Configuration Woes

Great! Now we know what IRQ’s are. Sow how does all of that help us when configuring devices or trying to solve conflicts? Well, the answer to that is best found by looking at the three basic methods of device configuration:


  • Hardware (legacy) configuration
  • Software (legacy PNP) configuration
  • BIOS (PCI) configuration

Hardware Configuration

As its name implies, this type of device configuration is accomplished through physical changes to the device (card). Typically, these changes involve setting jumpers or DIP switches. Figures 4 and 5 below illustrate typical jumper blocks and DIP switches.

Figure 4 – Jumpers – “Open” and “Closed” are often referred to as “On” and “Off”
Figure 5 – DIP Switches

Typically, the device manufacturer will provide configuration instructions that will indicate the correct jumper or switch positions for various available device configurations. As an example, a modem may have a Berg header consisting of two rows of eleven pins. The configuration diagram shows that the pairs are labeled JP1 through JP11, and that the default (factory shipping) configuration has a jumper placed on one pin of JP1and JP2, and on both pins of JP3 (see Figure 4), and that this jumper configuration (JP1 and JP2 open with JP3 closed) sets the modem to the standard COM4: resources of 2E8h and IRQ3.

Figure 6 – Typical ISA Modem
Figure 7 – Berg Header for Jumpers
Logical Port Hex (h) Address JP1 JP2
COM1: 03F8h to 03FFh ON ON
COM2: 02F8h to 02FFh ON OFF
COM3: 03E8h to 03EFh OFF ON
COM4:* 02E8h to 02EFh OFF OFF
JP # 3* 4 5 6 8 10 11
IRQ 3 4 5 7 10 12 15


Figure 8 – Jumper Positions vs. Resources – Asterisks denote factory default settings

Figures 6 through 8 illustrate this concept for a typical jumper-configured ISA modem. As stated earlier, this modem ships configured as a standard COM4: device. This means that the modem, once properly installed in the PC, will be hard-assigned the resources normally assigned to COM4:, and will therefore carry the COM4: designation.

Here is where ISA device configuration gets tricky. Suppose that you wish to use this modem in your PC, but that you are already using serial port devices attached to COM1: (a digitizing tablet) and COM2: (a plotter). Will this modem work using its factory settings? Yes and No! Sound confusing? Let’s see if we can make some sense out of this all.

The Yes Part

Remember earlier when I said that two devices couldn’t share a single IRQ and have both devices work properly at the same time? The operative phrase here is the “work properly at the same time” part. Sure, we could install the modem with its factory configuration, and Windows will even allow it to be assigned the same IRQ as COM2:, and yeah, it will work that way… until you need to use the plotter and the modem at the same time. Then, whichever device had the interrupt first would work, and the other one would not. Inconvenient? You bet! Then, to complicate the matter even more, if you should choose to use a background application such as a FAX manager, and have it launched when Windows starts, you would lose functionality of the plotter altogether, as the modem will then claim IRQ3 immediately when the FAX application is launched.

The No Part

As the preceding paragraph indicates, we probably don’t want to have unusable peripheral equipment, meaning in this case that we probably don’t want to have both COM2: and COM4: assigned to IRQ3. That means that you will have to select a different configuration for the modem – one where there will be no resource sharing or conflicts. Fortunately, this particular modem gives us several configuration options besides the standard COM port definitions from which we can choose.

Note that in Figure 8, the modem ships with jumpers in place on positions JP1, JP2, and JP3. Jumpers JP1 and JP2 are used to select the Input/Output (I/O) Address, while jumpers JP3 through JP11 are used to make the IRQ assignment. We will discuss I/O Addresses in another article, so for now I will simply point out that COM ports are not bound to any particular IRQ lines. Although we customarily associate IRQ4 with COM1: and COM3:, and IRQ3 with COM2: and COM4:, we are really free to assign any available IRQ to these ports that is supported by the design of the hardware. What does this doubletalk mean? Simply that if the maker of a hardware device such as a modem or a printer port provides for it in the design of the card, we can assign non-standard IRQ’s to these devices. Let’s look at what I mean with respect to the modem in Figures 6 through 8. If we want the modem to carry the COM3 designation while using IRQ5, Figure 8 tells us that jumper JP1 should be open, JP2 should be closed, and JP5 should be closed. To accomplish this, take the JP2 jumper that is currently only on one pin of that pair and place it on both pins, closing JP2. Now take the jumper that is on JP3 and move it to JP5. Voila! The modem is now set to COM3 and IRQ5.

The DIP Switch Part

Earlier, I said that DIP switches are also used for device configuration. It makes absolutely no difference which method is used, as they are essentially the same. A jumper can be either off (open) or on (closed), making it a simple switch. Whatever method is used, the manufacturer should provide the same basic information. The purpose of all user-configurable jumpers or switches should be clearly discernable, and the documentation should include a picture or diagram of the card, showing the relative locations of the jumpers or switches. In addition, many cards will also have the configuration information etched or screen-printed on the card itself; this information may be on either surface of the card. Some cards which use DIP switches will have the bank of switches located near the rear edge of the card, with an access hole stamped in the card bracket to allow reconfiguration of the card without removing it from the PC. On some of these devices, the switch functions will be marked on the card bracket as well. One very important caution is needed here… never change any jumper or DIP switch setting while the PC is powered up!

Don’t be afraid to return any hardware device that does not include adequate configuration and installation documentation. Also be sure to store any documentation that you do receive in a safe and accessible location, as you will probably need it if you decide to reconfigure your PC.


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