5 hours ago, Sandmage said:
I've just noticed that the Ulitmate1541 has a builtin modem now. I've been surfing BBSs the last hours, it's working great. It should be possible to hook the address- and databus to the cartridge. The Expansion ports expose the whole Address and Databus. Of course the cartridges software will not work, but it can be controlled via telnet, so that shouldn't be necessary.
That won't work. The memory map on the Commander is completely different, and the 8MHz clock would also necessitate an intermediary. By the time you're done, you'd have spend more time and effort building an interface adapter than just building a new I/O unit. The Ultimate should be able to work as an IEC disk drive, but you won't have access to the RAM expansion, SIDs, or Swiftlink modem.
I think the way to go for serial is to simply build a 6551-based Swiftlink clone. There are several people making them for the Commodore 64 and 128, so adapting that to the Commander should not be too difficult. The current implementations are interrupt-driven and can deliver 38.4K on a 1MHz 6510. So we should be just fine using that for up to 115K on an 8MHz 65C02.
At one point, I was also going to write a serial library for the User port. It looks like I'll be re-visiting that plan, as well. I don't really want to write a 2-wire RS-232 driver, since that's slow and requires exact timing. Instead, I'll build a parallel version that uses all of the GPIO pins on the User port. Since it's supposed to be Centronics compatible, we'll have at least 12 bits plus a clock line, and that's enough for what I have planned. (3 control lines and an 8 bit data bus.)