DB (D-Sub) Connectors and COM Ports A serial port on a PC is a male 9-pin connector (DE-9 D-sub). Early PCs had two 9-pin connectors or one 9-pin and one 25-pin (DB-25). On a PC, serial ports are ...

it may well have come with an adapter for a 9-pin RS232 serial port. Those mice from a decade or more ago would have contained the software to recognise the interface into which they were plugged ...