Overview of KVM Switch

KVM switch (with KVM being an abbreviation for "keyboard, video and mouse") is a hardware device that enables precise control of other computers from a single terminal – although more than one control point is possible. Used by computer engineers and IT support staff to manage multiple machines without having to connect a keyboard, mouse and monitor to each one, they have many practical applications in the communications and information technology support industry. Relatively recent developments in the technology have also seen the KVM add USB (universal serial bus) and audio sharing capabilities.

Broadly, several different types of KVM switches, which employ distinct core technologies, are currently available. These include Enumerated or USB Hub-Based KVM switches, which can be literally plugged in or out of different ports as USB devices can be. The full USB enumeration process must occur, however, when this type of switch is plugged in to another port.

Basic keyboard and mouse functions can be easily and relatively immediately supported by Emulated USB KVM switches, which are designed for use on dedicated USB console ports. These imitate distinct clusters of mouse or keyboard switching data in any of the targeted systems.

The True Emulation or DDM (Dynamic Device Mapping) USB KVM switch enables a full range of mouse and keyboard functions to each and every connected computer simultaneously, rather than the restricted span permitted by the Emulated USB KVM.

KVM switches are often used in situations where there are a number of machines required to achieve a certain outcome, but the use of input devices – keyboards and mice – and monitors are non-essential. An appropriate example might be the servers found in data centres. With potentially terabytes of information to manage, engineers need an effective way of accessing individual servers in the network in the event of service outage, data loss or connection issues. Using KVM switches means that they can control access to servers in the rack from a single point.

The technology also has several domestic uses.  Essentially, homes with more than one computer can manage other base-unit PCs or laptops and tablets from one place. In practical terms, this gives people the option of using more than one operating system without needing to partition the hard drive on a single machine.

In additions, computers using KVM switches can be connected using a number of different methods. This is largely device dependent, but native connections for input devices and monitors usually feature in standard configurations. In essence, the controlling device and its input peripherals (keyboard & mouse) and the monitor are connected to the switch.  A cable then runs from the switch out to the other computers, usually with separate connectors for the monitor, keyboard and mouse ports. Some switch and cable sets offer keyboard, video and mouse cables housed in one extension cable – although these are generally more expensive than other methods of connection.


The computer industry began to expand exponentially during the 1980s, but with the expansion came a number of problems: data centre server rooms became seriously overcrowded, as burgeoning numbers of monitors, keyboards and mice began to eat up increasingly scarce rack space. They also generated unpleasant amounts of heat, and larger data centres were faced with cumulatively complex server management issues. In those days, technicians had no option but to physically walk to every server they were required to work on.

It remains uncertain who invented the first KVM switch, although the very first such switches are more accurately depicted by the acronym KVS (keyboard video switch), as the computer mouse was not yet in common use. These early switches were rudimentary A/B push-button devices which permitted video and keyboard switching only, but they did allow operatives to control several central processing units (CPUs) from a single desktop computer.

They had a dramatic impact, vastly reducing the space requirements of the older, pre-KVM system and radically simplifying server management. In addition to slashing heat emissions, they also slashed costs – it was no longer necessary to buy dedicated monitors, keyboards and mice for each individual CPU.

These “single-user” KVM switches remain in use and are found in most data centres around the world.

A shortcoming of the single-use KVM switch, however, was its limited capacity to handle multiple servers, a problem which became impossible if more than one technician needed access at the same time. A new generation of multi-user KVM switches were subsequently developed to overcome these difficulties. With these devices, multiple technicians can remotely access each and every server they need to from a central Control Room or NOC, abolishing the need to physically walk to them.

Multi-use KVM switches typically come with inbuilt security screening technology, enabling discrimination between those users who are permitted to log-in and those that are not. 


Technical aspects

The earliest KVM switches were designed as mechanical, multi-pole devices and, despite their limitations, they are still widely used today, not least because they are exceptionally inexpensive. Typically, these “passive” switches, which require mechanical movement of a rotary dial to select different computers, can readily share 2 – 4 computers, although 12 is generally their upper limit.

Newer “active” KPM switches dispense with the mechanical movement of switch contacts and send electrical signals instead and can typically control considerably more computers than their passive predecessors.

Passive (mechanical) KVMs have several disadvantages, despite their economic price range. If a computer is not currently selected by the switch, it will not detect any mouse or keyboard connections. While this poses no problem in ordinary circumstances, booting up can be tricky: the computer tries to detect keyboards and mice during this process and, when not selected by the switch, will either abort the boot up procedure entirely, or alternatively boot in an undesirable mouseless or keyboardless state. It will also fail to find its monitor and ‘regress’ to a rudimentary graphic resolution. Machines which automatically reboot after an interruption to the supply of power are particularly unsuitable for these switches as a result.

Electronic or active KVM switches overcome most of these shortcomings, delivering full emulation for all peripherals (as mentioned earlier, passive KVM switches often offer only a limited span of functions). The ‘failure to detect peripherals’ issue is solved because active switches emit signals to both selected and unselected computers, ensuring that all can simulate connections with monitors, keyboards and mice. Frequently, they’re also equipped with troubleshooting functionality that delivers uninterrupted support to computers that are continually communicating with peripherals.

Recently, KVM software options are providing alternatives for at least some of the functions available on hardware KVM switches.


Where the KVM switch is used in manufacturing

KVM switches are widely used in data centres around the world, as well as other settings where there is little to no need for dedicated peripherals for each computer. They are also used in domestic settings, enabling users to operate mobile devices such as laptops or tablets from the full-sized desktop PC mouse, keyboard and monitor.

The vast majority of KVM switches are now operated non-invasively via hot-key commands such as Print Screen, Scroll Lock+Scroll Lock and Ctrl+Ctrl. Typically, an on-screen display will clearly indicate which computers are connected.


How the KVM switch differs from other switches

Unlike motion switches, light switches, dimmer switches and Reed switches, KVM switches are specifically designed to emulate the connection of peripherals (mice, keyboards and monitors) on multiple computers, each of which then becomes controllable from either a single point of operation or from several points simultaneously, as in an NOC or control room.