Serial port Yost standard combines DSR and DCD. Some serial ports on motherboards or add-in cards provide jumpers that select whether pin 1 of the DE-9 connector connects to DCD or a power supply voltage, and whether pin 9 of the DE-9 connector connects to RI or a power supply voltage. The power supply voltage can be +5V, +12V, +9V, or ground. (Selection varies by vendor.) The power is intended for use by point-of-sale equipment. Makers include Dell, HP, and others (This is not an official standard.) Operating systems usually create symbolic names for the serial ports of a computer, rather than
Serial port of the full standard: they traded off compatibility with the standard against the use of less costly and more compact connectors (in particular the DE-9 version used by the original IBM PC-AT). The desire to supply serial interface cards with two ports required that IBM reduce the size of the connector to fit onto a single card back panel. A DE-9 connector also fits onto a card with a second DB-25 connector. Starting around the time of the introduction of the IBM PC-AT, serial ports were commonly built with a 9-pin connector to save cost and space. However, presence of