The Busch 2090 Microtronic Computer System was released in 1981 in (West) Germany. It was my first computer on which I learned the basics of (machine language) programming and computer interfacing. This is the original:

The following picture shows my Microtronic. It was extended with the very rare 2095 Cassette Interface Module (on the left) to allow for cassette tape storage:
In 2016, I had created a talking Arduino-based emulator of the Microtronic system:

This predecessor project - The Talking Microtronic Emulator - also has a page here on Hackaday. Background info about the Microtronic (i.e., details about its virtual CPU and its instruction set) are provided as well:
https://hackaday.io/project/11560-the-talking-microtronic-computer-system-emulator
Even though the Microtronic uses a TMS 1600 CPU, it was actually not programmed in TMS machine language. Rather, the (mask-programmed) TMS 1600 was running a monitor program that emulated a much more versatile, powerful, and flexible CPU architecture much more suitable for an education system. It even included high-level instructions / op-codes for multiplication, division, time / clock functions, random numbers, display, digital input & output, keyboard input, etc. A very nice instruction set.
Back to 2020: The Next Generation Microtronic project (these pages) is the continuation, culmination and wrap up of this work. This time, I have a proper PCB with greater reliability and durability than my previous breadboard big-mess-of-wires prototypes, hence preserving my efforts for the future.
PCB Version 1 for the Nokia 5110:



PCB Version 2 for the SH1106 SPI OLED Display:




So, what has changed over the 2016 version? In a nutshell:
- Nokia 5110 LCD or SH1106 SPI OLED instead of the 7segment LED + Hitachi HD44780 display: the 7segment LED display was nice since it offered a very 1981-like authentic retro user experience. However, a multi-line LCD offering different screen modes that can display mnemonics, multiple addresses of the machine code program at once, etc., has big advantages. Especially for an educational system. A number of display modes are possible: a simple classic one-line Microtronic display, also in double size font; a mode that displays the memory contents (and optionally mnemonics) at the current Program Counter (PC) and its neighborhood locations; a mode that displays the contents of the work and extra-register sets, and so on.
- Real Time Clock: the Microtronic has time / clock functions built in (but no date / calendar). The ROM-program PGM 3 displays the current time, and the ROM-program PGM 4 is used to set the clock. The clock could also be read with a "Get Time" machine code instructions into the registers A to F, so alarm clocks etc. could be implemented. The original Microtronic clock was not battery backed-up though. I have fixed that by linking the PGM 3 and PGM 4 clock functions to a standard DS3231 battery backed-up RTC module. In addition, I have assigned a "set date" function to the built-in ROM program PGM 0. Program PGM 0 usually runs a self-test program, which is not needed here, so it made way for the display / set date function.
- Sound Output: in order to make a sound with the original Microtronic, one usually had to connect a transistor multi-vibrator or a Piezo buzzer to the digital outputs (in the former case, over a simple DA / resistor ladder). However, we figured it would be convenient to have a real SOUND op-code built in. Unfortunately, there are not unused op-codes available in the Microtronic that could be used to this end - every op-code from 000 to FFF is taken and already has a meaning. Fortunately, some of these op-codes are really non-sensensical and hence don't occur in actual existing Microtronic programs. These are the op-codes "MOV x -> x" (copy register x onto itself), "ADDI 0 x" (add 0 to register x), and "SUBI 0 x" (subtract...
Thank you! Yes, I was thinking about a FPGA version with video output and a better keyboard as well. Maybe a good 2021 project :-) My "Microtronic colleagues" mentioned in the description (Frank from Belgium and Manfred from Germany) have a better keyboard for the 2nd Generation Microtronic that installs into the Busch console. The telephone matrix keypads that I used for the prototype aren't bad either. Still, lots of ideas and things to improve and try! Regarding 80x25 screen, not so sure if this is needed given that this is only machine code programmable.
Regarding FPGA, you also have to consider the purpose and price of the project... the ATMega 2560 MiniPro literally costs me 3 or 4 USD. No FPGA is that cheap. If I wanted to sell these assembled or as a kit (after providing some translation of the Busch Manuals to English) I could do so for 30 USD on Tindie or so. That's only possible by using inexpensive stock components like the MiniPro. So I am not sure that FPGA adds anything here (other than price and maybe video output, which is not what I really need for this project).
Anyhow, thanks for the input and all the best for 2021!
Cheers Michael