Re: Is Vex Receiver compatible with other RC gear ?

Curt Welch wrote:
Buddy Smith <nullset.spamtrap@xxxxxxxxxx> wrote:
Curt Welch <curt@xxxxxxxx> wrote:

The RF receiver unit (small yellow box) uses a yellow 4 wire cable with
standard 4 pin module phone plugs on it. (the ones you find on
handsets, not the six pin connectors used for phone lines).
Correct. There is a GND, +5V, and signal. The other wire is "tether"
which is used to tell it it's operating from a tether.

The signal is a "standard" servo signal, it's just 6 of them in a row.

Ok, cool. I had forgotten about the fact you could tether the transmitter
dirrectly to to the controller.

How does it know which channel is which? Is there some type of
start marker in the signal or is it just the signal spacing where there's a
longer delay between groups than between pulses in the group?

In standard hobby radios, it is just a long "sync period" between pulses. Each channel follows the sync period in sequence. The sequence is not standardized between manufacturers.

If you look at the composite signal on a scope it will look something like this:
|<------------ 1 Frame ------------------------------------->|
_ _ _ _ _ _ _
_________| |___| |_____| |_____| |____| |____| |_____| |_______________|

| T1 | T2 | T3 | T4 | T5 | Tn | Tsync

T1..Tn are the periods of each pulse that will route to the individual servos. N may be as small as 1 or as high as 10. The frame period is about 20mS. The difference between the sum of all of the channel times and the frame time is the sync time. On "computer" radios I have looked at, the frame time is constant with the differences going into the sync period, The analog radios used a constant period sync time and the frame time varied as the sum of all the channel periods + the sync time. The decoders are made with parallel in/out shift registers or Johnson counters (see CD4017). The pulse train shown clocks the shift register or counter, while an RC time constant watches for a long pulse to reset the counter or parallel load a 1000000 pattern into the shift register. Individual servos are connected to the output bits on the counter or shift register. This arrangement lest you drive an 8 channel receiever/decoder with a 4 channel transmitter and not have stuff get confused.
What I have shown here is an approximation. The polarity of the signals varies with the manufacturers implementation, and it has been a couple of years since I looked at one and I may have swapped it. The basic content and format is right though.

And is there one standard in the R/C word for hobby stuff for what is
transmitted over the air or are there multiple standards?

Mostly one standard with the PWM/PPM gear. For FM gear, some vendors deviate + during a pulse and others deviate -, but the timing is pretty standard. The signal format is generally referred to as PPM, but it looks like PWM to me.

Anyone ever create a digital system for the hobby market instead of using
these analog pulse width system or is all the R/C world just using the old
pulse width system?

Yes, all off the major vendors have what is called PCM which is actually a digital code transmission. As far as I know, all of the PCM formats are manufacturer proprietary and not released. I have seen web sites where people have been reverse engineering the formats, but all are works in progress as I remember.

Good Luck,
Bob
.

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