|Operating system||Commodore BASIC 3.5|
|CPU||MOS Technology 8501
@ 1.76 MHz
|Memory||64 kB RAM + 64 kB ROM|
|Graphics||TED (320 × 200, 121 colors)|
|Sound||TED (2 channels, 4 octaves + white noise)|
The Commodore Plus/4 is 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".
Internally, the Plus/4 shared the same basic architecture as the lower-end Commodore 16 and 116 models, and was able to use software and peripherals designed for them. However, it was not compatible with the established Commodore 64.
While the Plus/4 had some success in Europe, it was a total failure in the United States, where it was derided as the "Minus/60"—a pun on the numerical difference between the Plus/4 and the dominant C64.
In the early 1980s, Commodore found itself engaged in a price war in the home computer market. Companies like Texas Instruments and Timex Corporation were releasing computers that undercut the price of Commodore's PET line. Commodore's MOS Technology division had designed a video chip but could not find any third-party buyers. The VIC-20 resulted from the confluence of these events and it was introduced in 1980 at a list price of $299.95. Later, spurred by the competition, Commodore was able to reduce the VIC's street price to $99, and it became the first computer to sell over 1 million units. The Commodore 64, the first 64-kB computer to sell for under 600 US$, was another salvo in the price war but it was far more expensive to make than the VIC-20 because it used discrete chips for video, sound, and I/O. Still, the C-64 went on to become a best-seller and was selling for $199 at the time of the Plus/4's introduction. Even while C64 sales were rising, Commodore president Jack Tramiel wanted a new computer line that would use fewer chips and at the same time address some of the user complaints about the VIC and C64.
Commodore's third salvo — which, as it turned out, was fired just as most of Commodore's competition was leaving the home computer market — was the C116, C16, and 264, which became the Plus/4. There were also prototypes of a 232, basically a 32 kB version of the Plus/4 without the software ROMs, and a V364 which had a numeric keypad and built in voice synthesis. The latter two models never made it to production. All these computers used a 6502 compatible MOS 8501 that was clocked approximately 75% faster than the 6502 and 6510 used in the VIC-20 and C64 respectively, and a MOS Technology TED all-in-one video, sound, and I/O chip. The Plus/4's design is thus philosophically closer to that of the VIC-20 than of the C64.
The Plus/4 was the flagship computer of the line. The Plus/4 had 64 kB of memory while the C16 and 116 had 16 kB. The Plus/4 had built-in software, whereas the others did not. The Plus/4 and C16 had full-travel keyboards; the 116 used a rubber chiclet keyboard that was still superior to the flat membrane keyboards used on less-expensive Timex-Sinclair computers. The C116 was only sold in Europe. All of the machines were distinguished by their dark gray cases and light gray keys. This was a reversal of the color scheme on the 64 and VIC, which used lighter cases and darker-colored keys.
The Plus/4 was introduced in June 1984 and priced at 299 US$. 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 from Commodore's major markets.
The Plus/4 was later used in Denmark, as part of a bundled product from the then-national telecompany (now TDC A/S) to help hearing impaired communicate over telephone lines. Outgoing calls were made from the Plus/4 via modem to a call center where a service assistant would read the written input from the user, call the other party and read the text aloud. Vice versa, incoming calls could be made from other users to the cal lcenter, who would dial the Plus/4 modem. A strobe light connected to the Plus/4 would notify the hearing impaired about the incoming call. The Plus/4 enjoyed lasting popularity in Hungary due to CBM's decision to saturate the Central European market with the failed product at a greatly reduced price. A number of unofficial ports of C64 games were produced by Hungarian users.
The TED offered 121-color (15 colors × 8 luminance levels + black) video, a palette matched only by Atari's 8-bit computer line 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, which used bank switching far more extensively than the C64, gave it a 56% 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. The 1551 plugged into the cartridge port.
The TED chip had identical resolutions and video modes to the VIC-II (bitmap or character graphics which could be high-resolution or multicolor), but lacked hardware sprites. It also had 128 colors (in practice 121 since several of them were identical gray shades) against the VIC-II's fixed 16 color palette in addition to blinking text. Its sound was a weak 2-voice square wave generator.
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's serial port uses (like C64's) 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 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.
The Plus/4 has one of the earliest examples of an Easter Egg. Entering the command SYS DEC("CDAB") (or SYS 52651) puts up the names of three programmers and a hardware designer: Fred Bowen, John Cooper, Terry Ryan and Bil Herd. Bowen's name will be in reverse-field and Ryan's will be blinking.
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 59 kB available, aided by its memory map that swapped the ROMs in and out of memory as needed, and that placed the memory mapped I/O registers, which all MOS-6502-based computers have to use, at the top of memory ($FD00), while in the C64 they had been located at the much lower address $D000. On the C64, a program that did not need BASIC was able to swap out the ROMs and the I/O registers manually and thus gain access to the full 64 kB, but this was not compatible with BASIC on that machine; on the Plus/4, on the other hand, most of the ROM area was automatically switched out when not needed, rendering the RAM existing at the same addresses accessible for BASIC programs.
