PicoRAM Ultimate Rev. 2

... both ET-3400A and ET-3400 ROM emulation are fully functional. Given that PicoRAM adds 2 KBs of emulated RAM / ROM, and the ROM starts at 0xFC00 and is 1 KB only, there was still some space to add my Towers of Hanoi program starting at 0xF800. PicoRAM's GAL was changed to map into the upper 2 KB = 0xF800 to 0xFFFF address range.

ROM emulation is running stable now - I disabled the Write Enable (/WE) signal detection in the firmware software, resulting in a stable ROM emulation: 

I will change the firmware such that it distinguishes between ".RAM" and ".ROM" files on SD card - ROM files will get the "automatic memory protection", RAM files obviously won't. New firmware version 1.8 soon - stay tuned! 

Not sure how stable it is - probably need to prevent the "write" signal to protect it, but some initial progress!

Is currently being made - this fixes the PCB RE signal flaw on the IO header, routes more address lines into the decoder for more fine-grained control over the PicoRAM address range, and bumps it up to 4 KBs. More isn't possible because of the GPIO shortage on the Pico, and the address bus is already multiplexed. I need the other GPIOs to drive the other functions (databus, SDcard, display, ...) 

... is indeed possible: simply change the address decoder. For example, by changing the GAL equation for the decoder from 

RAM_SELECT = !A11 & !A12 & !A13 & !A14 & !A15 ;

to 

RAM_SELECT = !A11 & !A12 & !A13 & !A14 & A15 ;

the emulated 2 KBs of RAM will appear in the 0x8000 - 0x87ff address range instead of the default range from 0x0000 - 0x07ff. 

This hence gives you 2.5 KBs, adding 512 bytes from the existing SRAM chips to PicoRAM's 2 KBs. Note that the 2 2114 chips are on board here, and don't conflict with PicoRAM. It is probably even possible to pull the ROM chip (0xfc00 - 0xffff) and hence change the monitor program if needed (i.e., by putting a custom "emulated ROM" in there that can conveniently be updated and developed without having to burn (E)EPROMs all the time): 

Unfortunately, the current version of PicoRAM Ultimate has a PCB bug: the position of the RE signal on the header is incorrect. Apologies for that.

You can see that in the schematics here:

When I wired up the IO header on my ET-3400, I flipped left and right and put it on pins 30 and 29 instead of 12 and 11, where they should have been.

I only noticed now, with my new ET-3400A, which already came with the RE signal wired up on the IO header (I only had to install the data lines), that the position didn't match. And indeed, the ET-3400A schematics lists the RE pin positions as follows - you see the difference:

There is an easy fix for this - don't use JP9, but a DuPont wire that runs directly into the breadboard RE signal, as shown in these pictures:

Eventually, I will create a new PCB version with this bug fix. But for now, the workaround is sufficient.

The new firmware supporting the ET-3400A is now online!

https://github.com/lambdamikel/picoram-ultimate 

... got my hands on a Heathkit ET-3400A. This one has the much faster 6808 CPU. It took me a week to get the timing right, but eventually, I got there:

Happy New Year - I had committed to build a PicoRAM Ultimate for a fellow ET-3400 enthusiast, and am wrapping up the year 2025 with three "first steps" videos that I made for new users: