Editing Commodore Plus/4 (section)

=== Input and output ===

Commodore released a high-speed [[floppy disk]] drive for the Plus/4, the [[Commodore 1551]], which offered much better performance than the C64/[[Commodore 1541|1541]] combination because it used a [[parallel interface]] rather than a [[Serial_communication#Serial_buses|serial bus]]. The 1551 plugged into the cartridge port.

The Plus/4 had a built-in [[MOS Technology 6551]] [[Universal asynchronous receiver-transmitter|UART]] chip that could perform up to {{nowrap|19200 [[Bit_rate|bit/s]]}},<ref>{{cite web|url=https://archive.org/stream/run-magazine-35/Run_Issue_35_1986_Nov#page/n103/mode/2up|title=RUN Magazine Issue 35|date=November 1986}}</ref> unlike the C64, which emulated the 6551 chip in software.{{citation needed|date=January 2023}} This allowed the Plus/4 to use high-speed [[modem]]s without additional hardware or software tricks (the C64 required specially written software to operate at {{nowrap|2400 bit/s}}), at a time when 300- or 1200-bit/s modems were more common{{snd}} and Commodore never released a 2400-bit/s modem{{snd}} so this feature went largely unnoticed. The Plus/4's serial port is the standard Commodore user port used since the PET, featuring [[Transistor–transistor logic|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.{{synthesis inline|date=January 2023}}

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.