I have an old MasterView CS104 KVM switch sitting around, still in working condition but without support for USB peripherals. The ports it offers are:

  • VGA
  • PS/2 or DIN keyboard (the switch acts as a passive adapter on each port, thus connections can be mixed)
  • PS/2 mouse
  • Serial mouse

Switching is done electronically: press a button on the switch to cycle through the connected computers. This can also be done via certain key combinations on the connected keyboard, which means the keyboard signal definitely gets intercepted in the switch.

No translation is done between the two mouse port types; the manual states that if the computers use a mix of PS/2 and serial mice, a mouse of each type must be connected to the switch.

Now I am wondering if there is a way to “bolt on” USB support by adding passive homebrew adapters to the mouse ports (either the PS/2 or the serial ones). Will this particular type of KVM switch pass the data lines as well as the supply voltage in the intended manner? Would I need to use specific pins for specific signals?

Note: I am aware of active adapters that allow a PS/2 keyboard and mouse to be plugged into a USB port (with protocol conversion), as well as the inverse for at least some mice, but this is not my question.


closed as off-topic by Chris Stratton, Elliot Alderson, Dwayne Reid, Bimpelrekkie, MCG Feb 5 at 11:16

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  • \$\begingroup\$ It may be possible, especially for low-speed devices like mice and keyboards. To be clear, you're talking about using the USB protocol through PS/2 (mini-DIN) connectors? \$\endgroup\$ – Hearth Jan 29 at 22:07
  • \$\begingroup\$ @Hearth Yes, USB protocol either through PS/2 or through serial DB-9 connectors. \$\endgroup\$ – user149408 Jan 29 at 22:37

Well, with a lot of luck this might work.

But really, that's only luck: USB does actually require some bounded impedance changes on its lines, and while just "any" through connection would work for USB low speed / full speed, you really can't expect things to be reliable, or faster USB drives (USB2 HiSpeed) to work.

Anyway, product page says:

Keyboard Caps Lock, Num Lock, Scroll Lock states are automatically saved and restored when switching among computers.

This means the thing "speaks" keyboard on its DIN port, and hence, won't deal well with USB.

The product page further suggest you can cascade it three levels deep – it wouldn't surprise me if there was slight amplification on the PS/2 port, too, so, that wouldn't be usable for USB passthrough, either.

Ah, even moreso, it speaks of "keyboard emulation" and "mouse emulation". This used to be a nice device, which "played" mouse and keyboard. It's not just a passthrough. Your plan probably won't work.

Let's face it: something for serial mice was simply useful in the early to mid nineties. Something with a DIN connector for a keyboard was useful in the late 1980s.

I'm personally happy the VGA days are gone, too. Modern LCDs are simply way, way crispier than even the midrange professional screens I used in that era.

I wouldn't go that route, but shell out 13€ or so for a USB switch. I just couldn't stand the excitement whether my mouse works reliable or not. You shouldn't, either.

  • \$\begingroup\$ Indeed, I am just seeing VGA resolution is limited to 1024×768. Which explains why I never got it to work with the 1280×1024 screen I was using back then… So even with a VGA monitor, this device is no longer up to today’s standards for screen resolution. \$\endgroup\$ – user149408 Jan 29 at 22:43
  • 2
    \$\begingroup\$ I'd argue that "VGA" and "up to today's standards" is already mutually exclusive, to be honest. \$\endgroup\$ – Marcus Müller Jan 29 at 22:44
  • \$\begingroup\$ I went from VGA CRT to digital LCD in 1998, then to VGA LCD in 2004 and back to digital LCD in 2008, I can say that the big gain is the display technology, not the interface. In my experience, at least with professional equipment, VGA still yields a surprisingly clear picture on an LCD, even at full HD resolution. And, thanks in part due to the 3 different digital interfaces with a total of some 5 different plugs, I still often have to resort to VGA when I’m on the road. It certainly isn’t the best or most modern solution, but it’s still alive and does the job. \$\endgroup\$ – user149408 Jan 31 at 12:09
  • \$\begingroup\$ Well, aside from full hd not being that great a resolution these days: Albeit there being really good screens, with really mature analog interfaces, to get 1920×1080 pixels across 60 times a second, you need a pixel rate of ca 120 MHz. To get sharp transitions without ringing, no matter how great your technology is, math says you need about 5 to 7 times the rate in signal bandwidth. Don't know, haven't found a GHz-specified VGA cable yet, so there's really limits to what VGA can do. \$\endgroup\$ – Marcus Müller Jan 31 at 16:22

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