You may also find computers that have PS/2 or USB connectors ... done using a 9-pin outlet on one computer connected to another computer through a null modem: a special, RS-232 serial cable ...

USB-to-serial adapters are available for old peripherals. 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 ...