While the RS-232 standard originally specified a 25-pin D-type connector, many designers of personal computers chose to implement only a subset 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).
Kuigi RS-232 standard määras algselt 25 kontaktiga D-type ühenduse, otsustasid paljud koduarvutite disainerid kasutada ainult osa standardist: nad valisid standardiga ühildumise asemel odavama ja kompaktsema ühenduse (Eriti D-Sub-9 versioon, mida kasutati originaal IBM PCl).