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Commodore VC-20 CR
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(pictures copyright by M.A.Grundke)

serial number: WG A137593

Overview of the ASSY 250403 board: this must be one of the first 'cost-reduced' boards, in which several 1Kx4 RAMs have been replaced by 2 pcs. 2Kx8. However, it still needs an external RF-modulator, which the later VIC-30 (aka C64) doesn't. Back to the board: left, beneath user-port, are the two CIAs (UAB1, UAB3). Beneath them the pair of 2Kx8 RAMs (U15,U14) mentioned earlier, and again three RAMs (UD2, UE2, UE1), this time 1Kx4, 2114-type, to their right. Next to them the VIC (UB7, together with some TTL video-logic covered by a shielding cage), with the character generator ROM (MOS 901460-03, UD7) and two MOS 65245 (UE8, UD8) below. Further right, the board's upper half is dominated by the massive cartridge interface (P6). Below that, among some 74xx logic and another 65245 (UF8), are CBM BASIC (UE11) and CBM KERNAL (UE12). Right from there, the MOS 6502 CPU (UE10), as well as power suppy- and joystick connectors

board silkscreening, part 1 (right under cartridge slot shielding): MDK 25 HB

board silkscreening, part 2 (board's lower side, which has a dark-green upmost layer): VIC 20 ASSY NO.250403 (C)1982 COMMODORE INTL FAB NO.251040-01 REV.D

The VC-20's CPU, MOS 6502A, made with CBM's NMOS-process and ancestor to MOS 6510 (64-series), MOS 7501/ MOS 8501 (264-series), MOS 8502 (128-series), and others. The MOS 6502, which was invented by Chuck Peddle and used first in the PET 2001, was probably the most successful 8 bit microprocessor of its time, head-to-head with the Zilog Z80, used in competing designs. Interestingly, there was also a MOS 6501, which was a 6502 with pinout compatible to the Motorola MC6800 (MOS and Motorola were direct competitors in the market of 8 bit CPUs these days). Its production, however, was stopped in early state because Motorola threatened CBM with legal action

The VC-20 CR's RAM bank, which is one of the most significant changes compared to the older machines. These came with 2114 (1Kx4) RAMs only. In this cost-reduced design, eight of the 2114 were replaced by two 2Kx8 (Fujitsu MB8128-15 in this case, left). Three 1Kx4 (here Matsushita MN2114-3, right side) remained, one of them being used as video RAM for VIC. Cost reduction, by the way, always was one of Commodore's design goals (and of other manufacturers, of course). That was especially necessary for RAM configurations, as in these early days of computing, RAM could be quite expensive. Thus, by replacing RAM chips with different types, one could, for example, use the cheapest chips available on the market. Another type of cost-reduction was to simplify the board design by integrating several chips into one; for example, older boards use lots of TTL ICs (i.e. 74xx-type). These 'TTL-graveyards', as some call them, became significantly cheaper when all these logic functions were integrated into one chip, a so-called gate array (because it has lots of gates)

The machine's graphics- and soundchip, VIC (aka MOS 6561 in the PAL-B version), surrounded by video-logic. 'VIC' means 'video interface chip; it's the ancestor of the 'VIC family', and was followed by VIC-II used in the C64 series and the VIC-III in the unfinished C64DX/ C65 prototypes (although sound capability was 'outsourced' in these machines, into the famous SID soundchip). All these are descendants of the very first and primitive standard CRTCs, that could merely generate display output. So it's even more interesting that Commodore integrated hi-res color graphics, four-voice sound and an A/D converter on one chip. This VIC's plastic, by the way, looks a little worn, so I guess it's getting quite warm under that shielding (a problem which the later VIC-II and TED chips shared - Commodore later used cooling paste and other 'tricks', like metal latches, to cool them down)

The two MOS 6522 'VIA' ('versatile interface adaptor') used for the VC20's I/O. This 'versatility' includes controlling keyboard, user- and controller-ports and the CBM serial-bus to drive floppies and printers. One of the reasons why the keyboard connector can be found next to them. Left here, the machine's 24-pin user-port, which was also known for its versatility. Lots of users built their own hardware for the user-port, such as for controlling- or measuring purposes

Here are two of the VC-20's ROMs, C901486-07 (CBM Kernal Rev.7) and C901486-01 (CBM BASIC V2). To their left, one of the machine's three MOS 65245 octal bus transceivers. They are just MOS-customized variants of the 74LS245, providing a bi-directional interface between data buses (e.g. the CPU's data- and address buses)

Here the machine's third ROM, the C901460-03 character generator. As the name implies, it creates all characters used for screen-output (i.e. if you press a key, the corresponding character is being generated and then displayed by VIC). Since the VC-20 was also sold to countries with different character sets, there have been different character generator ROMs, such as japanese or swedish/ finnish variants. However, unlike other Commodore computers (PET-series, Plus/4 or C128), the VC-20 was never available with a DIN character set for Germany. To its right, finally, the other two 65245 mentioned above

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