The Commodore Plus/4 was a home computer released by Commodore International in 1984. The "Plus/4" name refers to the four-application ROM resident office suite (word processor, spreadsheet, database, and graphing); it was billed as "the productivity computer with software built-in". It had some success in Europe, though it was a total flop in the United States.
The TED offered 121-color (15 colors × 8 luminance levels + black) video, a palette matched only by Atari Computers at the time, and 320×200 video resolution, which was standard for computers intended to be capable of connecting to a television. The Plus/4's memory map gave it a larger amount of user-accessible memory than the C64, and its BASIC programming language was vastly improved, adding sound and graphics commands as well as looping commands that improved program structure. Commodore released a high-speed floppy disk drive for the Plus/4, the Commodore 1551, which offered much better performance than the C64/1541 combination because it used a parallel interface rather than a serial bus. However, the Plus/4 did not have the parallel interface built-in; it was provided by a plug-in cartridge supplied with the drive.
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 19200 bit/s. 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 2400 bit/s). But most people could only afford 300- or 1200-bit/s modems in 1984—and Commodore never released a 2400-bit/s modem—so this feature went largely unnoticed. 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 C-64. Also, for serious programmers, the Plus/4 featured a ROM-resident machine code monitor, which rekindled a tradition from the first Commodore computers.
Source: Wikipedia, "Commodore Plus/4", available under the CC-BY-SA License.