KVM Switch - Passive and Active (electronic) Switches

Passive and Active (electronic) Switches

KVM switches were originally passive, mechanical devices based on multi-pole switches and some of the cheapest devices on the market still use this technology. Mechanical switches usually have a rotary knob to select between computers. KVMs typically allow sharing of 2 or 4 computers, with a practical limit of about 12 machines imposed by limitations on available switch configurations. Modern hardware designs use active electronics rather than physical switch contacts with the potential to control many computers on a common system backbone.

One limitation of mechanical KVM switches is that any computer not currently selected by the KVM switch does not 'see' a keyboard or mouse connected to it. In normal operation this is not a problem, but while the machine is booting up it will attempt to detect its keyboard and mouse and either fail to boot or boot with an unwanted (e.g. mouseless) configuration. Likewise, a failure to detect the monitor may result in the computer falling back to 640x480 resolution. Thus, mechanical KVM switches may be unsuitable for controlling machines which can reboot automatically (e.g. after a power failure).

Another problem encountered with mechanical devices is the failure of one or more switch contacts to make firm, low resistance electrical connections, often necessitating some wiggling or adjustment of the knob to correct patchy colors on screen or unreliable peripheral response. Gold-plated contacts improve that aspect of switch performance, but add cost to the device.

Most active (electronic rather than mechanical) KVM devices provide peripheral emulation, sending signals to the computers that are not currently selected to simulate a keyboard, mouse and monitor being connected. These are used to control machines which may reboot in unattended operation. Peripheral emulation services embedded in the hardware also provides continuous support where computers require constant communication with the peripherals.

Some types of active KVM switches do not emit signals that exactly match the physical keyboard, monitor, and mouse, which can result in unwanted behavior of the controlled machines. For example, the user of a multimedia keyboard connected to a KVM switch may find that the keyboard's multimedia keys have no effect on the controlled computers.