Ports, Serial Ports, Resources Used By Serial Ports, Resolving Resource Conflicts

Resources Used By Serial Ports

Remember that an IRQ points to a memory address in the Interrupt Vector Table or I/O Address Table. This table resides at the very beginning of the memory addresses and has memory address 0.

You normally don’t need to be concerned with an I/O address conflict if the IRQs for the two devices don’t conflict, especially for the first 16 IRQs. To see which I/O address is assigned to a particular port, view I/O addresses using the DOS or Windows MSD command or from Device Manager in Windows 9x.

Serial ports are used for various input/output data transfers that include:

Data transferred over modems to mice.

Data transferred over modems to printers.

Data transferred over modems to other computers.

To configure a serial port with a COM assignment, if it is on an I/O card, you would most likely set jumper switches on the card. If they are connected directly to the systemboard, most often the assignments are made in CMOS setup. Sometimes the setup screen shows the COM assignments, and sometimes you see the actual IRQ and I/O address assignments.

Resolving Resource Conflicts

It is possible to have more than one resource conflict when installing a new device. Plug-and-Play solves many of these conflicts, but there are also tools you can use to determine what resources are available and how to allocate them if you are not using Plug-and-Play.

To find out what resources are presently used by your system in DOS and Windows 3.1, use the MSD diagnostics software.

Keep in mind that MSD does not show everything. Keep a list of IRQs for each PC that you work on. Normally, you shouldn’t be concerned with an I/O address conflict if the IRQs don’t conflict.

The figure below shows the results of displaying the IRQ settings, and that IRQs 10, 11, and 12 have Reserved status. IRQ 11 and 12 are available, but IRQ 10 is being used by a SCSI host adapter.

Steps For Viewing Resources In Windows 9x

1. In Windows 9x, click Start, trace to Settings, then trace to and click Control Panel. 2. Double-click the System icon, then reveal the Device Manager card.

3. Click Computer, then click Properties.

4. Reveal the View Resources tab. The figure below shows the current IRQs that are in use. Note that IRQ 11 is available to be used by a new device.

5. Select Input/output (I/O) to display a list of I/O addresses currently in use, as shown in the figure below. Note that LPT1 is currently assigned the I/O address of 0378h.

6. Select Memory to display the current upper memory addresses in use by devices, as shown in the figure below. The A, B, and C range of upper memory addresses are normally reserved for video. The F range is reserved for system BIOS. Not all the memory addresses actually in use are listed on this screen, only those that are directly requested by a working hardware device.