The clockport is called like that because it was initially intended to carry the OKI6242 RTC in an Amiga 1200. However, since these RTCs were always contained in the obligatory accelerator cards, the port was free pretty much all the time.
Commodore made two address spaces available on that port: The "RTC space" and the "spare space". The RTC space was occupied by the RTC on the accelerator, so adding another RTC would have been pointless. I was the first to map "something" to this port with the Catweasel MK2. This product maps to the "spare space" of the port.
After the Catweasel MK2, I designed other things for this port, such as the Silversurfer, VarIO (special A1200 version). When the Retro Replay was in the test-phase with the Cyberpunx, we talked about the possibility of debug-access through RS232 at the Mekka/symposium party, which is where the idea of a clock-port for the C64 was born: It made an existing design (Silversurfer RS232) available to the C64, solving multiple problems at the same time (development, time-to-market, design verification, low quantity etc.).
The "clockport" of Retro Repaly and MMC Replay has limitations: Although it's called clockport, it's main use has always been the secondary use, which is "spare". One limitation of the C64 implementations is that ONLY the spare-space of this port is used. Another limitation is that only 14 of the 16 registers that the port has can be used. The other two addresses are occupied by the Retro Replay registers.
This makes only a few products available to the MMCR/Retro Replay/MMC64 implementations:
- Silversurfer (uses 8 addresses and NMI)
- RR-Net (uses exactly 14 addresses, no NMI)
- MP3@64 (uses 3 addresses, no NMI)
The Catweasel MK2 is NOT compatible, because one of the most important registers (the data FIFO register) is not accessible without additional interfacing. Also, the Subway (mentioned by GeilerMacker) will most probably not work, because certain banking registers are not accessible. Since there's the other limitation of "only spare-space available", any Amiga-RTC module will NOT work on a C64-clockport, because the RTC-space is not accessible by the C64.
There you have the weird essence of this post: The clockport is not compatible with an Amiga clock-module ![]()
Jens