Hi TND2000,

8 bits describes the instruction set... It's really an internal thing... An 8 bit processor uses an 8 bit instruction set, and has an 8 bit ALU etc.

The PCM output has 10bit resolution... That's a little trickier... It means it has a 10 bit counter (more accurate) and this is tied to a comparitor, and this is tied to the main CPU clock in a way (plus there's usually extra divider options in the input section). Then, the PWM can wait between 1 cycle and 2^10 cycles before it changes the output pulse. This is used to make the PPM output for the radio... The program would work out how long the pulse is, and send that value to the PCM output. The 10 bits means that there are 1024 steps between the thinnest pulse and the thickest, while 8 bits would mean there were only 256 steps... The simple answer is that the 10bit PCM output gives a reasonable resolution of for the pulses that control the servo motors. Like a 1400x1050 resolution laptop is better than a 640x480 resolution one.

It takes a few instructions to load a 10 bit counter in an 8 bit MCU. First you load in the lower bits, then the upper bits. There are two PCM outputs on the MCU.

I guess that might be confusing if you don't know a lot about MCUs.

The chip has 8 Analogue to digital converters... 8!... So it just seems a shame that it just uses 4 for channels, denying the other 4, but that's not really a criticism. It was
never designed to support more than 4, however since there are 2 switches and a pushbutton, it just seems a shame.

The toggle switches on the 4 channel TX only change how the stick responds, correct, but the toggle switches on the 6 channel do actually get transmitted as extra channels. True! Get a CRO and have a look yourself. The 4 channel is basic, but the 6 channel just transmits everything to the receiver, and lets the receiver figure out how to mix it all. Funny huh? They don't affect the analogue servo transmissions at all. One of the channels on the 6 channel also transmits an oscillating signal. But I didn't mention that before.

The transmitter also just pumps this as baseband into the TX Xtal to cause a frequency shift in the modulated signal... Real basic stuff.

I'm not sure if the receiver does an IF conversion. Personally, I suspect they just mix the LO with the input (Super-heterodyne) and just suck out the remaining signal, and
they probably just check it in a basic chip, since it's digital in nature and not analogue, but I don't have anything to support that hypothesis. But yeah, it would be low frequency enough to analyse with a CPU... However the output from the receiver is finally digital before it goes into the chip for processing. It's messy, but it's there. There's a test-pad on the receiver board where you can tap into it with the CRO.

You can't tell how many channels there are by hooking it up to FMS. FMS doesn't work that way. Also, it's a high-level code (Zhen Hua 5-byte protocol) not the PPM output that the parallel port interfaces read. Unless you make the 6 Channel mod I put on my website.

But there are extra null channels there! True!... Again, hook up a CRO and have a look.

My Futaba Skysport 4 puts out 5 channels... 4 useable.

But there are 3 extra pulses on the output from the ZH 4 channel...

And there are about 12 pulses on the 6 channel !!!! And most of them do something... The first 6 are the analogue inputs (5 and 6 and the pots at the top). The two switches
are channels, and a couple of the dipswitches are transmitted also !!! Not bad eh? A pity it doesn't work with cheap receivers.

I did write up the review from memory, and didn't take notes, so it might have a few innacuracies, but what I saw was interesting, so I thought I would post it

David