The interface described in this article is based on a Microchip PIC16c84 microcontroller.
A lot of different PC mouse designs were created and also a lot of them are still present. In short, you’ll find three main types of PC mice: serial, PS/2 and USB mice, connecting to the RS-232, a dedicated PS/2 mouse and the USB port, respectively. Some mice are ‘combo’, thus they can be connected to both serial / PS/2 or PS/2 / USB ports (usually, with a small adapter which is shipped with the mouse). The latters are clear designs, but the serial one is not so much obvious. The serial port was in general designed for communication and not for connecting a standalone external device. The mouse needs some current for its operation. Since there is no dedicated PS voltage on the serial port, the current is drawn from some unused signal outputs (that are set to the appropriate level by the mouse driver software on the PC). This is a bit suspectible, but since these mice are usually manufactured with low power technology, the supply current is relatively small, about some 10-15 milliampers.
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