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.

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