VIC20Reloaded-1.1-002

VIC-20-Reloaded

Reimaging of the original VIC-20cr ("cost-reduced") main board in a smaller footprint based on schmetics of the 251027-01D version as redrawn by Steve Gray. This should represent changes through ECO 830151 as of 4/12/1983.

Introduction

This project is a re-implementation of a VIC-20 based on the cost-reduced version from 1983. It's built using commonly-existing TTL (all of the original types of chips used are still generally available) and original core chips from a VIC-20 (the CPU, VIAs, VIC, and original mask ROMs) which were typically socketed. The memory chips (2114L and 6116), which are not typically socketed, are still available as used or new-old-stock.

How is this version similar?

Since it's based on the original schematics, it's identical operationally and 99% identical electrically (except as noted in the next section). I also tried to keep the parts placement relationally similar to the original, recognizing that the board size is different. Parts designators are the same.

How is this version different?

Size

My main goal was to reimplement the VIC in a smaller form factor. I was able to squeeze it {mostly} into an S-100 format (which is 10" x 5"; actual is 10.375" x 5.375") size. As such, this board isn't a drop-in replacement. However, the spacing and placement of the side (power/joystick) and the rear (expansion cartridge, video, serial IEC) are placed at the correct distances from the upper-right corner.

Ports

Due to the smaller form factor, the cassette port was eliminated (instead opting for a 6-pin header for an external driver) and the user port was moved to the left side. The orientation of the expansion connector was changed to vertical (more like the Atari 2600 which saved about 2" of board space). Using the expansion connector is limited to one cartridge, or an expansion board that uses two pieces connected by a cable (there is another repro here that has an example).

Power

Electrically, I opted for a single 5V power supply (9VAC wasn't needed as there is no cassette port), and the power switch is slightly different due to parts availability. The main fuse holder is different, but the same 5x20mm fuse is used. Large axial capacitor was replaced with smaller radial cap of the same value.

Other

Since the board is autorouted to a smaller footprint, circuit paths and keepout areas are different from the original.

Versions

• v1.0-001: Initial prototype based on original schematic; screen artifacts
• v1.1-001: Second prototype with a rearranged chip layout; screen artifacts.
• v1.1-002: Third prototype. A transcription error was found in the original schematic.
  Corrected. Works.
• v1.1-002E: Final version.

Known Bugs

Current Release (1.1-002E):

• The signal flag "S01" was incorrectly used for the CPU PHI1 output. The system works with this improperly connected signal, but the fix is to lift pins UE10.3 and UC3.3 and connect them with a jumper. The way the traces are routed make it hard to make a single-cut fix.

Prior Release (1.1-002D):

• An error was introduced in this version as a result of rearranging the data bus lines lines on IC UF8 (74LS245). D4 (pin 4) and D5 (pin 5) were reversed. There is a picture (ECO_001) which indicates a convenient place to cut two traces and cross-wire them. Will be fixed in the next version.