> now, for the board, i wonder this would be cheaper to make a
> single board for 4 and 6 ch, with the same micro-controler, same program inside.

You are right -- to save money, they use the same TX body and circuit board for different transmitters (just like all R/C manufacturers do). But programming the chip with different microcode costs nothing and the code must be different in order to have CCPM-mixed or un-mixed signals. CCPM mixing is merely a computation of an output signal as a linear combination of input signals, which has to be performed by the chip.
As the switches do not disable the CCPM mix (which is utterly silly!), connecting them won't change the way the microchip operates...


> ... it could be possible to transform your 4ch transmitter in a 6 ch one
Don't hold your breath!
As I've already wrote, "Channel Six" name is a lie! This so calles "Channel Six" transmitter is still a 4-channel TX with a hard-wired CCPM, which mixes channels 1, 2, and 3 into 1, 2, 3, and 6 (channel 5 in left unused). There's no control over channels 5 and 6 other than, for all these 4 output channels, changing mixing function from "normal" to "3D" (which only changes the coefficients in the mixing equations). Read the manuals for "real" transmitters--if Futaba calls its TX a 6-channel model, there are always controls for channels 5 and 6!


These are not just speculations--I own ESky "Channel Six" transmitter, which I thoroughly studied to extract parameters of the CCPM mixing. This was done to develop a new Serial PIC interface, which decouples CCPM mixed signals on the fly and feeds un-mixed channel data to a computer:
[link=http://www.milehighwings.com/]www.milehighwings.com[/link]


Walkera/Dragonfly/Esky "Channel Six" transmitter - what a joke!
[>:]