Commodore 16 & Plus 4 8501 to 6510 CPU conversion

C16/+4 8501 to 6510 CPU conversion

I have made it possible to convert the unreliable 8501 processor fitted to the Commodore 16 and Plus 4 computers to the more reliable, cheaper and widely available 6510 CPU, as fitted to the Commodore 64.

The conversion is very simple for the C16, just plug the small circuit board containing the 6510 into the 8501 socket and replace the Kernal ROM with my modified version.

C16 before

 

C16 after

 

The Plus 4 is a little more awkward. The conversion will work fine, however due to the lack of space inside, the keyboard will hit the 6510. The best way round this problem is to remove the 8501 socket and solder the conversion board straight to the motherboard.

Plus 4 before

Plus 4 CPU socket removed

Plus 4 after, conversion board soldered

The Kernal needs to be replaced to get the serial port working, otherwise it’s not possible to access a disk drive or the SD2IEC card reader.

With the 6510 and replacement EPROM installed, the C16 & Plus 4 will work as before. The only thing that has been lost is the ability for the computer to stop the tape deck. Which is not a problem for most games, as once they have loaded the tape will simply go to the end and stop. If a program requires further loads, the tape must be stopped manually.

Underside, motor link at bottom

I have added a small link to my board to enable the tape deck’s motor to run. If fitted the motor on the tape deck will always run, even if the play, rewind or fast forward are not pressed. In most cases this is not a problem. If you don’t have a tape deck plugged in, then it does not matter.

A diode can be added to the motherboard on both the C16 and Plus 4 which will enable the cassette motor to only run when play, rewind or fast forward are pressed.

A 1N4001 or similar should work fine between pin 5 of U9 (7406) and FB15 for the C16.

A 1N4001 or similar should work fine between pin 3 of U7 (7406) and FB13 for the Plus 4.

Don’t forget to cover any exposed wire to stop shorts and cut the link on the 6510 board.

The reason for the loss of motor control is due to the number of control ports on the CPU’s.

Basically, the 8501 has seven and the 6510 only has six. These ports control the disk and tape functions on the C16 and Plus 4. They are also numbered slightly differently, hence the need to rewrite some of the code in the Kernal. The tape filename found pause has also been reduced as the computer can no longer stop the cassette and could miss some of the data on the tape.

10 Responsesso far.

  1. Damien Stewart says:

    Awesome stuff. Perhaps you can license JiffyDOS for a turbo disk load. And where can we buy a complete kit? 😀

    • andy says:

      I am planning to sell the board with a 6510 and an EPROM on ebay.co.uk shortly for £25.

      I have just created a modified version of the circuit board to plug straight into the plus 4 and position the 6510 closer to the back of the computer.

      • Damien Stewart says:

        Not bad. I do have a concern with the missing port bit as with the bit positions reordered it will be incompatible with a lot of software and fast loaders. Even the 8502 doesn’t look like it would help as it still misses the MSB. There are some R6502 new (old?) stock on eBay. Looks cheaper then the odd 6510 I have seen. That coupled with some I/O chip mapped to location $00/$01 could be a good follow up. 🙂

        • andy says:

          The 6502 does work, but it’s a lot more work as it has no ports. A 6522 could be used for this. The other problem is that the 6502 does not disconnect from the data and address bus to allow the 8560 to access to memory. There is a circuit online which will achieve this. http://www.webalice.it/gratteri/65028501.gif

          I agree about the port bits. This design is a compromise. Tape fast loaders should work fine as bit 1 is used on the 8501 & 6510 for tape data. Disk fast loaders probably wont as IN, OUT, and Clock serial data are on different bits.

          My aim is to try and keep the C16 and Plus 4 alive. The 8501’s are becoming rarer by the day. I’ve bought a lot of C16’s and +4’s from eBay and at least 80% have a dead 8501. If you don’t use disk fast loaders, you should be fine. I think it’s a lot better to have a slightly slower disk load and have a working computer than no computer at all.

          Thanks for your comments
          Andy

  2. This is great. Thanks. I was planning the same by just simple leg-bending-and-extending before inserting the 6510 into the socket.

    Are you planning to sell this kit? Or was there any link that I missed to download the modified kernal?

    • andy says:

      I am planning to sell the board with a 6510 and an EPROM on ebay.co.uk shortly for £25.

      I have just created a modified version of the circuit board to plug straight into the plus 4 and position the 6510 closer to the back of the computer.

  3. Luke says:

    Hi Andy, very nice! I’m interested, one for C16 and one for PLUS4.

  4. Laszlo says:

    Hi Andy,

    First of all congratulations for this essential development.
    I think the CPU replacement is the Achilles corner of this Commodore line so I’m glad to read this post.
    Do you planned to sell this beauty at other places, such as Amibay ?

    Cheers,
    Laszlo

    • andy says:

      Hi Laslo

      I am planning to sell the board with a 6510 and an EPROM on ebay.co.uk shortly for £25.

      I have just created a modified version of the circuit board to plug straight into the plus 4 and position the 6510 closer to the back of the computer.

      Andy

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