM.DEB, or any filename with a .DEB extension.

4. Add the following line to the end of your AUTOEXEC.BAT file:


   The "> NUL" ending keeps the Debug script from echoing to the

  NOTE: Do not place this entry after a program execution line, such
  as WIN. If you do, the Debug script will not execute.

5. Save your AUTOEXEC.BAT file and reboot your PC.

You can view the BDA again to see whether the address changes have
   taken effect.

  NOTE: The port address setting on your physical serial device must
  match what you specify in the Debug script. If your hardware
  requires a nonstandard address, you can specify that address in the
  Debug script.


Some earlier BIOS versions only recognize devices installed on COM
ports 1 and 2. If your system BIOS does not recognize serial devices
on COM ports 3 and 4, you can use Control Panel to notify Windows that
you have devices installed at these locations. However, if your BIOS
registers such devices but incorrectly records them at different
addresses in the BDA (due to address shifting or packing), you cannot
use Control Panel to correct this situation. Instead, you need to
write a Debug script. Refer to the previous procedure.

To determine whether your system BIOS registers devices on COM port 3
or 4, you need to read the BDA. Refer to the procedure "To view the
BDA by using Debug."

  CAUTION: Setting the port address by using Control Panel does not
  override a BDA value. What the BIOS reads replaces any entries you
  make by using Control Panel, if there is a discrepancy between the
  two. This procedure is not a substitute for writing a Debug script
  if your BIOS incorrectly registers COM addresses.

To Register a Serial Device

1. In the Main group, choose the Control Panel icon.

2. In the Control Panel window, choose the Ports icon.

3. In the Ports dialog box, select the port you want to identify to
   Windows, and then choose the Settings button.

4. In the Settings dialog box, choose the Advanced button.

5. Open the Base I/O Port Address list box and select an address.
   Check your hardware documentation for the appropriate value.

6. Open the Interrupt Request Line (IRQ) list box, and select an IRQ.
   Check your hardware documentation for the appropriate value.

Selecting values in this dialog box places COMxBase and COMxIRQ
entries in your SYSTEM.INI file.

For more information on COMxBase and other SYSTEM.INI file entries,
refer to the SYSINI.WRI file in your Windows directory.


You need to specify the device contention options for each MS-DOS-
based communications application you are running under Windows.

To Set Device Contention Options

1. In the Main group, choose the Control Panel icon.

2. In the Control Panel window, choose the 386 Enhanced icon.

3. Select the port that is connected to the device you want to

4. In the Device Contention group, select the option you prefer. Each
   option is described below.

   Always Warn

   Select this option to have Windows display a dialog box whenever
   more than one application requests use of a COM port at the same
   time. The dialog box lets you specify which MS-DOS-based
   application should receive control of the COM port. In most cases,
   you should select Always Warn.

    NOTE: The Always Warn option does not replace IRQ sharing. When
    you specify that one application receive control of a COM port,
    the other application requesting to use the port is interrupted.
    Optimally, once the first application completes its operation (for
    example, a fax transmission), the COM port becomes available again
    and the other application can resume. However, sometimes giving
    control of a COM port to one application over another can
    indefinitely hang the application that is required to wait.

   Never Warn

   When you select this option, Windows provides COM port access to MS-
   DOS-based applications even if it detects that the requested port
   may already be in use by another application.

   Selecting this option may cause unexpected problems, such as data
   loss or an interruption during application processing.

   Idle (in sec.)

   Select this option to specify the number of seconds Windows should
   wait before giving port access to an application without notifying
   you. For instance, if you specify 10 seconds, Windows displays a
   dialog box when any application requests the use of a port that was
   occupied within the last 10 seconds. The dialog box lets you
   specify which MS-DOS-based application should receive control of
   the COM port. Otherwise, Windows provides COM port access without a

For more information about these options, see page 247 of Chapter 7,
"Non-Windows Applications," in the version 3.1 "Microsoft Windows
User's Guide."


The Windows 3.1 Setup program updates many drivers that were included
in Windows 3.0. Setup cannot update third-party drivers, however. You
should obtain updates from the manufacturer for any third-party
drivers that you are using under Windows 3.1.

