KVM Switches - What, Why and How

By Vaughn L. Treude
In my job, I work on multiple operating systems. Years ago, to save money, I set up my PC for "multiple boot", which meant that I could have up to four operating systems on the primary disk (providing there was sufficient room.) I did this because my work required that I use multiple versions of Windows. I was also interested in tinkering with other systems, like Netware, OS/2, Coherent (a Unix clone) and an upstart called Linux. There's a program (by V Communications) called System Commander that makes OS selection possible, but rebooting a PC has never been what I'd call "convenient" in any operating system. (System Commander not the only OS-selection program, but it's by far the best, being most flexible and easy-to-use ) Therefore I didn't tinker as much as I would have liked to, since my work and my day-to-day operations were on Windows; I tended to stay in Windows through sheer inertia.

Luckily, technology fixed that problem for me. PC's became cheaper, and though I worked hard to upgrade the ones I had, at some point it made sense to buy a new one. In the early days, I'd give the old system away to a computer-less family member, but soon they all had their own, so I'd just keep my old systems and add them to my network. As newer systems became more common, I began to accumulate older systems from family, friends, and colleagues, all of which which I'd upgrade and put into service. Soon it was no longer necessary to put multiple systems on a PC, because I had multiple PC's. That created a new problem - what to do with all those monitors, keyboards, and mice?

The answer was obvious: since one person can only work on one system at a time, the solution was some sort of a switch which would allow me to use a single monitor, keyboard and mouse to control multiple systems. These devices are called KVM switches (for keyboard, video, mouse) and were pretty primitive in the beginning. My first switch was just that: a totally passive device in a metal box, which could switch the required multiple lines in parallel. At the back of the box were three sets of connectors: three SVGA video connectors, three AT-style keyboard connectors, and three nine-pin serial ports. It allowed the user to connect one keyboard, monitor, and mouse to two different systems. It worked, but not well- the video would jump when I switched it, and the keyboard would sometimes get locked up, caused probably by switching in the middle of a clock pulse. Clearly, something better was required.

The next generation of KVM switches were active. That is, they contained circuitry to make the switchover from one system to another go more smoothly. Since the switch hardware was electronic rather than mechanical, the device could also be smaller. My first electronic switch was a four-port model by LinkSys. It worked quite a bit better than the mechanical version, avoiding most of the "jump" and "lockup" problems. It was, of course, more than twice as expensive than the simple "box," but it was worth it. And though it was electronic, it derived its power parasitically from the system (PS/2 ports have 5V power and ground lines) connections, so it didn't require an external power source. There were other advantages as well. Because it was electronic, it didn't require a big rotary switch; all I had to do was push a single button in front, and it would cycle the display between computers 1, 2, 3, and 4, and then back to 1. It was also set up to trap a key sequence so that a particular system could be selected from a keyboard. The sequences all started with Alt-Ctrl-Shift, where the three keys were not pressed simultaneously but in quick succession . The fourth keystroke was the number of the desired PC - 1, 2, 3, or 4. This made switching between different systems pretty convenient. Of course, I should note that technology had changed since I bought the original switch; this one now had PS/2 connectors for both the mouse and keyboard ports.
Linksys 4-Port Compact KVM SwitchFigure 1 - LinkSys 4-Port KVM switch with two systems attached.  Cables to monitor, etc. at back.

I still use that switch; though I've updated my newer systems to a Belkin 4-port switch (for technical reasons that I'll list later.) It's a great convenience to me. I have two tower systems, one running Windows 2000 and one running Linux. I also have two notebook PC's in this setup. One of the notebooks is the one I currently use while on the road; but when at home, it's nice to be able to use it with a better screen and a more ergonomic keyboard and pointing device (personally, I prefer a trackball to a mouse, but I admit that's an eccentric acquired taste.) The other notebook is so beat-up that it's no longer good for traveling, but it still works, and it's useful as a test platform.

