Tech Flashback: 2-port VGA PS/2 KVM Switch

Rummaging through some of my old equipment boxes, I found this old two-port KVM switch. KVM stands for Keyboard, Video and Mouse and these switches were used commonly where someone wanted to share one keyboard, mouse and monitor with two computers, allowing a user to switch between two machines without doubling up on monitors, keyboard and mice. This was especially popular in offices with isolated systems where staff would have to work on two separate machines. Unfortunately, this particular unit was missing its cables, so I couldn’t use it.

This belongs in the flashback category for the reason that VGA graphics and PS/2 connections are now considered legacy – replaced by DVI/HDMI/DP and USB. The relevance of KVMs are slowly being eroded given the remote administration software and the abilities of Lights-Out-Management solutions with virtual KVM functionality. As such, these units aren’t much of relevance, but it’s worthwhile taking a look at how it works.

The front panel has one button that allows you to switch the input from one PC to the other, and a reset button in case the KVM malfunctioned. There are two LEDs on the front panel to indicate which input is active. Inputs can also be switched by pressing Scroll-Lock, Scroll-Lock, [1,2], Enter on the keyboard.

The inputs on the back are two HD-15Ms for the video signal. Unused/unimportant pins on the HD-15M not used by the VGA signal are used to carry the PS/2 keyboard and mouse signals, and thus the cable needed is non-standard. The keyboard and mouse input is plugged right into the back of the KVM, and monitor output on the HD-15F. The whole unit is encased in metal to keep the RFI that could happen from high frequency video signals from leaking out or being interfered by external RF sources. Unfortunately, adding these KVMs (of questionable quality) often has deleterious impacts on high frequency VGA signals due to poor design. In order to reduce costs, they didn’t opt to use a power adaptor and instead used the 5v from each of the connected computers.

The insides of the KVM are extremely simple:

There’s a small front panel PCB connected by a rainbow wire. There’s a crystal and a PCT-21AK chip (likely an ASIC/micro-controller, no datasheet available) which controls the lot. There is a small beeper to alert us on each input switch. And there’s a smattering of 74HC chips which we don’t see in most modern designs – through hole and discrete logic are a dying breed. Specifically there are two Hitachi HD74LS125AP Quad 3-state Buffer and five Philips 74HC4053N Triple 2-channel analog multiplexer/demultiplexer chips.

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