Cumulus and Sedoric 3

Cumulus in Operation (1)

Cumulus in Operation (2)

Cumulus in Operation (3)

Cumulus in Operation (4)

Cumulus Rev A Closeup

In my post yesterday, I had said that Sedoric 3 disks were not working with Cumulus. Actually, that turned out not to be true, due to a stupid mistake on my part (mostly due to my being not very familiar with the Oric software environment).

The disk images I was trying to get to boot were actually “slave” disks, so they were not bootable at all. Since I had never tried those disks on an emulator (and never had a real Microdisc unit), the “DOS is altered” message misled me into thinking that Cumulus was at fault.

So, booting first with a Sedoric 3 Master disk gave the results in the shots above. To be able to switch disk images, I implemented a rudimentary method that goes through all images in the root directory of the SD card on press of a button.

Now that the basic functionality is working, it’s time to think about where to go next:

0. There is still a bug (probably with the Oric bus) that causes occasional boot failures or crashes. This is much less in frequency than before, but it still needs to be fixed of course.
1. There are quite a few commands missing from the WD179X implementation on the PIC. Also the whole behaviour of the floppy emulator could be made more “floppy-like”, with more accurate timings, for example. Multiple drive support is possible.
2. A proper UI that makes use of the LCD and buttons is part of the specs.
3. A new PCB design that consolidates all existing design elements into a compact, single board, and fixes existing problems (couple of missing traces, etc.) is necessary.

Cumulus 6502 Bus Working

Cumulus Test ROM running on Oric

Cumulus LA Debugging Setup

Over the last week I worked on debugging the Cumulus 6502 interface. One of the shots above shows a small test program (coloured bars in hi-res graphics mode) I wrote for testing. The program resides on the 28C256 EPROM on the Cumulus board and is executed by the Oric on reset.

The problem with the Oric bus turned out to be due to the 74HCT221-based circuitry on the Cumulus board. This circuitry generates a couple of signals from the 6502 O2 signal coming from the Oric. Those signals are in turn used for controlling some Oric bus-related signals such as 28C256 output enable. The pulses generated by the 221 were too long, actually about twice the length specified in the datasheet.

I managed to solve the problem by manually adjusting the component values until I got proper the pulse widths. The problematic circuitry actually comes straight from the original Oric Microdisc schematics, with one difference: The original circuit uses an LS variant instead of the HCT that I used. According to the datasheets, there does not seem to be a difference, but I may have missed something.

There are still some problems though. Sometimes the Oric does not properly reset, most likely due to Cumulus affecting the reset sequence. Sometimes there is some wave interference on the video output. Other times, the video output looks as if the Oric never switches to the high-res mode, remaining in text mode (I get coloured bars, but longer). I’ll probably conceive a shorter program to reproduce these issues and debug using my current setup with the logic analyzer.