# Connecting computers to single monitor/keybd? **URL:** \ **Category:** Factual Questions\ **Created:** [September 23, 2002, 4:38pm UTC](https://boards.straightdope.com/t/connecting-computers-to-single-monitor-keybd/129289 "2002-09-23T16:38:00Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![av8rmike](https://avatars.discourse-cdn.com/v4/letter/a/edb3f5/32.png) [@av8rmike](https://boards.straightdope.com/u/av8rmike)\ **Post date:** [September 23, 2002, 4:38pm UTC](https://boards.straightdope.com/t/connecting-computers-to-single-monitor-keybd/129289/1 "2002-09-23T16:38:00Z")
I have one or two older computers at home, along with a newer one that I’d like to be able to control from a single monitor, keyboard, and mouse. We have something similar at the office because I’ve seen one person using two machines at once, and I know there’s something in the server room. However, when I tried looking for it at [CDW](http://www.cdw.com), all I found were networking switches or things that cost \>$250. Does anyone know where I can find something like this that’s much less expensive? Any brand recommendations? Does it matter that my newer machine uses PS/2-style connectors and the old ones use serial & older-style connectors? ---
**Author:** ![NotMrKnowItAll](https://avatars.discourse-cdn.com/v4/letter/n/6bbea6/32.png) [@NotMrKnowItAll](https://boards.straightdope.com/u/NotMrKnowItAll)\ **Post date:** [September 23, 2002, 5:05pm UTC](https://boards.straightdope.com/t/connecting-computers-to-single-monitor-keybd/129289/2 "2002-09-23T17:05:00Z")
I have a Bytelan Keyboard-Video-Mouse Switch (KVM/4), and I am not exactly pleased with it. The way these things work is that there’s electronics in the box for each computer that pretends to be a keyboard and mouse. This is so when you switch over, your computer doesn’t detect the switch out as an unplugging. For my bytelan it is iffy. My Dell Linux box has no problem with the mouse being switch away and back, but my Gateway with 98SE does. Seemingly ramdomly, I’ll switch back and not have a mouse. I have a USB mouse on that machine as well, and pretty much ignore the “shared” mouse. An additional “gotcha ya” with this box is that after paying around $100, you have to buy somewhat expensive cables to patch each computer’s video, mouse, and keyboard into this box. For mine, it does support both types of keyboard connectors as well as PS/2 and Serial mice. Be sure to label each cable as to whether it’s mouse or keyboard. All said, the cost was comparable to the price of a cheap 17 inch monitor. So my only recomendation is to ask around on the internet about a particular device before buying. ---
**Author:** ![Duck\_Duck\_Goose](https://avatars.discourse-cdn.com/v4/letter/d/50afbb/32.png) [@Duck\_Duck\_Goose](https://boards.straightdope.com/u/Duck_Duck_Goose)\ **Post date:** [September 23, 2002, 5:13pm UTC](https://boards.straightdope.com/t/connecting-computers-to-single-monitor-keybd/129289/3 "2002-09-23T17:13:33Z")
IANA computer geek, but I think you’re looking for something called a “single user KVM switch”. [http://www.42u.com/kvm\_switch\_history.htm](http://www.42u.com/kvm_switch_history.htm) > [@](#): > > In the early 1980’s as the computer industry grew, many server rooms and data centers were faced with the problem of having dozens and even hundreds of monitors, keyboards, and mice, taking up valuable rack space, and adding unnecessary heat disbursement issues. They also created server management problems for larger data centers in which technicians had to physically walk to each server they needed to work on. > > Who created the 1st KVM switch (Keyboard Video Mouse Switch) is disputed. Actually the very 1st switches would more approximately be called KV switches. As both graphical environments and mice were not common in the early 1980’s, the 1st switches only supported keyboard & video switching. The first switching units were very basic A/B type push button switches, grew to hot-key controllable switches, and later added OSD’s (On Screen Displays). > > Addressing some of these issues, these KVM switching products allow a single user to access multiple CPUs from a single monitor, keyboard, and mouse. In addition to improving server manageability, heat disbursement issues, and the space savings, there was a huge cost savings from not having to purchase a separate monitor, keyboard and mouse for each CPU. > > Single user KVM Switches are widely used today and can be found in almost every data center. The best of these products have intelligent on-screen firmware, are software independent, and allow you to connect multi-platform CPUs - such as IBM PC and Compatible, Apple Macintosh, Sun, Silicon Graphics, IBM RS/6000, DEC Alpha, HP 9000. This website has them starting at $89.00. Just Google “single user KVM switch”, and not only are there tons of hits, but there are ads on the right-hand side, too. Shop around. [http://www.kvm-switches-online.com/single-user-kvm-switches.html](http://www.kvm-switches-online.com/single-user-kvm-switches.html) I have no clue on the “serial vs. parallel” stuff, sorry. ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [September 23, 2002, 5:55pm UTC](https://boards.straightdope.com/t/connecting-computers-to-single-monitor-keybd/129289/4 "2002-09-23T17:55:30Z")