But I must admit that there are drawbacks, tradeoffs and gotchas with the use of a KVM switch. It's probably not the right solution for everyone, and there are alternatives (though those have their drawbacks as well.)

Switch Pitfalls
    1. A KVM switch requires lots of cables. You need a set of three cables for each PC. This adds considerable expense, even when you buy them online or from a big discount store. To make matters worse, KVM's often require special "switchbox" cables with identical connectors on each end. That means that those extender cables you may already have (such as mouse and keyboard extenders) won't do the job. Switchbox cables are usually harder to find and more expensive than extender cables. Some manufacturers have wised up and now make devices that use extender cables instead of switchbox cables-at least for the video. Buyer beware- some computer stores really gouge you on cable prices, so shop around. You can easily spend $100 on cables, in addition to $80-$100 for a low-end switch, so when you consider the current low cost of CRT's, the KVM setup is more of a space saver than a cost saver.
    1. KVM switches lock you into a particular generation of hardware. Keyboard and mouse adapters keep changing. Keyboards aren't so bad, usually you can find an adapter (such as AT-to-PS/2) but the mouse has always been a royal pain. First of all, you can't really adapt an old serial-mouse machine to PS/2, because it won't work with your PS/2 mouse. You can't use a dual-purpose mouse, because the OS won't recognize its serial function through the switch. USB has made things worse. My Sony Vaio notebook has no PS/2 connectors, so I had to buy an expensive (circa $40) USB-to-PS/2 converter. I could go to a USB trackball, but the I'd have to replace the switch with a newer version that supports USB. (And I sometimes boot one or more of the machines into an older OS that doesn't support USB.) Only the video connection avoids these problems (unless you have an extremely old CGA system.) It's amazing that video cables have remained the same for as long as they have; I'm sure that PC manufacturers will need to change them soon.
    1. KVM switches don't usually incorporate audio. (Iogear's Miniview is an exception, but it only supports two PC's.) I had to buy a separate audio switchbox from Radio Shack so I could hook up my set of good speakers to three of the four systems. That was a mess, because this switchbox used RCA cables, rather than the stereo mini-phone plug used on a PC. I needed a male-mini-plug-to-RCA adapter for each PC, as well as a female-mini-plug-to-RCA adapter for the speakers. And the switch definitely doesn't handle other accessories, such as the microphone and web camera.
    1. KVM switches can sometimes confuse your operating system's hardware-detection routines. I mentioned earlier that I needed a USB-to-PS/2 adapter for my Sony notebook. These adapters usually come with two PS/2 ports, so you can hook up both a mouse and a keyboard. These work fine in a standalone setup, but KVM switches hate them. For some reason, I could never get Windows to recognize both devices if they were on the other side of the switch, even though the switch is supposed to emulate a typical keyboard and mouse. (I have not tried this with Linux, but device recognition is not Linux's strong suit, so I'd anticipate similar problems.) For this reason, I invested in a Belkin "Omni" KVM switch, which has the unique feature of two mouse ports for each PC. One is the standard PS/2 port; the other is a 9-pin serial connector, which is converted via the switch's firmware so that any or all of the PC's can "see" a serial mouse on its COM port when the actual mouse is PS/2. This is how I had to connect the Sony: the keyboard uses the USB-to-PS/2 adapter, and the mouse uses the serial port! It works, but note that you lose the use of the scroll wheel in this setup. My older Winbook notebook had a similar, maddening problem. Like most notebooks of its vintage, it has a single PS/2 port with a two-port adapter which you must use when you need both the external keyboard and mouse. This is another adapter that switches hate. For some reason, signals from the keyboard get sent to the mouse port, causing erratic cursor movement. One of the manufacturer's tech support people suggested using back-to-back PS/2-to-AT and AT to PS/2 adapters, with the idea that the sixth wire on the PS/2 connector (the AT connector has five) was causing the problem. This "fix" didn't work. Neither did using a USB-to-PS/2 adapter for one device and the regular PS/2 port for the other; that confused the OS. The two viable options were to connect the mouse serially, or to use an extra pointing device.
    1. Strictly speaking, the KVM switch isn't the only alternative to having a monitor, keyboard, and mouse for each system. Linux aficionados will be quick to point out that if the machines are networked (as mine are) you can log in remotely from one machine to any other on the network. I am not, however, one of those purists that thinks the command line ought to be the only game in town. I like the Gnome and KDE windowing systems- if they didn't exist I'd go back to Microsoft. Yes, I could set up an X-client on one machine and an X-server on the other, but it wouldn't be the same. I like to experiment with different Linux distributions and windowing systems; I need these different platforms to test my software thoroughly. A Windows user might note that Windows also has remote-control capability. There are programs (such as Symantec's PCAnywhere) which allow you to log in to another machine on the network and access its display via a window on your own. Microsoft's NetMeeting can also do this. These work well, but they are noticeably slower than having a local setup, at least on a standard 10-base-T network. Perhaps they'd be better on the fastest ethernet setup, but upgrading your router and network cards can be another major monetary investment. You also need a dedicated monitor for installations, and some older machines require a keyboard (or something that emulates a keyboard) in order to boot.

    What type would I recommend? As far as electronic switches, I have investigated several, but have actually used only Linksys and Belkin. Although this is by no means an exhaustive list, the two types illustrate some typical features and options.  KVM switches most commonly come in two or four-port versions, though I have seen up to eight ports on a single switch. You can utilize more switches by selecting a switch with a master/slave option. The Belkin model I have can be used to daisy-chain up to four banks of switches, for a total of sixteen PC's controllable by a single monitor, keyboard and mouse. Because of its additional features (especially the serial-to-PS/2 mouse converters) the Belkin requires its own power supply, a standard "wall wart" transformer.

    The Linksys switch was less expensive, but is not designed to be extendable. As I said before, it doesn't have its own power supply. But its simplicity allows for easier operation when switching between screens. Since it handles only four screens, the screen number is specified by a single digit (hot key sequence Alt-Ctrl-Shift-N) Because the Belkin can be extended, you must specify a bank number, even if there's only one bank, thus the more complicated hot-key sequence: ScrlLock-ScrlLock-Zero-N. Of course, you could extend the Linksys by connecting the output of additional switches to one or more of the its four inputs, but I wouldn't recommend it, since the switches would then have to draw their power from each other.

Belkin OmmiView SE 4-Port KVM SwitchFigure 2 - Belkin 4-Port KVM switch.  Note additional audio switch, video switch USB switch, and USB-to-PS2 converter.
    Physical configuration is another important option. The Belkin is much larger than the Linksys, but all of its connectors are on the back, making it fit conveniently beside the monitor on your desktop. (See Figure 2 above.) It's also easy to stack additional switches on top. The Linksys is more compact, but it achieves its smaller size by having connectors on three of its four sides. This tends to create a tangled mess of cables on your desk. Perhaps its best installation would be to fasten it vertically to the side of a monitor or a PC box, providing this causes no problem with electrical interference. (See Figure 1.)

    As I said before, another consideration is the vintage of your PC's. Since everything seems to be going to USB, newer KVM switches incorporate USB connectors instead of PS/2. Most still have SVGA connectors for monitors, but the newer monitors use USB as well. Many people may be tempted to use a simple four-port USB switch and daisy-chain the monitor, keyboard, and mouse on the other side, but I wouldn't recommend that. The switch must be able to emulate all of your USB devices so that when a particular PC is selected, the devices do not "disappear" for the other systems. And of course, if you want to add a Macintosh to the mix, you will require an adapter cable for the video, and (for older systems) ADB-to-PS/2 adapters for mouse and keyboard.

    So, although KVM switches have their problems, I'd definitely recommend them to anyone who is into multi-platform development, or even just experimenting with different distributions. They're also good if you end up installing or re-installing on a regular basis. Best of all, they save space, and don't require you to move from one desk to another when operating in cross-platform mode.

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