A bus mouse was a specific form of PC mouse used mainly in the late 1980s. Its connector was the same shape as the more common PS/2 mouse connector, but had a different set of pins. It shouldn't be confused with the universal serial bus mouse, or USB mouse, which was the most popular form of mouse connector as of 2011.
The bus mouse came into popularity in the late 1980s at a time when IBM PCs did not have a built-in connection to computer mice as standard. This led to the development of two ways to connect a mouse. The first was through a serial port, which was then a standard way to connect all peripherals to a computer. The main drawback of the serial port was that a computer could only receive information through one serial port at any precise moment, which could slow down the use of attached devices.
The second main method was the bus mouse. This used a dedicated plug and socket design. The design was also known as Microsoft InPort®, after a specific brand of mouse made by the software giant.
In time, the bus mouse lost popularity to the PS/2 system. This name refers both to a range of computers made by IBM, and to a specific set of keyboard and mouse ports designed for the range but adopted by many other PC manufacturers for the sake of compatibility. Although the PS/2 range did not go on to dominate the PC market as IBM had hoped, the keyboard and mouse ports became the closest thing to a standard in PCs for most of the 1990s.
The similarities between the bus mouse and the PS/2 mouse had the potential to cause problems. Both used a circular plug and socket that was 5/16ths of an inch in diameter. The pins used in the plugs were different, though; the PS/2 had six pins, roughly in a circle, while the bus mouse had nine pins in a shape similar to a toadstool mushroom. This could cause problems with users mixing up the two designs and attempting to jam the wrong plug into a socket.
By the 2000s, the bus mouse had almost disappeared, and the PS/2 mouse also fell in popularity. Both were widely replaced by the USB mouse, which plugged into a USB port. This removed the need for dedicated mice ports, which was a useful space-saving measure in portable computers. It also allowed the supply of power to a device, which allowed for the design of optical mice that did not need batteries.