After a couple of busy months, I finally carved out some time to have a proper think about PolyMod 2. I had been somewhat paralysed by indecision, given the sheer number of possibilities in terms of features, form factor, microcontroller, price point, construction method, etc. However, a late-night exploratory session on KiCad focused my mind, and I think I now have a reasonably firm idea of everything. Here is my current thinking:

• I am going to make the PolyMod system physically compatible with Eurorack, meaning the modules will fit into a standard Eurorack case
• For cost reasons, and to prevent confusion while using a mixed Eurorack/PolyMod system, patch cables will be basic breadboard jumper wires (this is not set in stone but is likely)
• The PolyMod system will require a single master module which contains the Teensy, a MIDI input, audio outputs, (probably) audio inputs and (probably) some Eurorack CV inputs and outputs
• The master module will be powered via a standard Eurorack connector, or via an external PSU, depending on user preference
• Other modules will interface with the master module via ribbon cables, likely to be identical to Eurorack cables because they happen to need the same number of pins
• The master module will contain an ATmega328 (basically an Arduino) alongside a Teensy, to handle all module communication and free up the Teensy's pins and runtime for more fruitful endeavours
• Adding the Arduino will mean that a Teensy with fewer pins could power the master module, so the forthcoming Teensy 4 should slot right in when the time comes

And what does all this mean for the specifications? Here are the current likely connectivity specs:

• Total maximum number of modules is 64, with some caveats
• Some modules with a lot of sockets, knobs or switches count as 2 or more modules
• The master module itself counts as 2 modules
• Other factors will probably prevent the PolyMod from ever getting up to 64 modules - firstly, that would be a lot of audio processing for even the Teensy 4, and also there may be issues with multiplexing speed (and other, currently unknown, electrical issues) as we add more and more modules
• The master module will theoretically be able to handle 8 groups of 8 modules
• For space and cost reasons, 2 of these groups will be housed on the master module, while the other 6 will require (fairly simple) expander modules
• This means that, without any expander modules, the master module will be able to support a total of 14 modules (i.e. 16, but 2 are used by the master module itself)

And the other features:

• Adjustable polyphony, perhaps up to 8 notes
• MIDI input
• CV in/out for interfacing with Eurorack modules
• Stereo audio in/out
• OLED display and menu system - allows saving/retrieving/recreating patches
• USB port for updating firmware and uploading new modules from module library

If anyone has any thoughts, please comment below - the project is still at a fairly malleable stage right now, but will crystallise over the coming weeks!