A serial port is a communication interface where data flows in a serial manner, one bit at a time. The term ‘serial port’ usually refers to the RS-232 standard compliant interfaces though many other types of interfaces like USB and Ethernet also utilize serial communication for data transfer. Originally defined as a 25 pin interface, designers have often implemented only subsets of the whole specification and developed interfaces like the 9 pin serial DE-9 connector and TIA-561 connectors.