PPM Signal

A PPM signal (or Pulse Position Modulation) is used by most transmitters and receivers to transfer information from the transmitter to the receiver.

The signal itself is a series of pulses of fixed length. In electronics these pulses are an electrical signal of varying voltage (somewhere between +2.5V and +5V seems to be normal). Between these pulses are pauses of varying length. In electronics these pulses are somewhere between 0V and +0.5V. In some equipment the pulses are low and the pauses high, this doesn't really matter though, you'll see why. The information on this type of signal is encoded as the length of a pulse plus the length of the following pause. Or in other words: the time between the start of two pulses (or end, doesn't matter since the pulse is of fixed length). This is also the reason it doesn't matter whether the pulse is high or low; you simply pick either the rising or falling edge of a signal and measure the time between two rising or falling edges.

Channels follow each other in chronological order in the signal; i.e. channel 1 comes first, channel N last. The time between the pulses is used to generate a [Servo Signal]. Pulses are usually around 500 microseconds in length, the pauses somewhere between 500 and 1500 microseconds. Add those together and you get channel values of 1000 to 2000 microseconds, which happens to be the same range as used in Servo Signals. After the last channel there's an End Of Frame (or Beginning Of Frame) pause. This pause is somewhere between 5000 and 20000 microseconds in length; receivers don't seem to be very strict with this. The pause may be of varying length, this way you can pad a frame to a fixed length.

The receiver will pick up either a rising or falling edge, when it does it'll put a high signal on the first channel's signal pin. When it receives the next edge it'll put a low signal on channel one and a high signal on channel two's signal pin. This continues until all channels have been handled. Then it will wait until it receives an End Of Frame pause. Visualized, it looks like this: