- A MOS Technology 8501 compatible CPU for your Commodore TED computer
- Uses a standard 6502 and standard electronic components
- Compatible CPU port, works with original KERNAL and fastloaders
Introduction
A few chips by MOS Technology have high failure rates. These chips have been reported to have been manufacturerd in a fab from Frontier Semiconductor, a chip manufacturer that Commodore did acquire in the early 80s. One of the chips that has a very high failure rate is the MOS 8501 CPU. This is the CPU used in the TED series computers, i.e. the Commodore 16, Commodore 116 and Plus/4. If you have a TED series computer that starts with a black screen, there there is a high chance that its 8501 CPU is deceased.The 6502to8501 is a device that allows you to use a 6502 CPU in your Commodore TED series computer, it allows you to bring a dead computer back to life. The device is an adapter that interfaces a 6502 to the 8501 signals with 74xx bus transceivers in between in order to implement the AEC funcitonality. Also, the CPU port is implemented with standard 74xx logic chips. A GAL16V8 is used as a controller for the CPU.
An NMOS 6502 CPU is required for a compatible replacement. Allthough NMOS 6502s are no longer produced, availability and pricing of them is good, allowing for a cost-effective CPU replacement. There exist 8501 replacements based on FPGAs but these are often expensive and many FPGAs have been discontinued in the past years, requiring continuous updates of the design.
All parts of the 6502to8501 are still in production (even the GAL, ATF16V8), but more important, they can
easily be found on the secondary market. It is quite plausible the 6502to8501 can still be built 20 years
from now.
Questions and answers
Q: What type of GAL should I use for the 5801 controller?A: 15ns or 25ns have been tested, but other speeds may work.
Q: Can I use both Lattice Gal16V8 and Microchip ATF16V8?
A: Yes, GALs from both vendors work fine.
Q: Is there any preference for the 74xx chips?
A: Experiments have been done with 74HC(T) and faster chips. The HC(T) chips got the best results, but faster chips did produce working CPUs as well.
Q: Do I need to a modified KERNAL?
A: No, this is exactly the point of the 6502to8501: It is compatible with the original 8501 and therefore does not need a modified KERNAL.
Q: Do games with custom disk loaders work?
A: Yes, this is exactly the point of the 6502to8501: It is compatible with the original 8501 and therefore games with custom loaders work just fine.
Q: Does the 6502to8501 fit into the Plus/4?
Yes, but it is extremely tight. You may want to omit the 6502 socket and solder the 6502 directly in order to have a little bit more margin in the Plus/4.
Schematics
Download the schematics:5801 controller
The brain of the 8501 CPU replacement is the 5801 CPU controller. It tells the transceivers on the board to switch on or off and tells the register of the CPU port to latch at the right time. Why 5801? Well, the 5801 CPU controller is implemented by a GAL16V8. in MOS Technology product numbers, chips with a model number 5000 range were gate arrays. For example the CIA replacement in the C128DCR 1571 is a 5701. The chip in the Commodore 1351 mouse has model number 5717 and Amiga Gary has number 5719. Considering that a GAL is about arrays, a number in the 5000 range fits well with MOS Technology chip numbering, and 5801 is 8501 with two digits reversed.The reason that I document the 5801 controller separately is that its capabilities aren't restricted to build 8501 CPUs. You can use the 5801 to build a 6510/8500, 8501 and a 8502. It can be used to build CPU replacements, but I can also imagine a modern TED/C64/C128 mainboard with a 6502 socket rather than custom CPU socket. This will make it way easier to find parts and as the required parts to build a CPU are easily available in through-hole packages, this will accomodate amateur construction of computers and facilite availability of low-cost parts.
The block diagram for a CPU would look as follows. We are now assuming a hard wired data direction register like the 6502to8501 adapater:
However, in case a fully functional data direction register is used, the 5801 controller can be used as well and the implementation of the CPU port can be changed as follows:
In summary, the 5801 controller is a very fexible component for a CPU solution for Commodore 8-bit computers.
You can download the following file:
8501_CONTROLLER_def.PLD | This include file contains the CUPL code that implements the 5801 controller. |
8501_CONTROLLER_def.jed | JED file to program into a GAL16V8 chip. |
Obtaining a PCB
You can order PCBs from me or produce them yourselves. I have uploaded the Gerber files to PCBWay, you can order the PCB here: PCBWay: MOS 8501 CPU replacement If you order from PCBWay, I will receive a small donation from PCBWay so you will support me that way. However, if you really want to manufacture the PCB somewhere else, I have enabled the Gerber download button on PCBWay, so you can download the Gerber files there and produce it at the PCB manufacturer of your choice.Obtaining a CPU
If you are interrested in a 6502to8501 for your Commodore TED computer, please fill in the following form:(the 6502to8501 is not yet available on SellMyRetro, it will be in the future)
Contact
Do you have any questions or would you like to contact me for some other reason? You can e-mail me at daniel.mantione at freepascal dot org, or use the following form:Thanks
(C) 2023 by Daniël Mantione