USB stands for "Universal Serial Bus," and some newer PCs take the universal part very seriously: These machines may have no parallel (printer) port, no serial (comm) port, no keyboard port, no mouse port, no game (joystick) port, etc. Instead, the system typically will have two or four USB ports, and that's that.

Some PCs don't go quite that far, and instead offer a hybrid combination of ports. They may, for example, provide older PS/2-style (round) mouse or keyboard ports, but have no classic serial or parallel port, using USB for these functions instead.

Still other PCs (from just a few years ago) may lack USB ports altogether, instead providing only classic-style ports.

All these cases can lead to port mismatches: You may have no good way to connect a perfectly good printer, modem, mouse, keyboard, scanner, camera (or whatever) to your PC. The problem can be at either end of the connection: in going from a USB device to a non-USB port, or from a non-USB device to a USB port.

Even if you can make a physical connection between your external hardware and your PC, you may still run into software trouble. For example, if you ever use an older operating system (like DOS) on your newer PC, you may discover that none of your USB devices work: On its own, DOS doesn't know about USB devices, and has no intrinsic way of making them work.

Fortunately, there are work-arounds that can help you make working connections between USB and non-USB systems and peripherals, and even get USB devices working in DOS. With a little cleverness--and maybe an adapter--you probably can connect just about any piece of hardware to just about any PC, and make it work. Here's how:

Connecting A USB Device To A Non-USB PC
Sometimes, it's amazingly easy to connect a USB device to a non-USB PC--at least, when the USB device is a simple one such as a basic mouse, keyboard, joystick, and the like. In cases where the USB device isn't making use of USB's more-advanced capabilities and is simply transmitting exactly the same data as the non-USB version of the device, all you may need is the right physical plug to pass the signals through. Many vendors include these simple "pass-through adapters" in the box; and if one is available for your plug-in device, it's about the easiest way there is to overcome a port mismatch.

If your vendor didn't provide an adapter plug, you may be able to buy one for just a few dollars. For example, this Google search http://www.google.com/search?num=50&hl=en&lr=&ie=ISO-8859-1&newwindow=
1&safe=off&q=%22ps%2F2+to+usb%22&btnG=Google+Search shows several merchants selling PS/2-to-USB mouse converters for under $9. Of course, keyboards and mice are inexpensive devices in themselves; it may make more sense simply to buy a replacement that has the plug type you need.

More-complex USB devices (such as external USB hard drives, CDs, and the like) can't make use of a simple pass-through adapter; they must be plugged into a true USB socket. What do you do then?

The answer is an add-on USB host adapter card. These cards start at around $25 for the most basic models, and let you add two or four USB ports to PCs (and laptops) that originally shipped without USB hardware support. See this http://www.google.com/search?q=USB+card or this http://www.google.com/search?q=add+usb+ports for many inexpensive examples.

Either an adapter plug or add-in host controller will take care of the physical and electrical connections between a USB device and a non-USB PC, but what about the software?