PPM

PPM (Pulse Position Modulation) or CPPM is a type of electronic signal which encodes information about multiple channels with a single wire. For this purpose, the signal is divided into about 20 ms frames with as many pulses as there channels plus a sync pulse. Sync pulse is the longest pulse which marks the end of a frame. Pulses are delimited by signal low state, which is maintained for about 0.3 ms. Duration of signal low plus the duration of a subsequent pulse determines duration of PWM pulse for each respective channel (typically 1-2 ms every 20 ms to control position of servos).

Some manufacturers (e.g., FrSky) have used frames shorter than 20 ms, which resulted in the sync pulse being too short and the CPPM protocol losing reliability.

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