Plug and Play: The Technology Explained

     Before Plug and Play was developed; each I/O device such as a modem, video card, network card or sound card had top be configured manually.  Each one of these devices had either a series of DIP switches (Dual In-Line Position) or there were some jumpers on a circuit board somewhere.  A user had to study the instructions, hope you had the right ones, and set the switches or jumpers accordingly.  If they were set wrong, you would get a conflict between two or more devices and your computer could crash.  It may even fail to boot entirely.  A person had to keep trying different combinations until all of the individual components worked together without a problem.  What a hassle!

     Why does a computer work in this manner?  Each device works by using an “interrupt”.  The primary purpose of the interrupt is to make the computer run more efficiently.  Instead of having the CPU check each device over and over even when there is no I/O activity would waste a lot of CPU cycles for no reason.  Instead, an I/O device will literally “interrupt” the flow of data to communicate with the CPU.  Every keystroke on your keyboard and every movement of the mouse is an example of an interrupt.  The interrupt is simply telling the CPU to stop what its doing and communicate with a particular device such as the keyboard.  To use an analogy, it is like jumping ahead of someone else in a long line at the supermarket.  You will be serviced faster.  But what happens if two separate device interrupts occur simultaneously?  Since each interrupt is assigned a number; the one with the lowest number will always be taken first. To make this all work, there is a set of standardized hardware guidelines and IRQ numbers that manufacturers have agreed to follow.  However, many cards can still be manually configured to use settings other than the default.  You may for instance run two network cards.  Each card would need to be on a different IRQ (Interrupt setting) so that the system could tell them apart.

     The innovation of Plug and Play has to be one of the more significant technology leaps in recent computer history.  Configuring devices manually was fine for computer geeks, but the average person was hopelessly confused and frustrated by it.  With the advent of Plug and Play; a user can purchase a new device such as a modem from the store, install it into the PC with some simple instructions and it will be magically discovered by the operating system.  Wow!  The OS may or may not recognize the new device specifically; but it WILL know some new piece of hardware has been added.  The operating system will either load the driver if it’s already included with Microsoft Windows or it will prompt the user for the location of the driver software.  The CD or floppy that contains the driver software is put in by the user and everything is installed.  This procedure is a LOT easier than trying to fumble with the intimidating jumpers and switches!

     Plug and Play (PnP) works because of industry standards that have been developed for the sake of compatibility and ease of use.  Manufacturers figured if they could make a computer easier to use, they could sell more.  They were right!  As a result, every device manufacturer began to adhere to the industry standards.  With PnP, the name and other information about each component device are embedded within the chips on that device.  When the computer is turned on, the CPU will scan the system bus.  Anything that the CPU comes in contact with will be recognized and configured in the OS as a part of the system hardware.  This would include disk drives, CD-ROM’s, keyboard, mouse, ZIP drive and so on.  Likewise the Windows operating system has been designed to specifically recognize each of these devices.  The OS knows what driver to use based on the unique identification it finds on each device.  Microsoft maintains a very extensive “Hardware Compatibility List” (HCL) that can be found on the www.microsoft.com website.  The list contains literally thousands of items by hundreds of manufacturers.  If your device is on the list, you can be sure it will work with Windows.  If it’s not on the list, it still may work but hasn’t been tested by Microsoft.  PnP continues to evolve and improve as each manufacturer creates new products to follow these guidelines.  In the early days of PnP, many users actually called it “Plug and Pray” because quite often, it didn’t work nearly as well as it does now.

           In summary we can say that Plug and play was created to provide compatibility, make computing easier and make the installation of components easier.  PnP also reduces maintenance costs and frustration because much less time is spent working on hardware.  In my view, anything that makes life easier is a good thing!

Source

Windows 2000 Resource Kit, Device Management
http://www.microsoft.com/resources/documentation/Windows/2000/server/reskit/en-us/Default.asp?url=/resources/documentation/Windows/2000/server/reskit/en-us/prork/prdh_dmt_dtjr.asp

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