Unlike the C64 which emulated the 6551 chip in software, the Plus/4 had a built-in MOS Technology 6551 UART chip that could perform up to . This allowed the Plus/4 to use high-speed modems without additional hardware or software tricks (the C64 required specially written software to operate at ), at a time when 300- or 1200-bit/s modems were more common and Commodore never released a 2400-bit/s modem so this feature went largely unnoticed. The Plus/4's serial port is the standard Commodore user port used since the PET, featuring TTL voltage which is incompatible with RS-232. This requires a voltage converter to use modems or other serial devices from non-Commodore vendors. The Plus/4 keyboard had a separately placed directional "diamond" of four cursor keys, presumably more intuitive to use than the VIC's and C64's two shifted cursor keys. A reset button was added on the right side of the system, a feature lacking on the C64. The Plus/4 also revived the built-in machine language monitor from the PET days, a feature missing on the VIC-20 and C64.