PPM or PWM signal - advanteges, disadvantages

[lukasz]

For time being in my RC garage I didn't find any receiver from which i could pull out PPM signal.

Till I find one I'm thinking of using PWM signal from the receiver.
Just to be 100% sure - PWM signal is the one that comes directly from the RC receiver and is passed to servos?

On the Gluonpilot board we have 3 PPM signal input connection + 4 PWM input connection.
Reading Tom's page I dind't find anywhere under which PPM/PWM connections are what servo channels.
The only thing i found is when I change the configuration to read PWM signals is that the 1st channel will be under PPM input on the board. What is the order of remaining 4 channels on PWM input?

Also, what are the advantages and disadvantageous of using either PPM or PWM signals?
The only thing that comes to my mind is that when using PWM we need more cable connections

[Mitch]

Hi Lukasz,

I edited this post when Tom advised me that newer boards have 5 total pwm inputs!

You are correct above. PWM is data to control only 1 servo. PPM is a data stream containing all of the channels from the transmitter.

If you look at the bottom right of the circuit diagram, you will see the ppm1 input connector has 3 pins (input signal IC1, +5, and ground). The ground is closest to the board edge and the input is furthest from the board edge.

The ppm2 connector is 3 pwm inputs IC2, IC3, and IC4 on first revision boards. Or 4 pwm inputs for newer boards.

Using ppm connection - connect only the three pins on the ppm1 connector (ppm,+5,gnd)

Using pwm connections - connect the three pins on ppm1 (pwm1,+5,gnd) and connect the other three (or 4)pwm connections from your receiver to the ppm2 connector pins (pwm2,pwm3,pwm4...). On servo wire connections these will be white, yellow or orange wires. The power and ground wires are almost always red and black. Additional power and ground wires to the receiver for each channel are not necessary. Which channel goes where is not critical. These can be configured with gluonconfig (there is no standard between different brands of rc equipment). I would make up a harness to simplify the pwm connections.

Advantages of ppm:
fewer cables is a huge one as you get into more complex installations
more reliable due to fewer cables
you can use a very cheap receiver (say a 3 channel) and extract all of the transmitter channels. i.e. a 3 channel receiver could be used for 6 or more channels
uses only 1 microprocessor input pin

disadvantage of ppm:
have to hack a receiver
spread spektrum receivers seldom have a ppm stream on one connection

I just discovered that contrary to tom's getting started page, my board only has 4 pwm inputs with header connections - edited - older boards have 4 total and newer boards have 5 total. Firmware is set up for 5 inputs on processor pins RD8-12. Another reason to use ppm!

[lukasz]

Thanks Mitch for replay.

So the only disadvantage is the number of cables connected between the receiver and gluonpilot.

[Mitch]

I don't think there will be any performance differences. You will have to make up a connector/adapter that runs 3 pwm connections from the receiver to a single connector on the gluonpilot board.

[lukasz]

Ok.
I will try this out till I find a receiver that I can hack.

I will make proper connections and try out if this works with my Jeti receiver.

[Mitch]

The ground and power wires are the ones that will cause problems so be careful to get those right (the polarity). A few years ago I destroyed the servo amps on 4 very expensive digital servos when I connected a board with reverse polarity. I hadn't built any reverse polarity protection into the board for prototyping. Miraculously - the board and the pic survived!

[lukasz]

I'm aware of this

Will double or triple check before I power it on

If I connect FTDI cable, servos, RC receiver, GPS, will I need to solder jumper SJ2 and SJ3 to have all working when connected to FTDI?

[Mitch]

Tom provided a lot of flexibility here. This is how I elected to do it.

I am not using the battery connector, but elected to always use one of the servo connectors to power the board (three position connector). This is to prevent accidental reverse polarity as I mentioned above and because the servos are the main current users. I flowed the solder joints for +5 and ground between the 6 servo connectors to minimize losses in the pcb foil (this is really only necessary if you were using some bigger high current servos). Also, when using an electronic speed control, the board is powered this way through the bec connector.

When no servos are attached the ftdi can power the entire board through its connector, but with servos and a receiver the current will be too hi for the usb power supply.

It is a good practice to avoid having both powering the board at the same time (they will fight each other).

In short jumper SJ2 and SJ3, and remove the +5 wire from the ftdi connector so it is not powering the board when the board is being powered elsewhere.

[Tom]

Yes, I need to update the wiki with some circuit diagrams

PPM: PPM1 = GND, 5V, PPM signal (starting from border to center)
PWM: PPM1 = GND, 5V, PWM channel 1
PPM2 = PWM channel 2, 3, 4, 5
Just connect the PWM channel x signals to a signal pin on your RC-receiver, the GND and 5V can be used to power your receiver.

Pro PPM:
- Fewer cables
- The autopilot can know when the signal is out of range and act accordingly (return home or whatever you program it)

Pro PWM:
- No need to hack your receiver
- PPM only works on PPM receivers (no 2.4GHz or other fancy stuff)


Mitch,
You have gluon first revision board. The 2nd revision has 5 PWM inputs in total (the PPM2 connector has 4 pins).

[lukasz]

Thanks Tom and Mitch for replay.
Because I can't wait till first flights I will use PWM signal for time being.

Once I find a receiver that I can hack I will use PPM signal.