Microsoft KB Archive/83449

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Windows 3.1 Serial Communications Q & A Part 1

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Q83449

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The information in this article applies to:


 * Microsoft Windows 3.1

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SUMMARY
This article contains specific questions and answers regarding the Microsoft Windows operating system version 3.1.

MORE INFORMATION
 Q. How does Windows 3.1 improve serial communications over Windows 3.0?

A. Most of the serial communications problems experienced by users of Windows 3.0 were corrected in Windows 3.1. The following is a summary of the changes:

 COM port addresses and IRQs are configurable in the Control Panel. Windows 3.0 uses preset address values for serial ports. Windows 3.1 allows any port address and IRQ. This corrects problems using ports COM3 and COM4 (Windows 3.0 uses non-standard default values).

NOTE: If the communication settings in Terminal do not match those specified in Control Panel, Terminal's settings take precedence. The Advanced port settings, however, affect all applications including Terminal. Therefore, if you experiencing problems when using Terminal, verify the I/O base address and IRQ settings in Control Panel. Faster bit rates. Windows-based applications can now set speeds higher than 19.2 kilobits per second (Kbps) and up to 57.6 Kbps. Contact the application manufacturer for an update to support this new feature. Reduced system overhead. Windows 3.1 is fine tuned for better overall performance, including faster throughput and fewer errors during serial communications. A new driver interface allows Windows to pass data in blocks to the driver, rather than one character at a time. See the Windows 3.1 Device Development Kit (DDK) for details. 16550 UART support for Windows-based applications. This is described in detail below. EnableCommNotification API. Windows-based applications can register to be notified by a window message when serial port events occur, rather than performing constant polling. (Constant polling reduces overall system performance.) Serial ports on the same IRQ can be used in rotation. On most personal computers, serial ports that use the same IRQ cannot be used simultaneously; however, Windows 3.0 sometimes prevented them from being used in rotation, one after another. This problem was corrected in Windows 3.1. Full duplex improvements. Some users experienced lockups during full-duplex transfers at fast bit rates with Windows 3.0. These problems were corrected in Windows 3.1.</li></ol> </li> Q. How many serial ports does Windows support?

A. The Windows 3.1 communications driver provides access to standard MS-DOS serial ports (COM1, COM2, COM3, and COM4). Third- party serial adapters with more than four ports can be used in Windows with special drivers available from their manufacturers. Windows allows a maximum of nine serial ports (COM1-COM9).</li> Q. What base addresses and IRQs does Windows use for serial ports?

A. Windows must know the base I/O addresses and interrupt request lines (IRQs) of the serial ports on a personal computer. Windows determines this as follows:

<ol style="list-style-type: lower-alpha;"> Windows looks in the BIOS data area (40:0) first for port base addresses. Most personal computers specify address values here for built-in COM ports, but not for add-in adapters. If Windows finds an address here, it uses this, rather than any other defaults or settings.</li>  Serial ports commonly use the following base I/O address and IRQ values: <pre class="FIXEDTEXT">        Port     Address  IRQ --- ---         COM1 *    03F8      4 COM2     02F8      3 COM3     03E8      4 COM3(PS/2)3220     3 COM4     02E8      3 *If the BIOS data area specifies the COM1 address as 02F8, Windows uses IRQ3 instead of IRQ4. </li> Control Panel Ports Advanced dialog box. The addresses and IRQs of ports can be identified, although they are not specified, in the BIOS data area. Addresses and IRQs are modified by opening the Control Panel, choosing the Ports icon, and selecting the Advanced button. Here you can set the COMxBase and COMxIRQ values in the [386Enh] section of SYSTEM.INI file. IRQ values 8-15 are allowable entries. Setting the port address in this dialog box does not override a BIOS data area value.

For more information on COMxBase and other SYSTEM.INI file switches, refer to the SYSINI.WRI file in the WINDOWS directory.</li></ol> </li> Q. Windows 3.0 did not recognize the COM3 and the COM4 ports. The COM3Base and COM4Base settings in SYSTEM.INI have been changed; however, this has not helped. Is this problem fixed in Windows 3.1?

A. Yes. The problem in Windows 3.0 is that the communications driver (COMM.DRV) and virtual communications device (VCD) weren't using the same values. Now, both modules use the specified values. The SYSTEM.INI file should no longer be modified manually. Instead, use the Advanced dialog box in the Control Panel (accessed through the Ports icon). These settings apply to both standard and 386 enhanced modes.</li> Q. When can the same IRQ be used for different serial ports?

A. In general, two devices should not be used simultaneously with the same IRQ. For example, if COM1 and COM3 ports both use IRQ4 (the common value), problems arise if a serial mouse is on COM1 and a modem is on COM3.

Some personal computers and serial port adapters support IRQ sharing (the ability to have multiple ports using the same IRQ). This is common on MicroChannel (MCA) bus architecture computers, such as IBM PS/2s and some EISA bus computers. Windows 3.1 fully supports IRQ sharing serial adapters. To use them correctly, be sure that the IRQ values have been set correctly in the Advanced dialog box. Also, if the computer does not have MCA or EISA architecture, the setting &quot;COMIrqSharing=TRUE&quot; must be set in the [386Enh] section of the SYSTEM.INI file. For more information on COMIrqSharing and other SYSTEM.INI switches, refer to the SYSINI.WRI file in the WINDOWS directory.</li> Q. Problems occur when using COM4. What's wrong?

A. Check the settings in the Advanced dialog box in the Control Panel (accessed through the Ports icon) to make sure that Windows knows the correct port address value and IRQ settings for the COM4 port. The address for COM4 is not usually specified in the BIOS data area of most computers because it is not a standard port.

Many serial adapters use a COM4 default base address of 02E8. Unfortunately, this address value conflicts with some peripherals, including 8514/A video adapters and certain network adapters. To work around this problem, reconfigure one of the peripheral adapters to use a different address. If the COM4 address is changed, be sure to reset the address value for COM4 in the Advanced dialog box.</li> Q. What is &quot;device contention&quot;?

A. As a multitasking operating system, Windows must handle simultaneous requests for devices, such as COM ports, from different applications. &quot;Device contention&quot; refers to the rules Windows uses to determine how applications receive control of a device. The 386 Enhanced dialog of the Control Panel allows the specification of how COM ports are allocated to applications.

The Always Warn option means that Windows displays a dialog box asking whether or not an MS-DOS application should receive control of a COM port that it detects may be currently in use by another application.

The Never Warn option means that Windows always provides COM port access to MS-DOS applications, even if it detects that this port may currently be in use by another application.

The Idle (in sec.) option means that Windows displays a dialog asking whether or not an MS-DOS application should receive control of a COM port that it detects was in use as recently as the number of seconds specified. Otherwise, Windows provides COM port access without a warning.</li> Q. With Windows 3.0, use of different serial ports on the same IRQ in rotation was not possible. Was this problem corrected in Windows 3.1?

A. Yes. Serial ports that use the same IRQ (for example, COM1/3 on IRQ4 and COM2/4 on IRQ3) may be used one after the other, but not at the same time (unless the serial adapter supports IRQ sharing). However, some MS-DOS communication applications may leave a serial port in a state that prevents Windows from using its IRQ for other ports. The &quot;stuck&quot; communication IRQ may be reset by opening and closing the original COM port with a Windows communication application, such as Terminal.</li></ol>

Additional query words: 3.10 COMMQ&A winfest

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Technology : kbWin3xSearch kbWin310