Multiplexing and demultiplexing
Much of the complexity in a KVM extender’s design involves multiplexing and demultiplexing the RGB (separate sync) video and various data streams (PS/2® keyboard/ mouse and serial data) over the four twisted pairs within a CATx cable.
The figure below illustrates one of the most common schemes.
The red, green and blue (RGB) components of the analogue video signal are multiplexed with the horizontal and vertical sync pulses (polarity maintained) so that all five video components can be transmitted across three of the twisted pairs. The remaining pair is used for a bidirectional data channel onto which keyboard, mouse and serial data are multiplexed in a proprietary format.
To maintain signal quality and minimise potential interference between adjacent pairs and externally, all signals are sent differentially.
For each video signal, the extender contains a high-bandwidth active balun circuit, while the data channel can use a system such as RS-485 or a Current Loop.