A fully capable hotplug system includes hotplug system hardware (server), hotplug-aware software and operating system (OS) support, hotplug-capable device drivers, and hotplug-aware PCI user interface (see picture below). The hardware and software components work together to ensure that any hotplug activity occurring on the PCI bus is benign to other devices on the PCI bus. The vast majority of industry-standard PCI adapters can be made hotplug aware through the installation of hotplug device drivers. PCI adapter vendors usually provide these device drivers.

System hardware

While the PCI hotplug specification identifies the technical requirements of a hotplug capable system, it doesn’t specify the implementation of this technology. In the specific implementation that Compaq developed, the PCI hotplug hardware isolates a single PCI slot from all other devices on the PCI bus.

The PCI hotplug system hardware performs two main functions: Powers down a single adapter slot, allowing insertion and removal of adapters; and protects the system and other adapters from the electrical effects of hot-plug operations.

Hotplug electronics consist of two separate elements: the hotplug controller and the slot-specific power control. The hotplug controller is designed to manage the following components:

PCI Bus The controller communicates with isolation devices on the PCI bus to electrically isolate a single PCI slot from the rest of the system. Slot isolation permits the insertion or removal of an adapter without interruption to the server or other active adapters.

Power The controller receives a command from the OS to power up or power down a single PCI slot. To perform this function, the controller uses the slot-specific power control. The slot-specific power-control electronics allow proper power sequencing on the PCI bus and guarantee safe control of power to individual PCI adapters.

Slot LED indicators The hot-plug controller also governs the slot LEDs. In one specific implementation of hotplug hardware, each slot has a green and an amber LED to indicate slot status. The green LED indicates power to the slot and flashes while performing a power state change; the amber LED indicates that the slot requires attention.

PCI hotplug button This feature is available only on one specific vendor’s PCI hotplug servers. The button is pressed to signal the software to initiate a power state change. While the button is more convenient, the same functionality can be provided through the software interface. Each slot has its own button to indicate which slot is to be addressed by the supporting software.