To Check for Third-Party Communications Drivers

1. Open your SYSTEM.INI file by using any text editor, such as Windows

2. Under the [boot] section, look for the following entry:


   If this entry reads differently, then your system is using a third-
   party Windows-level driver.

3. Under the [386Enh] section, look for the following entries:


   If either of these entries reads differently, or if there is an
   additional entry, then your system is using third-party
   communications drivers.

To determine whether you need an update, contact your manufacturer.

To use third-party drivers that have not been updated, you need to add
the following line to the [386Enh] section of your SYSTEM.INI file:


Once you obtain an update for your third-party driver, you can remove
this entry from your SYSTEM.INI file.


This section can help you to identify what is causing a problem and
then point you to the appropriate section in this Application Note for
an explanation of how to resolve the problem.


Are you using MS-DOS-based communications software? Refer to the "MS-
DOS-Based Communications Software and Windows 3.1" section in this
Application Note.

Related procedure:
"To set device contention options"

Is the serial device using an available IRQ? Refer to the "Resolving
IRQ Conflicts" section in this Application Note.

Related procedures:
"To use MSD to determine available IRQs"
"To reassign the IRQ from Control Panel"


Is the serial device using a unique IRQ? Refer to the "Resolving IRQ
Conflicts" section in this Application Note.

Related procedures:
"To use MSD to determine available IRQs"
"To reassign the IRQ from Control Panel"

Is the serial device on COM4? Refer to the "Serial Devices on COM3 and
COM4 Do Not Register" section.

Related procedure:
"To register a serial device"


"The COM Port is Currently Assigned to a DOS Application..."

If you receive this error message, refer to the procedure "To write a
Debug script."


Does your system BIOS recognize your internal modem? Refer to the
"Address Packing and MS-DOS-Based Applications" section for
information about reading the BDA.

Related Procedures
"To view the BDA by using Debug"
"To interpret the BDA COM address entries"
"To register a serial device"

Do you have a modem on COM3 and a mouse on COM1, or a modem on COM4
and a mouse on COM2? Unless your machine or serial I/O card supports
IRQ sharing, this configuration is not recommended. Refer to the
"Resolving IRQ Conflicts" section in this Application Note.

Related Procedure:
"To reassign the IRQ from Control Panel"

Do you have an internal modem and a serial I/O card? Have you observed
any of the following symptoms:

   The modem dials and rings but does not connect.
   The system reboots when the modem attempts to dial out.
   The system reboots when the modem should have connected.
   The cursor does not work in Window Terminal.

If your internal modem uses a COM port that is addressed by your
serial I/O card, you may experience IRQ conflicts. To correct this
problem, disable the COM port setting on the serial I/O card. For
example, if your internal modem is set to use COM2, and you have a
serial I/O card that recognizes COM2, you may need to disable COM2 on
the serial I/O card for the modem to work correctly.


General Guidelines

The mouse uses a different driver for MS-DOS-based applications than
it does for Windows-based applications. In general, all mouse drivers
should come from the same vendor. For example, do not use a Microsoft
driver as the Windows mouse driver and a third-party driver as the MS-
DOS mouse driver.

Do not install a modem on COM3 if you have a mouse on COM1, or a modem
on COM4 if you have a mouse on COM2, unless your machine or serial I/O
card supports IRQ sharing. Refer to the "Resolving IRQ Conflicts"
section in this Application Note.

Mouse Doesn't Work in Enhanced Mode

If your PC has no COM1 or if you have disabled COM1 and you have a
mouse installed on COM2, you may experience problems using the mouse
in 386 enhanced mode. Refer to the "BIOS Data Area and Address
Packing" section in this Application Note.

Logitech(TM) Mouse

The Logitech virtual mouse driver (LVMD.386) is hard coded for the
standard base port addresses and IRQs. If you use the Logitech mouse
with a nondefault port address or IRQ, it may fail to work in 386
enhanced mode.

