Pred ten ten "seriovy port" bych radeji predradil slova "IEC" nebo "IEEE-488" nebo "Commodore " https://en.wikipedia.org/wiki/Commodore_bus
Pod pojmem seriovy port vetsina lidi chape RS-232 a ten je prave k dispozici na user portu (i s podporou v BASICu pres zarizeni cislo #2). Takze rs-232 modemy se pripojovaly k user portu https://www.c64-wiki.com/wiki/User_Port ale chtelo to adapter https://www.c64-wiki.com/wiki/RS-232.
Taky klasicke LPT tiskarny z PC se pripojovaly na user port pasivnim adapterem.