Microsoft KB Archive/92447
Windows 3.1 and Serial Communications
The information in this article applies to:
- Microsoft Windows versions 3.1, 3.11
INTRODUCTION ============ 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. WHAT'S NEW IN WINDOWS VERSION 3.1 ================================= 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 performance. 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 sharing.) 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. SERIAL VS. PARALLEL PORTS ========================= 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). PORT ADDRESSES AND IRQS ======================= 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.) 16550AFN UART CHIP SUPPORT ========================== 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. MS-DOS-BASED VS. WINDOWS-BASED APPLICATIONS =========================================== 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. IRQ SHARING =========== 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. BIOS DATA AREA AND ADDRESS PACKING ================================== 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 COMMUNICATIONS SOFTWARE AND WINDOWS 3.1 ==================================================== 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 Windows?" 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. THIRD-PARTY COMMUNICATIONS DRIVERS ================================== 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 update. 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 note. 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. RESOLVING IRQ CONFLICTS ======================= 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 procedure.) 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 marks). 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 available). 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 results. 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 number. 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: COMIrqSharing=TRUE 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 specifications. ADDRESS PACKING =============== 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: E40:0 F8 03 F8 02 E8 03 E8 02 q 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: DEBUG < FIXCOM.DEB > NUL The "> NUL" ending keeps the Debug script from echoing to the screen. 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. SERIAL DEVICES ON COM3 AND COM4 DO NOT REGISTER =============================================== 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. DEVICE CONTENTION ================= 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 control. 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 warning. 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." THIRD-PARTY DRIVERS =================== 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 Notepad. 2. Under the [boot] section, look for the following entry: comm.drv=comm.drv 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: device=*vcd device=*combuff 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: COMMdrv30=TRUE Once you obtain an update for your third-party driver, you can remove this entry from your SYSTEM.INI file. TROUBLESHOOTING COMMUNICATIONS PORT PROBLEMS ============================================ 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. COMMUNICATIONS APPLICATION CANNOT ACCESS A SERIAL PORT ====================================================== 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" PC LOCKS UP WHEN ACCESSING SERIAL PORT ====================================== 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" ERROR MESSAGES ============== "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." MODEM PROBLEMS ============== 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. MOUSE PROBLEMS ============== 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 type. 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 using. 4. Choose the OK button to close the dialog box. COM4 PROBLEMS ============= 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 note. 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 RUNNING WINDOWS IN 386 ENHANCED MODE ============================================= 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 AUTOEXEC.BAT file 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." --------------------------------------------------------------------
The following file is available for download from the Microsoft Download Center:
For additional information about how to download Microsoft Support files, click the article number below to view the article in the Microsoft Knowledge Base:
Q119591 How to Obtain Microsoft Support Files from Online Services
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Additional query words: gpf 3.10 comm port lpt 1.0
Keywords : kbfile win31 kbgraphxlinkcritical
Issue type : kbinfo
Technology : kbWin3xSearch kbWin310 kbWin311
Last Reviewed: June 13, 2001