Model Forum / Radio Controlled / Air Models / September 2006

You should be able to encode a fairly large number of functions onto a
single channel with a PIC on each end. You should easily be able to
transmit and receive 16 different pulse widths, allowing for 4-bits to
be encoded in each pulse. You could reserve one bit to make the
beginning of a block, and one bit to alternate between high and low (so
the receiver will know when the data has changed) and pass two bits of
data in each framed chunk.

For example, if you want 6 on-off switches, you can simply send this
block:

1 0 S1 S2
0 1 S3 S4
0 0 S5 S6

Notice how the first bit is set on the first block. That allows you to
identify the first block. Notice how the second bit alternates. That
allows you to know when you have a new block (otherwise you might not
know of there are two blocks with the same value or one block with that
value). The last two bits hold the data.

Each combination of 4 bits would be encoded as one of 16 pulse widths.

If you just need to send changes, you can reduce the latency with a
protocol that consists of so many bits of data to indicate which
control has changed and so many bits to indicate the new position. You
can encode the beginning of the block and when there's new data the
same way.

How much data do you need to send? How many signals and how many values
for each signal? I can whip up an encoding scheme for you very easily.

You should try to confirm two things. First, what is the update rate
with your receiver and transmitter? That is, how many pulses per second
does it send (minimum and maximum, it may vary based on the input).
Next, what is the resolution of your receiver. That is, how many
distinct pulse widths can it reliably send? I believe most modern
transmitters and receivers can handle at least 32 positions, so you
should be able to get 16 reliably (with safety windows around each
valid position to detect an invalid signal).

DS