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Commodore PET 2001 ROM Replacement

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MOS 6540 ROM from a Commodore PET 2001
A MOS 6540, the 28-pin mask ROM of the early PET 2001, with corrosion on some of its pins

The original Commodore PET 2001 static board holds its BASIC, screen editor and operating system in seven ROMs at UH1-UH7 and its character set in a character generator ROM at UA2. Boards 320008 and 320132 use the MOS 6540, a 28-pin 2K × 8 mask ROM; boards 320081 and 320137 use the 24-pin 2316B in the same positions.[1][2] The 6540 is pinned unlike any standard ROM, so a failed one cannot be replaced by a modern EPROM directly; adapter boards and a ROM/RAM replacement board do the job instead.[3][4] Tynemouth Software regards the 6540, like the 6550 RAM, as unreliable.[5] The procedure below finds a failed ROM and replaces it.

Symptoms

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  • A screen of random characters at power-on that never clears: the processor is not running its start-up code, usually because of a ROM or RAM fault. Ken Shirriff's PET started this way with two of its seven system ROMs failed.[5][3]
  • Characters that are not quite right, such as an E with an extra bar: the character ROM at UA2.[4]
  • Characters that keep changing shape, or vertical lines in the same place in every character row: the character ROM or the shift register that takes its data.[6][1]
  • A 6540 that runs very hot. Tynemouth's advice is to remove it; a hot 6540 or 6550 slowly damages the chips working alongside it.[7]
  • A PET that starts but behaves wrongly, printing garbage or crashing in BASIC. Shirriff traced this to a single unreliable byte in a newly programmed replacement ROM.[3]

A garbage screen also comes from RAM faults; see Commodore PET 2001 RAM Replacement and the Commodore PET 2001 Troubleshooting guide.

Safety

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Warning: the mains switch, fuse and transformer primary are live whenever the PET is plugged in, and the CRT anode runs at about 10 kV and can hold its charge after switch-off.[8][9] Unplug the machine before working on it and see the CRT Discharge Procedure.

Tools and parts

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  • A ROM reader or chip tester that knows the PET ROM contents. Shirriff used a Retro Chip Tester, which identified the two failed 6540s.[3]
  • For each failed 6540: an adapter board that carries a 2716 EPROM in the 6540's socket, and a programmer able to program the 2716 properly. Shirriff's first replacement was programmed by a programmer that could not supply quite enough voltage for the 2716, and it gave one bad byte.[3]
  • For the character ROM, Tynemouth makes an adapter for 2001-8 machines with a 6540 at UA2; on most other PETs a 27C16 or 2816 fits directly.[4]
  • Alternatively a PET ROM/RAM board, which plugs into the processor socket and replaces the system ROMs and main RAM together.[7][10]
  • ROM images. Tynemouth built his EPROM images from the files at zimmers.net.[11]
  • An IC extractor. Commodore fitted all seven system ROM positions and UA2 with sockets: 28-pin for the 6540, 24-pin for the 2316B.[1]

Which ROM is where

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Commodore PET 2001 schematic 320026 sheet 2, 6540 ROMs H1-H7
Commodore's schematic 320026, sheet 2: the seven 6540s with their address ranges. Each is selected by one of the block lines SELC to SELF together with BA11.
System ROMs on the static PET 2001 board (Commodore)[2][1]
Position Address 6540 Commodore part 2316B Commodore part
UH1 C000-C7FF 6540-019 901439-09 2316B-09 901447-09
UH5 C800-CFFF 6540-012 901439-05 2316B-02 901447-02
UH2 D000-D7FF 6540-013 901439-02 2316B-03 901447-03
UH6 D800-DFFF 6540-014 901439-06 2316B-04 901446-04
UH3 E000-E7FF 6540-015 901439-03 2316B-05 901447-05
UH4 F000-F7FF 6540-016 901439-04 2316B-06 901447-06
UH7 F800-FFFF 6540-018 901439-07 2316B-07 901446-07
UA2 Character generator 6540-010 901439-08 2316B-08 901447-08

The part numbers are as printed in Commodore's parts list, which gives two of the 2316B numbers as 901446-xx and the rest as 901447-xx.[1] The UH1 entry is the revised ROM, -019 or -09.[1]

Procedure

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Open the PET and check the sockets

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  1. Unplug the PET and raise the top on its hinge and stay.[12]
  2. Reseat every ROM, pressing it down and checking for pins out of the socket. PETs are known for socketed chips working loose.[6]
  3. Check each ROM is in its own socket. On a later 3032 board Tynemouth found ROMs fitted in the wrong positions by a previous owner.[13]
  4. Feel for a hot 6540 with the power on and remove it.[7]

