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Schematic 320026 (Sheet 2 of 3, Board 320008)

Revision as of 14:01, 1 October 2026 by Josh (talk | contribs) (Rewrite from the drawings: sheet 2 has the board's power input (J8, CR1-CR3, VR3-VR6, TP1-1..4); asterisk note is '8K machine only'; 555 at UDE9 is the reset timer; transformer and 23,000 uF cap are in the case; cite PET-Shack wiring and zimmers source)
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Sheet 2 of Schematic 320026 is the memory section of Commodore's main logic schematic for the static Commodore PET 2001 board assembly 320008, the version with 6540 ROMs and 6550 RAM. The title block reads "PET Computer Main Logic, Memory Section, pertaining to board assembly #320008", Commodore Business Machines, Inc., Palo Alto, California. The image here is a separate monochrome scan of the sheet (5,096 × 3,296 pixels) from the PET 2001 schematics directory at zimmers.net.[1] The same sheet is page 2 of the colour PDF at Schematic 320026 (Complete, Board 320008), which also has sheet 1.

What the sheet shows

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The seven 6540 program ROMs, H1–H7, occupy C000–E7FF and F000–FFFF. The sixteen 6550 RAMs are in two rows, I1–I8 for data bits D0–D3 and J1–J8 for D4–D7, two chips to each 1 KB from 0000 to 1FFF. Two 74LS244 buffers, H8 and H9, connect the RAM data lines to the bus.

The chips for 1000–1FFF (I5–I8 and J5–J8) are marked with an asterisk, and the handwritten note beside the title block reads "All ckts marked * are for 8K machine only". A link on the SEL1 line is labelled "Jumper for 8K only".

The sheet also carries the logic board's power input. Connector J8 feeds rectifiers CR1–CR3 (1N5402), C50 and C51 (0.02 µF 500 V), C11–C13 (1.0 µF 25 V) and four three-terminal regulators, VR3–VR6. Each regulator has a 47 µF 16 V output capacitor (C7–C10) and a test point, TP1-1 to TP1-4. The regulator type is not printed on the drawing, and Commodore's parts cross-reference omits VR3–VR6, C50 and C51.

The mains transformer, a 1.8 A fuse and a 23,000 µF capacitor are in the case, not on this board; the transformer's 15.7 V AC centre-tapped winding comes in at J8.[2]

Commodore's note on the sheet reads: "All locations are same on all boards; only the specific components will differ on boards numbered 320132, 320081, 320137, 320008. The identifying board number is in upper left corner." A position read here applies to all four static boards; the full list of boards and their fitted parts is on Schematic 320026 (Complete, Board 320008).

The drawing has no parts list. Commodore part numbers are in System Board Parts List (1 of 3) and 2001 Main Logic Assembly.

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Commodore drawing 320026, sheet 2 of 3, PET Computer Main Logic, Memory Section, board assembly 320008.

Download: 320008-2.gif

References

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  1. ↑ /pub/cbm/schematics/computers/pet/2001/—link(accessed 2026-10-01) Directory listing: the four board PDFs (dated 2020-02-20), the 1998 GIF scans of sheets 2 and 3 for each board, and 320130-1.gif, described as sheet 1 of 3, "applies to all" four static boards.
  2. ↑ PET-Shack (P.S. Software House), PET 2001 schematic and data-sheet bundle, c. 1978, hosted at File:PET-Shack PET 2001 Bundle.pdf, PDF p. 4, "PET console wiring": power switch, 1.8 A fuse, transformer with a 16.2 V AC winding "to video monitor" and a 15.7 V AC centre-tapped winding to the logic board connector, and a 23,000 µF capacitor. Third-party, not a Commodore document.
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