PS2-Serial-Mouse-Adapter

Many retro PCs were used with serial port mouse and don't have any PS/2 ports. Unfortunately most of the serial mice are broken or expensive today. Furthermore they were mechanical devices with a heavy steel ball to track the movement. This mechanics got dirty quite fast and was not the best user experience anyway. A better solution are optical PS/2 mice, most of which unfortunately would not work on a serial port. This adapter can be used to connect such a PS/2 mouse to a serial port of a retro PC.The Adapter is made from through-hole parts only, so it is very easy to solder and due to Arduino based solution, it is also a very nice device for tinkering, experimenting and learning about mouse protocols. The serial port on the computer communicates with the mouse using basically two pins - RTS and RX. RTS is used for notification and RX for the actual data transfer, which contains mouse position, buttons state etcetera. A standard PS/2 mouse on the other hand uses completely incompatible protocol and transfers the data between the mouse and the computer using the Data and Clock pins. If we want to connect a PS/2 mouse, to a serial port, we need some instance in between, to translate between the two different protocols. In this adapter the translation is done using an Arduino Mini. However, there is another problem. PS/2 mouse protocol uses so called TTL logic, which simply means, that the voltage of 0V means a logical zero and 5V means a logical one. TTL logic is what Arduino uses as well, but a serial port, or also called RS232, uses another voltage levels. Any voltage below -3V is a logical one and a voltage above +3V is a logical zero. So, it is not possible to connect Arduino output directly to the serial port. It simply will not work and needs another translator, which converts TTL voltage level to RS232 levels. In this adapter such conversion is made by a well known MAX232 IC. The software is written in C/C++, it is a simple Arduino project.