Plug and Play - Plug and Play

Plug and Play

In 1995 Microsoft released Windows 95, which tried to fully automate device detection and configuration as much as possible, but could still fall back to manual settings if necessary. During the initial install process of Windows 95 it would attempt to automatically detect all devices installed in the system. Since full auto-detection of everything was a new process without full industry support, the detection process constantly wrote to a progress tracking log file during the detection process. In the event that device probing would fail and the system would freeze, the end-user could reboot the computer, restart the detection process, and the installer would use the tracking log to skip past the point that caused the previous freeze.

At the time, there could be a mix of devices in a system, some capable of automatic configuration, and some still using fully manual settings using jumpers and DIP switches. The old world of DOS still lurked underneath Windows 95, and systems could be configured to load devices three different ways:

• through Windows 95 device manager drivers only
• using DOS drivers loaded in the CONFIG.SYS and AUTOEXEC.BAT configuration files
• using both DOS drivers and Windows 95 device manager drivers together

Microsoft could not assert full control over all device settings so configuration files could include a mix of driver entries inserted by the Windows 95 automatic configuration process, and could also include driver entries inserted or modified manually by the computer users themselves. The Windows 95 device manager also could offer users a choice of several semi-automatic configurations to try to free up resources for devices that still needed manual configuration.

Also, although some later ISA devices were capable of automatic configuration, it was common for PC ISA expansion cards to limit themselves to a very small number of choices for interrupt request lines. For example, a network interface might limit itself to only interrupts 3, 7, and 10, while a sound card might limit itself to interrupts 5, 7, and 12. This results in few configuration choices if some of those interrupts are already used by some other device.

The hardware of PC computers additionally limited device expansion options because interrupts could not be shared, and some multifunction expansion cards would use multiple interrupts for different card functions, such as a dual serial port card requiring a separate interrupt for each serial port.

Because of this complex operating environment, the autodetection process sometimes produced incorrect results, especially in systems with large numbers of expansion devices. This led to device conflicts within Windows 95, resulting in devices which were supposed to be fully self-configuring failing to work. The overall device installation process could be so complex and frustrating for end-users that Plug and Play was sometimes referred to as Plug and Pray.

Up until about 2000, PC computers could still be purchased with a mix of ISA and PCI slots, so it was still possible that manual ISA device configuration might be necessary. But with successive releases of new operating systems like Windows 2000 and Windows XP, Microsoft had sufficient clout to say that drivers would not be provided for older devices that did not support auto-detection. In some cases the user was forced to purchase new expansion devices or a whole new system to support the next operating system release.