While the Plus/4's CPU could run about 75% faster than the C64's, the computer was still designed with a shared memory architecture, in which screen data resided in main memory. This means that the video chip has to access the memory while it is displaying the picture, in effect slowing down the CPU to less than half its full speed in this screen area. Only during those periods when the video chip is displaying the screen border or putting out the horizontal and vertical retrace signals is the CPU able to run at full speed. This means that, on average, with standard 40x25 screen the Plus/4's CPU runs only about 25% faster than that of the C64. However, as mentioned the Plus/4 can be 75% faster than a C64 when using screen blanking mode. The Plus/4's PAL model may also be switched to NTSC mode by disabling the screen which sets the CPU frequency to 2.22 MHz (this is 115% faster than C64's maximum speed). So a Plus/4 (PAL) is one of the fastest 6502 based computer for raw calculations. This is true for programs in machine code but the Plus/4's Basic is a bit slower than the C64's, the operations with strings are up to 100% slower. This result is caused by RAM/ROM switching procedures which should be called for with every byte of any Basic program. The NTSC model of Plus/4 is about 10% slower than the PAL model displaying a standard screen but slightly faster with screen blank.
The Plus/4's TED has several advantages over C64's VIC+SID. All TED registers that are available can read and write. TED may realize the blinking cursor and the characters in the reverse-video mode. It may display 256 characters in the text mode. It may use graphics split by raster interrupt and show pictures at 320x248 resolution. This with interlaced mode gives possibility to show 320x496 images. TED also proves to be a very effective means when it comes to creating 4-bit digital sound.
"The PLUS/4 was derived from an existent commercial program “TRILOGY” published by Pacific Tri – Micro. Under license to CBM a total of 600,000 units were built. It was warmly received and critically acclaimed in Europe." "It was translated into 3 languages English, French and German. Commodore dropped support of the PLUS/4 when Jack Tramiel left to run Atari Corp. Pacific Tri Micro continued to support PLUS/4 users until 1988. PLUS/4 users were a loyal group and remember fondly, their running, of their small business with the built-in programs. In those days it took about 20 minutes to load “TRILOGY” from tape, but with the PLUS/4 it was reduced to several seconds." David W. Johnson, developer of the PLUS/4 software
The Plus/4 had three main shortcomings, which proved fatal: unlike the C64's VIC II, the TED 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 equally more capable than the C64. The TED chip also experienced poor reliability.
Another problem that kept the Plus/4 from selling was that even though the three machines (116, C16 and Plus/4) were 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 C116 and C16, which were intended to be the largest selling machines in this series, most software was designed to run in 16 kB 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 the less-lucrative European markets. North American developers continued to concentrate on the booming C64 market.
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. Combined with the 64's greater abilities and broader software base, most buyers opted for the older model.
The Plus/4 does not support analog devices via the joyports like the C64 mouse making programs with a GUI such as GEOS less friendly to use. However there is an unfinished project that allows the use of a standard IBM PC compatible serial mouse.
The Plus/4, unlike the C64 and most other computers of its time (with the notable exception of the Coleco Adam), was equipped with ROM-resident application software (developed for Commodore by TriMicro). 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 equivalently-priced systems, including the C64. Since IBM compatibles were quickly dominating the small business market, the Plus/4 had no realistic chance of breaking into that. Further dividing the market was that once the user had created data using many of the built-in software packages, the result could only be saved to a connected disk drive—much of the software did not support tape. Thus, tape-based home users, the only users who might still have been interested in the Plus/4's less-capable but built-in and instantly ready software, were shut out from the package.
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 TED series (Plus4) was specifically designed to not encroach on the successful C64, it was designed to sell for 49 US$ and to go head to head with the Timex/Sinclair computer line, specifically the color Timex (Spectrum?). 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 Plus/4 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, urged on by the computer press 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.
- CPU: MOS Technology 7501, 1.77 MHz (PAL) / 1.79 MHz (NTSC)
- RAM: 64 kB, of which nearly 60 kB is available to BASIC users. There are known RAM expansions with 256 kB and 1 MB
- ROM: 64 kB including Commodore BASIC 3.5, a machine code monitor, and TRI-Micro's "3 Plus 1" (word processor, spreadsheet, database, graphing). It is possible to add up to 64 kB more ROM with cartridges
- Text mode: 40×25 characters (PETSCII). There are three text modes: standard, extended color, multicolor
- Graphics modes: 160x200 (multicolor) / 320×200 (hires), 121 colors
- I/O ports:
- Tape connector (for Commodore 1531 Datassette with 7-pin mini-DIN; incompatible with C64)
- ROM cartridge slot (incompatible with C64)
- Two 8-pin mini-DIN game controller ports (incompatible with C64)
- Commodore serial bus (compatible with C64)
- User port (for modems and nonstandard devices, incompatible with C64)
- Composite video connector including S-Video and mono audio signal (compatible with C64)
- RF modulator to TV antenna connector (obviously compatible with C64)
- The power connector is compatible with C64 power supplies in some Plus/4s and takes the same 9V AC and 5V DC voltages, but uses a non-standard "Square DIN" plug, like the C128, in most Plus/4s.
See also 
- "Compute's Gazette Issue 12 June 1984 Pg 147".
- "RUN Magazine Issue 35".
- "6502 actual speed at lemon64".
- "The examples of Plus/4 graphics".
- "Plus/4 - RS232 mouse project".
- "Plus/4 and C-16: Disappointingly Mediocre".
- Commodore Plus/4 description
- Plus/4 World – A comprehensive C16 and Plus/4 Game and demoscene site along with books, manuals, PRG files and TAP images of C16 and Plus/4 files
- Commodore16.com – A C16 and Plus/4 website that offers a software database, Documentation, busy forum, scanned books & Hardware shop
- Commodore "TED" 264 Series: The Beginning of the End – From Canadian CBM resource site www.commodore.ca
- Commodore V364 information – From Bo Zimmermann's CBM collection
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