Why is an extra board needed for SVD Apple 2 support?
The extra board, or dongle as it is called, converts the Apple 2 floppy control signals to the signals used by the “standard” 5 1/4 floppies of the day. For a bunch of information about the dongle, see here. Here is a quick overview of the signals that are converted: Head Stepping The Apple 2 uses low-level stepper motor phase pulses to manually advance the track head through the separate tracks of the floppy. The dongle converts these phase pulses into a direction signal and a step pulse based upon the direction of the progression of the pulses. In addition, the dongle ensures that the phase pulses coming from the Apple are the appropriate width since, when booting, the Apple actually holds the phase 0 pulse high for an extended period. Signal Polarity The logic sense (polarity) of the Apple 2 signals are often the inverse of those for the “standard” floppy controllers. This includes inverting the read and write lines as well as creating a “motor_on” signal based upon the drive select
