KVM switch

A KVM switch is a computer accessory that allows a Keyboard, Video display, and Mouse (or other pointing device) to be switched among multiple computers. A common application is in administration of a "farm" of servers, when work will only be done on a single server at any given time. The KVM switch saves the space and cost of individual KVM components for each computer in the farm. Another application might be when a user has several independent computers but room for only one set of KVM components, and, again, will use only one computer at a time.

While it is possible to use virtual terminal applications to take software control of one computer for another, that assumes a windowing operating system. If the computers being managed might need to have boot-mode or other low-level information entered for administration, they may not be running the windowing or remote access application, and thus the direct interface remains necessary.

The first KVM switches were mechanical devices, with pushbuttons or a rotary switch used to select the computer desired. There has been a trend to smaller switches controlled by "hot keys", but the hot key requires software that may or may not be available. Hot key software for Microsoft Windows, for example, might not run on LINUX.

There has been a push to make mechanical KVM switches smaller, and one approach has been to change the connectors on the switch itself. Previously, KVM switches had a separate keyboard, video, and mouse connector for each computer that could be selected by the switch. By making special cables that do have separate connectors at the computer end, but come back to a single high density connector at the switch, the switch indeed needs less "real estate" for the connectors. The cables, however, are specific to each switch manufacturer, since the high-density connectors are not standardized. These special cables can indeed become more expensive than the switch itself.

With large numbers of computers, it sometimes is efficient to make a "tree" of multiple levels of KVM switches, so the KVM switch selects not computers, but compatible KVM switches. Pushing button 1 on the top-level switch does not take the commands to computer 1, but to switch A, which next receives commands to its connected computers.

While the original KVMs used serial interfaces for the keyboard and mouse, some now use a Universal Serial Bus adapter for these, which again may be smaller but may not always adapt to as many computers. Video becomes more of a challenge with high-definition connectors being used rather than the standard VGA family connectors. These are all details to watch when choosing a switch.

There are techniques for remotely operating a KVM switch using proprietary software, which sends KVM commands over Internet Protocol to the actual computer location.