PWM Versus PPM
If you have shopped around for RC transmitters before, you
have likely seen the terms PWM and PPM associated with them.
They stand for
pulse-width modulation (PWM)
and
pulse-
position modulation (PPM)
, respectively. These labels identify
different communication protocols between your receiver (see
) and the components on your aircraft. The main
technical difference between the two is that PPM is digital while
PWM is analog. As a builder, you will notice that the biggest
install difference between these two is that PPM communicates
up to nine channels of control across one wire (digital serial
communication), while PWM requires a single wire for each indi-
vidual channel (analog communication). It is important to con-
sult the documentation for both your receiver and autopilot to
know which protocol you should use. If PPM is available, we
highly suggest that you use it, because it will clean up your
install dramatically with fewer wires. However, not all hardware
supports the PPM protocol, so consult your owner’s manual for
further reference. At the time of writing this book, APM does not
support digital communications protocols on its main firmware
fork. As far as we can tell, some developers have been able to
create custom firmware that supports it, but we do not advise
you to install untested firmware from indie developers as a new-
comer to this hobby. Stick with the official releases for now, and
you will be much happier in the short term!
Digital by Any Other Name
Many manufacturers have developed their own pro-
prietary versions of digital serial communications
and give them their own names. For example, Futaba
calls its version SBus.
Transmitter 109
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