The Goal is to control many steppers without using all Pins from the Raspberry. the AVR has 35 IO pins but I only use 32, so you can use 8 4-pin stepper or 16 2-pin stepper. (And of course mixed variants)
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The AVR is starting and sends an aline message, then you can add stepper with 2 to 4 pins. The pins are chooseable so if I have a free pin I can use them (where ever the pin is)