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What the heck is an IRQ?
There are so-o-o-o many devices that one can connect to your computer and most computers have several expansion slots just waiting to be used. The problem is that while this list of devices has grown our computer's resources have stayed the same, In this article I will focus on the interrupt request, or commonly more known as IRQ's
Your computer has 16 IRQ addresses named easy enough 0-15, now the problem arises because the computer needs these for its own resources. The remainders, if any, can be used for your add-on cards. Although you might have 4 free expansion slots on the back of your computer you may only have 1 or 2 extra IRQ's. Almost all devices don't like to share IRQ's, they are very, very jealous little things!
Can you add more IRQ's to your computer? Nope, you just have 16 to work with! What are we to do? We want to add more things to our computers!
A lot of devices can simply be daisy chained to your printer port. Although not the fastest, it sure is easy! For example, you could get an external Zip drive. Since both devices use your printer port no additional computer resources are needed, no jumpers to set. You don't even have to open the computer case. Just load the software and go!
If you have at least one IRQ free, you could use it for a SCCI card. A SCCI card transfers data faster and allows you to daisy chain devices off of it similar to your printer port, although you will have to open your computer case for the one time installation of the card. The card comes in many varieties and flavors. With all those choices, it can be hard deciding which card would be right for you.
Your third choice is the Universal Serial Bus (USB). Usually you can find these as standard equipment on new computers. These ports also let you daisy-chain devices together, another advantage is the device can get its power from the port as well - no need for another power adapter under the desk. Unlike other devices, you do not need to power down your system to switch devices. Installation is very simple, just plug in your device and your computer should recognize it, you will however need Windows'95B version or Windows '98. I ran out of IRQ's in a system that was using a ps/2 mouse, I swapped it out with a USB mouse and disabled the ps/2 mouse in CMOS and freed up another IRQ!
By understanding your IRQ useage you will be able to decide what you can and what you can't add to your existing system. It is more than just looking on the back of your computer and seeing room for an additional card. Just because there is a place to put it doesn't mean that there is an address for it. Without an address the card will be invisible to your computer. That is the same reason we leave the light on for the pizza delivery driver - we don't want to be invisible!