Keyboard, video and mouse switch technology
Written by Tomi Engdahl 2005
Introduction to KVM technology
KVM siwtch is short for keyboard, video, mouse switch. It is a a hardware device that enables a single keyboard, video monitor and mouse to control more than one computer one at a time. A user connects their monitor, keyboard and mouse to the KVM device instead of directly to the computers themselves and then uses special cables to connect the KVM device to the computers. Control is switched from one computer to another by the use of buttons on the KVM device, with the KVM passing the signals between the computers and the keyboard, mouse and monitor depending on which computer is currently selected. Most KVM switches have front panel controls (buttons or switch) or have allow control to be switched through keyboard commands (such as hitting a certain key, often Scroll Lock, rapidly two or three times.
Cheap two PC KVM switches are popular among users who have upgraded their home PC systems and want to still use their old computers but do not want to invest in a second keyboard, monitor and mouse. KVM switches are also used in business applications when one person needs to use more than one computer, or example applications where software developers have two computers on their desks running a different operating system (for example one for normal office document work and other for actual software development). KVM switches are also popular in computer center "computer rooms" and in server farms where it is only necessary to periodically access each separate server in the equipment rack one at a time. Devices differ in the number of computers that can be connected, with anywhere from two up to 32 computers possible. Enterprise-grade devices can also be daisy-chained (or "stacked") to allow even greater numbers to be controlled from a single set of a keyboard, video and mouse.
Popularity of KVM switches has increased with the use of Microsoft server operating systems such as Windows 2000 and Windows Server 2003, as these operating systems cannot be easily administered without access to a keyboard, mouse and monitor.
Remote KVM devices are also available that allow multiple computers to be controlled remotely across a wide area network, local area network or telephone-line using the TCP/IP protocols and suitable control software (could be proprietary application, a web browser, VNC client etc.. depending on the device). KVM switch doesn't depend on a software component running on the remote computer, thus it allows doing everyhting that you can do with a local keyboard controls (even editing of BIOS settings remotely).
Technology behind KVM switches
Monitor signal switching
Most KVM products are targeted to be used with computers that have VGA connectors. The idea in VGA signal switching is that the signal siwtching matrix routes the RGBHV signals from one PC input to the monitor output. This switching of the signal can be implemented with mecahnical switched, relays or using electronic components (analogue switches, controllable amplifiers etc.).
Here is the pinout of VGA connector:
Pin New VESA DDC Old VGA 1 Red Red 2 Green Green 3 Blue Blue 4 No Connect Reserved 5 Ground Ground 6 Ground Red Ground 7 Ground Green Ground 8 Ground Blue Ground 9 No Connect No connect 10 Ground Ground for syncs 11 No Connect Monitor ID 0 (ground) 12 DDC DAT Monitor ID 1 (no connect) 13 Horizonal Sync Horizonal Sync 14 Vertical Sync Vertical Sync 15 DDC Clock No Connet
For monitor switching you need to switch at least pins 1,2,3, 13 and 14 to make the monitor signal switch nicely. You can generally leave the ground pins (6,7,8, 10) connected all the time. With a modern plug&play monitor it is a good idea to connect also pins 12 and 15 so that the monitor-computer DDC communications works well (this is used by plug&play / plug&display features).
A typical mechanical switch works around this way: Mechanical changeover switches/relays to select between two input signal sources. For VGA connection geneally pins 1,2,3,6,7,8,13,14, and 10 of the monitor connector are switched (usually also 12 and 15 so that DCC for plyg&play monitor connection works). There are some variations what pins are switched (some implementations do not switch the ground lines, they keep then always connected. some other switch also those lines).
A mechanical switch will work for VGA signals quite acceptably when the internal wiring on the switch box is well made (preferably 75 ohm coax).
Electronic switches comprise a video switch that switches analog video and sync pulses between shared monitors and computers. Many modern VGA switching system use electronic switching of VGA signals.
Simplest keyboard siwtches can use mechanical changeover switches/relays to select between two PCs which one is wired to the keyboard connector. Using mechanical switch is almost like unplugging keyboard from one computer and hooking it up to another. Usually this wont cause serious problems but sometimes it can cause strange problems (erratic keyboard operation, noise characters to PC, even system crash with some computers). So mechanical switch is an inexpensive solution what works if the demands are not very high.
Electronics can be used to make things work more smoothly. Modern electronics KVM switch has a microcontroller that is all the time connected to two PCs plus the keyboard. This microcontroller emulated the real keyboard operation to the PCs (so the PCs think that they are connected real keyboard) and at the same time communicated with the real keyboard (received information on keys pressed and sends them to PC that should receive them at this time). In this way you can switch the keyboard control between tho PCs smoothly without any problems.
A modern PS/2 type mouse uses similar connector and quite similar signals as a keyboard. The siwtchign of mouse signals is typically implemented quite much in the same way as the keyboard switching.
There are also some new switches that have USB switching in them.
Different KVM switch implementation
There are different KVM switch implemtations using different technologies. Simplest one are just mechnical switches that switch the needed signals between PCs. The other end is a fully elctronic switch.
There are also switches that are built as a conbination of electronic and mechanical switches. Some KVM switches use mechanical switching for video signals and electronic switchign for keyboard/mouse, thus having the benfits of keyboard/mouse emulation combines with cheap mechanical switch construction.
A simple purely mechanical KVM switch does not need an operating power. All the KVM switches that have some electronics in them need some power to operate. How this operating power is genrated varies between different switch models. Many simple two port KVM products take the power from the computers connected to them (+5V power from keyboard connector). Larger ones with many computer connections typically use a "wall wart" or have a built-in power supply (device plugs directly to mains outlet).