Next I was trying to connect some simple Electronic circuits from the Microtronic Manuals to the emulator, using the original Busch Electronic Kits. I had built the transistor-based sound oscillator and when I connected it to the Arduino, there were some leakage currents. I hence added for signal diodes such that current can only flow "out" of the outputs, and not back in and through to another output.

For another experiments, I needed the four digital inputs of the Microtronic. The experiments from the Busch Manuals require the input levels to be roughly TTL compatible. I experimented with various pin modes on the Arduino, including INPUT_PULLUP, and INPUT, and figured that the inputs will only work correctly and reliably if I added external pulldown resistors and used INPUT pin mode.

So, the outputs required 4 signal diodes to prevent leakage currents, and the inputs required external pulldown resistors. This also had the nice effect that the inputs would correspond to non-inverted TTL levels, and the circuits from the Manuals worked. I soldered the signal diodes and pulldown resistors on two little perboards and mounted them on the base board (notice that the Arduino only has pullup resistors build in).

This step is documented here: