I recently bought a TrendNet TK-207K KVM switch. After hooking up my mouse and keyboard, I was suprised to find out that the setting did not work. The mouse and keyboard would act strangely and sometimes one would stop working altogether. At first, I thought that I might have gotten a defective unit. But I quickly realized that it was a power issue — The USB port was not able to provide sufficient power to both my mouse and my keyboard.
Even though the USB 2.0 specification states that a USB port should be able to suplly 5 unit loads (500mA) but in practice I have seen this varies greatly from computer to computer. In my particular case, while the combined current draw for my mouse and keyboard is only around 300mA, the voltage dropped below 4.5V when measured at the KVM’s USB connector side.
The solution to this problem is simple enough. If you already have a powered USB hub lying around, use it and the problem will go away. Otherwise, you can build a simple externally powered USB connector like this.
Since I have spare USB connectors lying around, I build an externally powered USB connector like the one shown above. By plugging in either the keyboard or the mouse to this powered USB connector, the problem went away and the KVM switch worked beautifully.