Retro Home Computers – Commodore Plus/4

Commodore Plus/4,

The Plus/4 had 64 KB of memory and had built-in software and full-travel keyboards. The Plus/4 was introduced in June 1984 and priced at US$299. It was discontinued in 1985. It is not completely clear whether Commodore’s intent was to eventually totally replace the C64 with the Plus/4, or whether they wanted to attempt to expand the home computer market and sell the Plus/4 to users who were more interested in serious applications than gaming. However, the Plus/4 succeeded at neither and quickly disappeared.

Commodore Plus/4

Above: Commodore Plus/4 competition for the Timex/Sinclair

Color Pallette

It offered 121 colour 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 layout 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.

Unlike the C64, the Plus/4 had a built-in MOS Technology 6551 UART chip. This allowed the Plus/4 to use high-speed modems without additional hardware or software tricks. However, since most people only could afford 300- or 1200-bit/s modems in 1984, and Commodore never released a 2400-bit/s modem, this feature went largely unnoticed.

The Plus/4 keyboard had a separately placed “diamond” of four cursor keys, presumably more intuitive in use than the VIC’s and C64’s two shifted cursor keys. Also, for serious programmers, the Plus/4 featured a ROM-resident machine code monitor, which rekindled a tradition from the first Commodore computers, the PET/CBM series.

While the C64 had the advertised 64 KB of RAM installed, only about 38 KB was available for BASIC programs. The Plus/4’s BASIC V3.5 made 59KB available, aided by its memory map that placed I/O at the top of memory ($FD00). In addition, the Plus/4’s CPU was about 75% faster than the C64’s.

No Sprite Ability

However, the Plus/4 had three shortcomings which proved fatal: unlike the C64’s VIC II, the Plus/4 had no sprite capability, which strongly limited its video game graphics capabilities. Also, its tone generator was much closer to the VIC in quality than to the C64’s SID, which, again, made the Plus/4 less attractive to game developers.

Finally, the lack of these capabilities made C64 software compatibility impossible. Commodore may not have believed this to be a problem, as the successful C64 was incompatible with most VIC-20 software — but the C64 had developed a large software library by 1984, and while the C64 was a significant upgrade to the VIC-20 in almost every way, the Plus/4 was not.

Another problem that kept the Plus/4 from selling was that Commodore had releases the Commodore 116, C16 along with the Plus/4, and while all compatible with one another, developers tended to write programs for the lowest common denominator in a computer family. So as not to alienate buyers of the 116 and C16, which were intended to be the largest selling machines in this series, most software was designed to run in 16k and the extra memory on the Plus/4 was not as widely supported as it could have been. Also, most development for these machines was in Europe. Few North American developers leapt at the chance to write programs for these machines.

Incompatible ports

Peripheral compatibility with the C64 was inconsistent. The Plus/4’s serial, user, and video ports were compatible with the C64, but the Datasette port was changed, rendering previous units incompatible without third-party adapters that only became available later.

This also posed a problem for the many third-party C64 printer interfaces that allowed one to connect a standard Centronics parallel printer to the Commodore serial port. Since most of these interfaces connected to the Datasette port to get +5 volts for power, they were incompatible with the Plus/4 unless the user modified the interface and risked voiding the warranty.

For a computer intended to be used for productivity applications, this was a heavy weakness. Additionally, with the Plus/4, Commodore abandoned the Atari-style joystick ports used on the C64, replacing them with a proprietary mini-DIN port that was said to be less prone to emit RF interference. While this may have been seen as an advantage by the Federal Communications Commission and other regulatory agencies, end users did not share this view.

This made upgrading to the Plus/4 from the VIC-20 or C64 more expensive, since the user in many cases would have to buy new peripherals in addition to the new computer. It also made the Plus/4 less attractive to new buyers, since VIC and C64 peripherals were more plentiful and less expensive than their Plus/4 counterparts. The street price for a complete C64 system was lower than that of a comparable system based on the Plus/4.

The Plus/4, unlike the C64 and most other computers of its time, was equipped with ROM-resident application software. Unfortunately, the application suite, featuring a word processor, spreadsheet, database, and graphing, was completely inadequate for the Plus/4’s originally intended market of business and professional users. Better business software packages were available for other systems, including the C64.



Above: Timex/Sinclair

Most of the developers of the Plus/4 also worked on the later Commodore 128 project, which was much more successful. The lead hardware designer Bil Herd commented directly on the wikipedia article adding: The Plus4 was specifically designed to not encroach on the successful C64, it was designed to sell for $49 and to go head to head with the Timex/Sinclair computer line, specifically the color Timex. Targeting the office more than the game market, the smallest version of the computer had a total of 9 IC’s, cheapness was the main metric as defined by Jack Tramiel.

After Tramiel left Commodore, the remaining management seemed to not know what to do with the Plus4 line which resulted in untold variations and lack of focus on the targeted market. Since most of the management at that time had only experienced the C64, they tried to market it as another C64 which was exactly what Tramiel had set out not to do.

This shortcomings of the end product were the inspiration for the C128 series as the designers calculated that if they created a computer that was compatible with the C64 that ultimately management and marketing could not damage the C64 software base (much) in spite of how they were to take the product to market.