ISA Plug and Play

Plug and Play (PnP) is an add-on feature to the IBM PC/Intel ISA bus that allows the addition of a new peripheral without requiring that a user choose an unoccupied I/O address or IRQ nor manual configuration of device drivers. PnP (capitalized) is a specific ISA-add-on technology, and should not be confused with the generic term plug and play, which both predates and has expanded beyond (see hotplug).

The PnP process is invoked at boot time: when the computer is first turned on, PnP devices are identified and assigned non-conflicting addresses and IRQ numbers.

The term Plug and Play was first used by Microsoft in reference to their Windows 95 product. Other operating systems such as AmigaOS Autoconfig and Mac OS NuBUS system, had already supported such features for some time (under various names, or no name [ [http://www.douglasadams.com/dna/980707-03-a.html] "We call this principle Plug and Play. Or at least, Microsoft calls it that because it hasn't got it yet. In the Mac world we've had it for so long we didn't even think of giving it a name." - Douglas Adams] ), but the term "plug and play" gradually became universal due to worldwide acceptance of Windows and related terms referring to market supremacy of this operating system.

Typically, non-PnP devices need to be identified in the computer's BIOS setup screen so that the PnP system will not re-assign those devices. Problems in the interactions between legacy non-PnP devices and the PnP system can cause it to fail, leading to this technology historically been referred to as "plug and pray" [ [http://www.pcguide.com/ref/mbsys/res/pnpPray-c.html Plug and Play ] ]

Requirements

A PnP-compatible computer must meet three requirements:
#The OS must be PnP-compatible.
#The BIOS must support PnP.
#The device to be installed must be a PnP device.

Hardware identification

Plug-and-play hardware typically also requires some sort of ID code that it can supply, in order for the computer software to correctly identify it.

This ID code system was not integrated into the early Industry Standard Architecture (ISA) hardware common in PCs when Plug and Play was first introduced. ISA Plug and Play caused some of the greatest difficulties that made PnP initially very unreliable. This led to the derisive term "Plug and Pray", since I/O addresses and IRQ lines were often set incorrectly in the early days. Later computer buses like MCA, EISA and PCI (which was becoming the industry standard at that time) integrated this functionality. (Note that none of these buses were "true plug-and-play" buses, because of the lack of safe hot-plugging support).

Finally, the operating system of the computer needs to be able to handle these changes. Typically this means looking for interrupts from the bus saying that the configuration has changed, and then reading the information from the bus to locate what happened. Older bus designs often required the entire system to be read in order to locate these changes, which can be time consuming for lots of devices. More modern designs use some sort of system to either reduce or eliminate this "hunt"; for example, USB uses a hub system for this purpose.

When the change is located, the OS then examines the information in the device to figure out what it is. It then has to load up the appropriate device drivers in order to make it work. In the past this was an all-or-nothing affair, but modern operating systems often include the ability to find the proper driver on the Internet and install it automatically.

Plug and Play compatible devices

* Universal Serial Bus
* FireWire
* Modem
* CD-ROM drives
* DVD drives
* Computer printer
* Network card
* Computer keyboard
* Graphics card
* Computer display
* Game controller
* Computer mouse

ee also

* Autodetection
* Autoconfiguration
* Autoconfig (Amiga)
* Hot plugging
* Direct Data Channel
* Universal Plug and Play (UPnP)
* USB flash drive
* User friendliness
* Advanced Configuration and Power Interface (ACPI)
* Extended System Configuration Data (ESCD)
* Option ROM

References

External links

* http://www.microsoft.com/whdc/resources/respec/specs/pnpisa.mspx (P&P in ISA)
* http://www.microsoft.com/whdc/system/pnppwr/pnp/pnpid.mspx (P&P ID)