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Advanced serial communications troubleshooting tips.

If you have not yet covered the basics listed in the "How do I troubleshoot the serial communications process?" FAQ, do so now. This information is not for you. Otherwise, feel free to read on. Note that the information is not necessarily presented in any particular order.

DOS Debug:

In Windows 95, select Start/Run. Type the word DEBUG and click OK A DOS box will open with a "-" and a blinking cursor. Type D40:0 and press ENTER. The first line should start out as something like

0040:0000  F8 03 F8 02 E8 03 E8 02 - 78 03 00 00 00 00 06 02

We're only interested in the first part of this line:

F8 03 F8 02 E8 03 E8 02

What are these cryptic entries? This will tell you how many COM ports have been assigned to devices on your system. In this case we have all 4 COM ports shown.

COM 1: F8 03
COM 2: F8 02
COM 3: E8 03
COM 4: E8 02

On your system, you may only see F8 03 F8 02 00 00 00 00

This means only 2 COM ports are being recognized - COM 1 and COM 2. COM 3 and COM 4 are unavailable to you. If you have serial ports that are supposed to use these ports, you may have configured them incorrectly. Consult the appropriate manual or your computer manufacturer for more information on why they may not be appearing properly.

Another possibility you may see is F8 03 F8 02 E8 02 00 00

This type of reading could present a problem. The E8 02 (COM 4) is not in its proper place. It SHOULD read F8 03 F8 02 00 00 E8 02 If COM 3 doesn't exist on the system, 0's should mark its absence. However, they do not in this case. Instead, COM 4 displays "Address Packing". Its address is being packed down into COM 3's area. This packing may confuse Windows programs. Contact your computer manufacturer to see whether an upgrade to the PC's BIOS is available that would prevent this address packing.

To exit from Debug, type Q at the "-" prompt and press ENTER.

IRQ Settings:

IRQ stands for Interrupt Request. A computer device must "interrupt" the processor, in order to have the CPU perform a task for it. PC's have interrupts from 0 to 15. Most are reserved for specific devices, such as the hard drive, video card, system clock, etc. and cannot be shared by anything else. A few IRQs can be "shared". In particular, IRQ 3 and IRQ 4. These 2 are generally used by devices that access a COM port. COM1 & COM3 share IRQ 4. COM2 & COM4 share IRQ 3. This sharing doesn't necessarily work if the two devices are trying to use the IRQ simultaneously and is most apparent when a modem is present on the system.

To get a complete IRQ listing on your Windows 95 system, select Start/Settings/Control Panel/Device Manager. Highlight Computer and select Properties. Locate the appropriate COM port and IRQ. Check to see whether another device is trying to access this IRQ. If so, you may need to direct this device to a different IRQ. For more information on this procedure, consult the device's documentation or your computer's manufacturer.



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