VIC-20 Technical Details and Trivia

Connectors

  • a serial bus (a serial version of the Commodore PET's IEEE-488 bus) for daisy-chaining disk drives and printers - 6-pin round DIN.
  • a "user port" - a TTL-level port providing 8-bit programmable I/O RS-232 and Centronics signals, e.g. for serial modems - 12-pin male edge connector.
  • a single joystick port on the right-hand side, fully compatible with Atari game console joysticks and with C64 joysticks - 9-pin DB9 plug.
  • a cartridge slot for memory expansion or software cartridges - 48-pin female edge connector.
  • a composite output socket (both audio and video) - 5-pin round DIN (an RF modulator can be fitted to this when using a TV for display). This differs from the C64 video output socket, which has 8 pins to support separate chroma/luma signals.
  • a cassette port - 12-pin male edge connector.
  • a power supply socket - 7-pin round DIN (9V) which is powered by a regulated 9V DC and 5V AC power supply (similar to the one for the C64).
    NOTE:
    Older VIC-20s use a 2-pin power socket - this outputs 9v AC, not DC, from its PSU. These were used on VIC-20 revisions A and B.

 

Hover over the image for connector locations

Technical Facts

  • It came with 5K of RAM, but 1.5K of this was reserved by the system for use with things such as video display and dynamic aspects of the ROM-resident BASIC interpreter and kernel (low level OS).
  • The RAM was expandable using plug-in cartridges, and Commodore released Port Expander boxes that allowed more than one cartridge to be fitted at the same time. Commodore RAM cartridges were available in 3K, 8K or 16K, with or without a BASIC ROM extension. Later, third-parties released 32K RAM expansion cartridges. Whenever a RAM cartridge was installed, the VIC-20's internal memory map was reorganised to cater for the expansion. This meant often that programs were incompatible unless a very specific amount of RAM was detected.
  • BASIC programs running on a fully expanded VIC-20 could access at most 24K of RAM. Any extra would occupy memory space used by ROM cartridges, for example games and other commercial software.
  • Graphically, the VIC-20 could display up to 128x128 pixels resolution with the unexpanded 5K of RAM. When expanded to 8K or more, it could display up to 192x200. There was actually a further 1K on the board (on top of the 5K) used as video RAM for the VIC chip.
  • the VIC chip (aka MOS 5651 in PAL-B form) was followed by the VIC-II in the C64 and the VIC-III in the unfinished C64DX/C65 prototypes, although sound generation was taken out in these two later versions with the new SID chip. The VIC chip would actually get rather hot (as did the VIC-II and TED chips of other CBM machines), and Commodore used several techniques at trying to keep it cool, including thermal paste and metal latches.
  • The standard 'text' mode displayed 22x23, although with a few tricks, it could display up to 29x35.
  • There were many storage options on the VIC-20 - a Commodore VC-1530 "Datasette" (model code C2N) cassette recorder, a serial floppy disk drive such as the Commodore 1540 or Commodore 1541 both of which stored up to 170K per disk (85K per side), other floppy drives usable via the IEEE-488 interface, and later on, 3.5" floppy drives such as the CBM 1581, which stored 800K per disk.
  • The VIC-20 is for the most part, compatible with all Commodore 64 peripherals as well as Commodore PET peripherals.
  • The default screen resolution provided by the VIC-1 chip is 22x23 text characters, or 176x184 graphics pixels. However, with an NTSC VIC-1 (the 6560), it can actually support (by removing all borders) 24x29 text characters or 192x232 pixels. The VIC-1 also supports a virtual screen which can be "panned" so the physical screen becomes a "window" into the virtual screen. The maximum scrollable virtual screen on NTSC is 28x32 text chars, or 224x256 pixels.
  • The one key capability of the VIC that makes full screen hi-res graphics possible is that it can be made to have each character cell use up 8x16 pixels instead of the standard 8x8 pixel character cell (without this hi-res graphics would only fill half the screen).
  • The VIC-20's tape format was deliberately kept identical to that of the Commodore PET, so that PET owners who were upgrading to the VIC-20 could load their PET programs. Compatibility is very high, and if by any chance problems are found, it is usually just a few code lines that need to be altered to have a PET program running perfectly.
  • The user port and cassette port on the VIC-20 were also kept identical to those on the PET, so many plug-in peripherals for the PET will plug directly into the VIC as well.
  • Commodore sold RAM expansion in 3K, 8K and 16K cartridges only. 32K and 64K units were available, created by third party companies.
  • The VIC-20 keyboard is electronically and physically compatible with that used in the Commodore 16 and Commodore 64 model computers (although the C16 keyboard has some keys labelled differently).
  • The VIC-20 was released in three revisions: A, B, and C. Revision A had the old 2-pin power socket with a left/right on/off rocker swtich. Because the power supply only produced 9V, the main board had a 5V converter with a large black heatsink on it. These boards filled the casing fully. Revision B kept this power arrangement, but got some other minor revisions, including a change of logo nameplate on the top. Revision C was a "Cost-reduced" model, which benefited from RAM replacement: one 4 KB×8 chip replaced eight 2114 (1 KB×4) chips. Also a change in power supply socket to be more compatible with the C64. Due to the lower power rating of the VIC power supply, it is NOT recommended to be used with a C64. A C64 power supply can however be used safely with a Revision C VIC-20.

