Plug-and-play

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Plug and Play (PnP) is a computer feature that allows the addition of a new device, normally a peripheral, without requiring reconfiguration or manual installation of device drivers. PnP is a process the computer runs through when it is first turned on, involving the boot process and power-on self-test. Therefore, the computer must be turned off before installing a PnP device into an expansion slot. Hot swapping is a similar feature that allows adding and removing devices while the computer is on.

The term Plug and Play is most associated with Microsoft, who started using it in reference to their Windows 95 product. Other operating systems (OSs), such as Mac OS, had already supported such features for some time (under various names), but the term gradually became universal over time. At the time of its initial offering by Microsoft, it was criticized by some as not being "proven" technology, which did not always work as it should. Detractors at the time of the earliest offering spoofed the functionality as "plug and pray" because "one never knew if in fact the thing would work."

Contents

1 Requirements
2 True hot-swapping plug-and-play vs. \"Plug and Play\"
3 Hardware identification
4 Plug and Play compatible devices
5 See also
6 External links

Requirements

A PnP-compatible computer must meet four requirements:

The OS must be PnP-compatible.
The BIOS must support PnP.
The device to be installed must be a PnP device.
32-bit or better device drivers must be available to support the device.

True hot-swapping plug-and-play vs. \"Plug and Play\"

True PnP requires support both in hardware and in software. First, and most importantly, the hardware must be able to have new devices added without causing physical or electrical damage, or unrecoverable glitches on interfaces. For example, some electrical interfaces are not capable of being safely plugged and unplugged without risking permanent electrical damage to components. Often, this involves electrostatic discharge hazards, or poor sequencing of connection and disconnection of electrical contacts. For example, the Apple Desktop Bus was self-configuring, but not hot-swappable: hot-swapping ADB connections risked damaging the motherboard interface in a way that would require motherboard replacement to fix.

Another ability needed in the hardware is the ability of the computer bus in question to recognize changes to its configuration when devices are added or removed. More modern systems, notably USB and FireWire, deliberately designed such support into the bus.

Without the hardware support for hot-swapping, a limited form of plug-and-play is still possible, which might be more correctly described as "power down, plug, power up, and play".

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 PCI (which was becoming the industry standard at that time) integrated this functionality. (Note that neither of these buses was a "true plug-and-play" bus, 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 (and others).

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

External links

http://www.pcguide.com/ref/mbsys/res/pnp.htm
http://www.microsoft.com/whdc/resources/respec/specs/pnpisa.mspx (P&P in ISA)