samples joystick and switch positions. It then
changes them into an RF signal which is
picked up by a receiver that outputs a
sequence of modulated pulses. The
important thing to consider for our purposes
is “how often” and “when” the transmitter
samples the switches. If you are just sending
a single switch position like “gear down,”
one can assume it will get there in this
sample cycle or the next (since it’s only a
matter of milliseconds anyway). Using the
channels to send data bits efficiently and
accurately needs to be either more precise or
spread across multiple samples.
For more precision, it makes sense to
find a time base so that the processor can
be sure to put the bit on the switch at the
exact right moment for every sample loop.
In developing this idea, I tried poking around in the
transmitter for a clock to sync the sample timing to the
master clock. I successfully synced the systems this way, but
it lead to the discovery that noise protection circuitry in the
transmitter software would not let a switch be toggled that
fast. So, the next approach was to stretch the time. If I put
a bit on a channel over multiple samples that was slow
enough to get by the noise canceling electronics and
software, then I could send the data.
As it turns out, the sample speed is different for
different transmitters and modulation types. The speed is
an integer multiple of the cycle time for all of the
transmitter’s channels. On my JR, that’s about 22 µs. For
SCHEMATIC 2. Transmitter/Encoder
switch and LED board.
eight bits, that works out to 176 µs. Typically, it will run
higher than that depending on the noise squelchers. I’ll
warn you now — the response time on this will seem slow
because it is. This will not work well for time-critical things
like firing water guns. It works great for mode switching
and things like turning lights on and off, or switching
camera views. Keep in mind that the remaining channels
still have very quick response and you can use the switching
to divert their control to any channel.
Surgery on the Transmitter
We need to find control lines to hijack. The JR-XP6102
SCHEMATIC 3. CPU, EEPROM, servo inputs,
and 3. 3 volt power.
SERVO 04.2010 57