 

Hover your mouse over the circuit board for a description of the components

Component details reproduced with friendly permission from Sothius' Home, www.sothius.com (shame you closed your site, Sothius, it was great! Ed)

Trivia

  • In 1983, in an effort to make the VIC-20 more of a professional machine, Commodore had already begun developing a new 40-column capable VIC chip (to be marketed as the VIC-40), the MOS 6562 - remember the standard VIC chip can only support 22 columns. However, development ceased when the VIC-30 project (soon to be known as the C64) began, with it's VIC-II chip.
    Unlike the PET, Commodore never produced BASIC 4.0 upgrade ROMs for the VIC-20.
  • The VIC-20 was designed by Bob Yannes who also created the SID chip for the Commodore 64. He later joined Ensoniq to design synthesizers.
  • The VIC-20 was codenamed 'Vixen' during development, and 'MicroPET' when first shown to the public, because its final name had not been thought of at that time.
  • The VIC-20 required an external RF modulator (part number CBM1001027-03 for PAL machines) to run with a standard TV.
  • The cassette connector on the VIC was the same as that taken off the Commodore PET, called the "Datassette".
  • When the VIC-20 was being prepared for its first public appearance at the CES in 1980, not one but two prototypes were developed: Bob Yannes had hacked together a minimal working prototype using spare PET/CBM parts. The second prototype, brought to the show by Bill Seiler and John Feagans (who was also the mastermind behind the Commodore kernel) had been put together after some preliminary discussions with Bob Yannes.
  • Commodore computers prior to the VIC-20 all used the IEEE bus to connect to peripherals such as disk drives and printers. The supply of cables for these became very scarce as there was only one supplier of them, Belden Cables. Because this put Commodore in a difficult position, Jack Tramiel decreed that for future CBM computers, they would "get off that bus! Make it a cable that anyone can manufacture.". And so, starting with the VIC-20, the serial bus was born. It was designed to be just as fast as the IEEE-488 that it replaced.
  • The VIC-20 was named after the VIC-1 chip (which provides the VIC-20 with its graphics capability). The "20" is the amount of RAM rounded down (~22K). Michael Tomczyk, who got stuck with the job of deciding on the name, did the rounding. In Germany, the VIC-20 is called the VC-20 because the pronunciation of VIC in German is a naughty word!
  • The Commodore C2N cassette recorder actually records two copies of a program onto the cassette, so even if there is an error with the first load, it records it into main memory and the second load is then used to verify against the first in memory.
  • The official colour of the VIC-20 case is called "Ivory". The regular typewriter keys are in "Chocolate Brown", and the function keys are in "Mustard" !
  • Commodore produced all the important chips used in the VIC-20, since they had their own chip manufacturing facility, MOS Technology, who they bought a year or two earlier. By not having external suppliers for these chips kept the manufacturing cost of the VIC very low. Commodore even sold the 6502 CPU to Apple and Atari for use in their own computers.
  • The VIC-20 could actually use Atari 2600 paddles and joysticks, and vice versa.
  • An IEEE-488 interface, which was standard on the PET, was available as an add-on for the VIC.
  • Apparently the VIC-20 came with five 1K RAM chips because Commodore still had tons of these chips from their calculator days.
  • The VIC was branded the VC-20 in Germany, and in advertisements it referred to the "VolksComputer". In Japan it was branded the VIC-1001.
  • The first VIC-20s keyboard prints were very wide and blocky and rough to touch (the font was called Microgramma Extended), very similar to the PET keyboard lettering. 2nd generation VIC-20s used the more square Eurostile font. Later models used the C64 keyboard font, similar to Helvetica Narrow. At the end of the VIC-20s production life, Commodore shipped some VIC-20s with C64 function keys, which were grey!
  • William Shatner was hired as the official spokesperson for the VIC-20, appearing in a number of U.S. adverts.


This page was last updated on 3rd March 2016.

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