100%-Microsoft-Compatible Mouse

Any "100% Microsoft compatible" mouse that you purchase from your
dealer is not interchangeable with the Microsoft mouse. If your 100%-
Microsoft-compatible mouse comes with a driver disk that contains both
Windows and MS-DOS mouse drivers, install these in place of the
Microsoft mouse drivers. Installing such drivers places a new setting
in the Mouse section of Windows Setup. After you install a 100%-
Microsoft-compatible mouse, use Windows Setup to select your mouse

Microsoft Mouse

If you are using the Microsoft Mouse with Windows 3.1, make sure each
of the following is true:

 - You are using version 8.2 or 8.2a of MOUSE.COM and MOUSE.SYS (the
   mouse drivers for MS-DOS-based applications). Version 8.2 or 8.2a
   of the driver is included with Windows 3.1.

 - MOUSE.DRV (the mouse driver for Windows-based applications) is in
   the Windows SYSTEM subdirectory, is the only copy of this file on
   your system, and is the most current version of the driver.

 - The mouse is assigned a unique IRQ (unless the serial port supports
   IRQ sharing). For more information, see the "IRQ Sharing" section
   in this Application Note.

 - The mouse is not installed on COM3 or COM4.

To see what version of the MS-DOS mouse driver you have

At the MS-DOS command line from the MS-DOS mouse directory (usually,
MSMOUSE), type:

   mouse /?

To see what Windows mouse driver you are using

>From the Main group, choose the Windows Setup icon. The mouse type
should be "Microsoft, or IBM PS/2."

To change your Windows mouse driver

1. In the Main group, choose the Windows Setup icon.
2. From the Options menu, choose Change System Settings.
3. Open the Mouse list box and select the type of mouse that you are
4. Choose the OK button to close the dialog box.


If your communications software doesn't recognize a serial device on
COM port 3 or 4, you may have a system BIOS that was manufactured
before those COM ports became standard. Refer to the "Serial Devices
on COM3 and COM4 Do Not Register" section earlier in this application

The COM4 default address of 02E8 conflicts with some peripheral
devices, including the 8514/A, Ultra (ATI), and S3 (Orchid Fahrenheit
1280 STB WIND/X, Diamond Stealth VRAM) display adapters, and certain
network adapters. Do not readdress COM4 in this situation. Contact
your hardware manufacturer for information about how to reset the
adapter's default address.


Problems of this type may be related to a number of causes, including
outdated driver files or IRQ conflicts. You can try the
troubleshooting steps outlined below if you experience any of the
following problems when trying to run Windows in 386 enhanced mode:

 -Windows stops running or returns to the MS-DOS prompt
 -General protection (GP) fault
 -Windows defaults to standard mode

If the steps outlined below do not help, then the problem is probably
not caused by a serial communications conflict. Refer to the Windows
Resource Kit for more information about problems running Windows in
386 enhanced mode.

To Check for Outdated Driver Files

Before starting Windows from the MS-DOS prompt, change to the Windows
directory, and then start Windows.

If Windows runs in 386 enhanced mode only when you start it from the
Windows directory, you may have an outdated driver file in some other
directory. When Windows starts, it looks for the files it needs in the
following order:

   The current directory
   The Windows directory
   The Windows SYSTEM subdirectory
   All the directories listed in the PATH statement in the
   All the directories in a network path

If you suspect that you have outdated driver files, you need to delete
them from your system.

To Delete Outdated Driver Files

1. Use File Manager to find all versions of the driver file(s) on your
   system. Compare each version by date.

2. Delete all other versions of the driver file(s) except the most
   current version. Retain a single copy of the most current version
   in the Windows SYSTEM subdirectory.

To Identify an IRQ Conflict

1. Remove any serial hardware you have installed on your system
   (mouse, network card, fax board, modem, and so forth) and then
   restart Windows.

   If Windows now runs in 386 enhanced mode with no problems, the
   problem may be caused by an IRQ conflict.

2. Reinstall each separate piece of serial hardware, one at a time,
   restarting Windows between each addition.

   This isolates the hardware that is causing the IRQ conflict. When
   the problem recurs, you can assume it is caused by the hardware you
   have just reinstalled.

3. Reassign the IRQ for the piece of hardware in question.

   Refer to the procedure in this Application Note on "To reassign the
   IRQ from Control Panel."

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