Beware that KVM switches are only guaranteed to work with standard mice and keyboards. Anything odd (like the ergonomic mouse I ordered) probably won’t work. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [September 23, 2002, 5:57pm UTC](https://boards.straightdope.com/t/connecting-computers-to-single-monitor-keybd/129289/5 "2002-09-23T17:57:00Z")
You can shop by entering KVM switch into Google. I just bought one factory direct from [D-Link](http://www.d-link.com/). It’s a 2-port KVM (2 computers, 1 Keyboard/video/mouse). It was $50. I bought 2 six-foot cables for $15 each. These are way cheaper than anything I was able to find. This particular switch was mentioned in a recent issue of PC Magazine where they give answers to all kinds of “How can I. . .” questions. The switch has active electronics so uses a power supply, included. It saves the state of num lock/caps/scroll lock for the keyboard. The cables combine video and 2 PS/2 ports in one combo cable. Since your older computer uses what sounds like a serial mouse and an older keyboard port, you might need either a converter cable or a different kind of switch. Also, for your computer to be able to support the kind of switch I bought, you have to make sure you can unplug and replug your keyboard and mouse without having to reboot. Sounds like you probably can’t use that. There are switches that don’t require the computer to support this like the kind \*\* NotMrKnowItAll\*\* discusses. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [September 23, 2002, 5:59pm UTC](https://boards.straightdope.com/t/connecting-computers-to-single-monitor-keybd/129289/6 "2002-09-23T17:59:01Z")
> [@](#): > > \*Originally posted by ultrafilter \* > \*\*Beware that KVM switches are only guaranteed to work with standard mice and keyboards. Anything odd (like the ergonomic mouse I ordered) probably won’t work. \*\* I can’t dispute that some won’t work, but this surprises me since I thought the signals to the port are basically the same. I have a wireless mouse and it works just dandy with my switch. ---
**Author:** ![happyheathen](https://avatars.discourse-cdn.com/v4/letter/h/b5e925/32.png) [@happyheathen](https://boards.straightdope.com/u/happyheathen)\ **Post date:** [September 23, 2002, 6:54pm UTC](https://boards.straightdope.com/t/connecting-computers-to-single-monitor-keybd/129289/7 "2002-09-23T18:54:58Z")
I have 3 machines (2 AT, 1 ATX) connected through $15 switches and $8 cables). Works. No electronics, just rotary switches. This is WIN9x, don’t know about the newer versions. ---
**Author:** ![buckgully](https://avatars.discourse-cdn.com/v4/letter/b/eb9ed0/32.png) [@buckgully](https://boards.straightdope.com/u/buckgully)\ **Post date:** [September 23, 2002, 8:29pm UTC](https://boards.straightdope.com/t/connecting-computers-to-single-monitor-keybd/129289/8 "2002-09-23T20:29:46Z")
I have some of the cheap rotary switch types that happyheathen describes, and they work ok for the keyboard and mouse, but the video coming out of them is generally pretty poor. They introduce a lot of signal noise and echo, so high contrast areas have little shadows follwing them around. They work fine for my linux servers that live in my closet. I wouldn’t want to use them for a machine I was working on all day long, though, as the blurryness would give me headaches. Another downside of these type is that when you reboot a machine, you need to have the KVM switched to that machine, otherwise you’ll get keyboard errors, since the machine won’t think it has a keyboard connected to it. I also have a 4 port Belkin USB kvm switch which is very nice and has no noticeable effect on the video. It also tells the machines connected that they have keyboards and video even when those computers aren’t the active ones, which means you can reboot a computer connected to the switch while still working on another machine. The downside of that one is that it cost around $300 when I bought it. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [September 23, 2002, 9:11pm UTC](https://boards.straightdope.com/t/connecting-computers-to-single-monitor-keybd/129289/9 "2002-09-23T21:11:17Z")