Identify the failed ROM

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  1. Remove the ROMs and test or read each one. Compare the contents against the correct image for that position, by Commodore part number.[3][1]
  2. Without a reader, a ROM/RAM board shows whether the fault is in the ROMs: with it fitted and set to replace the ROMs, the board's own images run in place of the fitted chips.[11][10]
  3. For a fault in the character set, look at the screen: a wrong shape on certain characters points to UA2.[4]

Replace a system ROM

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  1. Program a 2716 EPROM with the image for that position.[3]
  2. Verify the EPROM against the image before fitting it. Shirriff's lesson from the weak byte was to test replacement ROMs before they go into the machine.[3]
  3. Fit the EPROM on its adapter board and plug the adapter into the empty 6540 socket, the same way round as the original chip.[3]
  4. On a 2316B board, fit a 2316B of the same Commodore part number in the 24-pin socket, or use a ROM/RAM board.[1][7]

Replace the character ROM

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  1. On a 2001-8 with a 6540 at UA2, fit an EPROM on a character ROM adapter.[4]
  2. On a 2316B board, fit a 27C16 or 2816 programmed with the character set.[4]

Replace all the ROMs at once

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A PET ROM/RAM board replaces all the system ROMs and the main RAM from the processor socket. With it, a character ROM adapter and a video RAM replacement, Tynemouth notes that every 6540 and 6550 can be retired, leaving a more reliable PET drawing less than half the power.[4][10]

Testing

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  1. Power up: the random characters should clear and the start-up message appear.[5]
  2. Check the whole character set on screen against the keytops.[4]
  3. Type in and run a short program and check its output. Shirriff's machine started with the weak EPROM fitted but printed numbers instead of the string his program asked for, and stray characters appeared on the screen.[3]
  4. Leave the machine running and check that no ROM gets hot.[7]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Commodore Business Machines, 8K Main Logic Assembly Parts Cross Reference for the PET 2001, hosted as File:Commodore PET 2001 Main Logic Assembly Parts List.pdf, pp. 2-3: UA2 6540-010 901439-08 or 2316B-08 901447-08; UH1-UH7 6540 and 2316B ROMs and part numbers; 28-pin and 24-pin sockets for UA2 and UH1-UH7; character generator shift register UB2 74LS165.
  2. ↑ 2.0 2.1 Commodore Business Machines, PET Computer Main Logic, Memory Section, drawing 320026, sheet 2 of 3, for board assembly 320008, hosted as File:Commodore PET 2001 Main Logic Schematic (320008).pdf: H1-H7 and their address ranges, and the note that component locations are the same on boards 320008, 320081, 320132 and 320137.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 Ken Shirriff, "A tricky Commodore PET repair: tracking down 6 1/2 bad chips", April 2025: the Retro Chip Tester, two failed MPS6540s, 2716 adapter boards and the weakly programmed EPROM.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Tynemouth Software (Dave Curran), "Commodore PET video RAM faults", August 2017: character ROM faults, the 6540 character ROM adapter, 27C16 and 2816 replacements, and retiring the 6540s and 6550s.
  5. ↑ 5.0 5.1 5.2 Tynemouth Software (Dave Curran), "Commodore PET 2001 Repair - Garbage screen with noise", July 2017.
  6. ↑ 6.0 6.1 André Fachat, "PET repairs", 6502.org: socketed chips, character ROM symptoms.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 Tynemouth Software (Dave Curran), "MOS 6550 RAM chips", June 2024: hot 6550s and 6540s, and the ROM/RAM board.
  8. ↑ PET-Shack (P.S. Software House), PET 2001 schematic and data-sheet bundle, c. 1978, hosted as File:PET-Shack PET 2001 Bundle.pdf: console wiring drawing with the mains switch, fuse and transformer.
  9. ↑ Commodore Business Machines, Schematic, Service, Display Circuit for the PET 2001, hosted as File:Commodore PET 2001 Display Circuit Schematic.pdf: the anode supply of about +10 kV.
  10. ↑ 10.0 10.1 10.2 Tynemouth Software (Dave Curran), "Commodore PET 2001 Repair Part 1", February 2024: running a 2001 board from a PET ROM/RAM board.
  11. ↑ 11.0 11.1 Tynemouth Software (Dave Curran), "Commodore PET Repair Part 3 - Power, Reset, Clock", August 2014: the ROM/RAM board set to replace ROM and RAM, and ROM images from zimmers.net.
  12. ↑ Marcin Wichary, Commodore PET 2001 with lifted top, Wikimedia Commons, CC BY 2.0.
  13. ↑ Tynemouth Software (Dave Curran), "Commodore PET 3032 Repairs - Intermittent Problems", January 2018.
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