The keyboard should work fine. You are going to have a problem with the mouse, however. There are basically two types of old keyboards, XT and AT. XT keyboards use a slightly different protocol and only work with XT class computers (like the original PC and XT, and most 8086 clones). Since XTs are dinosaurs, we can pretty much ignore them. PS2 keybaords, despite having a different connector, are AT keyboards. So, you can use them with a regular KVM switch, although you will have to have a connector adapter in there somwhere to handle the different size connector. For the mouse, you’ve got a problem. A PS2 mouse uses the same sort of signals as a PS2 keyboard, just a slightly different protocol going back and forth. A serial mouse on the other hand uses RS-232 signals, which are completely different than the TTL level signals used by they keybaord and PS2 mouse. A simple connector change won’t do it. You can change to a serial mouse on the newer computer, then use a KVM switch designed for a serial mouse and PS2 keyboard. You can’t use your existing mice though. They are just incompatible with each other. PS2 keyboards are basically using TTL level digital signals with the microcontroller in the keyboard wired directly to the microcontroller on the motherboard, with no safety or buffering circuits. They aren’t designed to be hot swapped (unplugged and plugged back in while turned on). The $80 switches have buffering circuits in them that handle the hot swap issue. The $15 switches don’t have any ciruits in them, they are just switches. If you just use one computer at a time and turn them off before switching the keyboard from one to the other then you can use the el cheapo switches. If you switch them under power though you risk damaging the keyboard or motherboard. The risk is a lot smaller than it was back in the days of XT and 286 computers (because of advances in microcontroller designs, so they aren’t quite as fragile as they used to be) but its still a risk. Serial mice, on the other hand, use RS-232 protocol, which is designed for hot swap. USB keyboards and mice are also designed for hot swap. Another solution, depending on what operating system you are using, is to use software called terminal server, which basically allows one computer to log into another computer. You get a window which looks like the desktop of the other computer, and while that window is selected your keyboard and mouse operate the other computer. If you make the window full screen then it looks like you are at the other computer (which causes all kinds of fun where I work… oops, I rebooted the wrong computer!). The down side of this is that the “server” has to be running server software, i.e. NT 4 server, windows 2000 server, etc. The operating systems most people run at home (9.x, NT workstation, 2000 and XP home and pro) don’t have the software to do it. There are 3rd party software programs like PC anywhere which you could use. ---
**Author:** ![av8rmike](https://avatars.discourse-cdn.com/v4/letter/a/edb3f5/32.png) [@av8rmike](https://boards.straightdope.com/u/av8rmike)\ **Post date:** [September 23, 2002, 9:28pm UTC](https://boards.straightdope.com/t/connecting-computers-to-single-monitor-keybd/129289/10 "2002-09-23T21:28:50Z")
Wow… lots of suggestions, and even a few opinions. Thanks so far, looks like I need to go home and digest. **e\_c\_g** , suppose I just dig one of my old serial mice out of the trash? Would that be a better idea than the black magic required to get the box to talk to the older computer? ---
**Author:** ![Mr.Slant](https://avatars.discourse-cdn.com/v4/letter/m/c57346/32.png) [@Mr.Slant](https://boards.straightdope.com/u/Mr.Slant)\ **Post date:** [September 23, 2002, 10:26pm UTC](https://boards.straightdope.com/t/connecting-computers-to-single-monitor-keybd/129289/11 "2002-09-23T22:26:35Z")
If you buy a KVM switch, make sure you can return it. Half of the ones under $250 are garbage. All of the ones under $250 will throttle your maximum resolution and refresh rate relative to what they could be. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [September 23, 2002, 11:07pm UTC](https://boards.straightdope.com/t/connecting-computers-to-single-monitor-keybd/129289/12 "2002-09-23T23:07:44Z")
A serial mouse should work easily on both computers (except some of the newest ones that aren’t coming with serial ports). That and a KVM switch and you should be all set.