Open Pilot receiver connections
|Transmitter||Air side Modulation||Receiver|
Connect all channels from the receiver to the JST-SH 8pin PPM/PWM input
A receiver with a PPM output stream combines all output channels into a single wire, so you should only connect the connector with the Black, Red & White cable.
In the case of a Spektrum satellite, you need to connect the receiver to the Mainport or Flexiport.
The Futaba S.Bus connection can only be connected to the Main port.
|OPLink||Can be connected to either Flexi or Main ports. The Revo has an onboard OPLink. Can be used as a transmitter and receiver. Further information at the OPLink Wiki pages.|
Air side Modulation
The modulation type has nothing to do with the receiver type selection in the GCS. The modulation type is the way the signal is transmitted from the transmitter to the receiver. This can be either a PPM, PCM or other signal. Different types of modulation offer different possibilities. PCM modulation is a different method and offers, for example, extra information to be sent from the transmitter to the receiver, such as failsafe settings, error recognition etc.
The receiver type can be selected directly from the HW Settings page on the C within the OP GCS.
Receiver type information
Receivers may have different outputs available. In order to connect your OpenPilot board to your receiver, the correct communication must be specified. In the GCS this is referred to as receiver type & one can choose between PWM, PPM, Spektrum or S.Bus.
PWM (Pulse Width Modulation)
This is actually the most common receiver output. The receiver sends a signal of varying length towards the servo or ESC. The length of the pulse specifies the servo output or throttle position. So in practice, every servo or ESC has its own connection cable with its own dedicated PWM signal.
If you use a standard receiver like this then you must specify PWM (default in the Vehicle Setup Wizard as your receiver type, or on the Hardware page of the GCS.
The PWM signal is usually sent over the lightest colored cable from the servo or ESC. The other two wires are the plus & minus needed to power your receiver or servo. The red (positive) cable is always located in the middle for a good reason. If one accidentally plugs in the connector reversed, it doesn't harm anything, as the positive wire will remain the middle wire.
The receiver cable delivered with OpenPilot boards has one connector which caries 3 leads, while the other connectors only have one lead. The connector with 3 leads carries the 5V voltage (positive & ground) while the other connectors only carry the PWM signal. Since the ground & positive pins on your receiver are all in parallel, it is not necessary, nor advised, to connect all of them.
Pulse length info:
The length of the signal pulse normally varies between 1000 & 2000µs (micro seconds), with 1000µs being the minimum & 2000µs the maximum.
These values can be monitored and measured, and they have a real physical meaning. Hence, this is the reason why these units of measure are also used in the GCS.
PPM (Pulse Position Modulation)
OpenPilot can work with a PPM signal from the receiver or other device. A PPM signal (often referred to as a PPM stream) is basically a series of PWM signals (described above) sent one after another on the same wire. So instead of continuously sending the information for 1 channel or servo, the information for all servo's or ESCs is sent in a row on the same wire.
The advantage of such a PPM signal is that only one signal wire is needed instead of all the individual wires.
Many transmitters and receivers communicate between themselves via PPM and will say "PPM" somewhere on the box or transmitter. This does not mean the receiver actually outputs a PPM stream or signal. If you are using one servo plug per channel, your receiver is probably of PWM type, NOT PPM. Some receivers have the option to output also a PPM stream apart from the normal individual PWM outputs, and will then be a single output from your receiver for all channels. (This statement does not apply to S.Bus or Spektrum satellite transmitter/receivers.)
An example of such a receiver and the signal outputs can be seen below. On channel 8, the combined PPM stream is available. In this case you should only connect the 3 wires (Black, Red & White) to the JST-SH 8 pin port & choose PPM input.
Spektrum (Satellite connection)
A third receiver option is the Spektrum Satellite, also described here. It allows a Spektrum or OrangeRX satellite receiver to be connected to OpenPilot.
Note that the Spektrum adapter is designed to operate from a power supply of 3.3V, an adapter must be used.
The S.Bus is another communication protocol from Futaba that can also be used on Taranis radios. It also combines all output channel information into one lead. You will need to fabricate your own lead to use this system.
FrSky PPM signal Warning
Some older FrSky receivers have a bug that affects PPM operation so please ensure that your receivers are running the most current firmware.
The problem is that the PPM stream is missing it's sync pulse when all the channels are maximized. In this case the FC cannot longer recognize the first pulse of the first channel. This results in a fault on the OpenPilot boards & will most likely shutdown the aircraft.
The following applies for example to the 4 channel FrSky V8R4 receiver.
If we look at the PPM stream in detail in below image. One can clearly see the 8 channels on the left & a big sync pulse on the right. The software recognizes this sync pulse and knows that after this pulse the information for channel 1 is coming. It also searches continues for this pulse so it knows that the information received before this pulse was from the last channel.
In below example everything is still ok, the pulse lengths from the individual channels are longer indicating that the sticks are all centered. Note how the sync pulse is already shortened.
The complete information from all channels and the sync pulse is located in a total frame length of 18ms for FrSky. Other brands like Futaba & Spektrum use different total frame lengths of 20 or 22.5ms or even longer.
So here it goes completely wrong. Once all channels are maximized, then the individual channel pulses are too long. Since FrSky want to put all this information of the 8 channels and the synch pulse in the 18ms total frame length there is no room enough for the sync pulse. The sync pulse has now the same length of the other channels. This means that the software cannot longer distinguish the sync pulse from the channel pulses. In other words, it has lost track of the individual channel information & causes an error.
In above image one can clearly see that the channel pulses & sync pulse are all the same. This only happens when all 8 channels are used & are high. A problem on an OpenPilot board can also occur when only 7 channels are used on the complete maximum. You can reach the complete maximum for example when you set the channel endpoints in your radio at 125%. (The exact max. depends on the radio of course).
In normal circumstances where you only use 5 channels you will not run into this issue. If you decide to use the remaining channels to control the pan & tilt of your camera or to use them for the tuning of the PID's via the TX (TxPID module) then you may run into this problem.
So please be warned if you are using the older stock FrSky receivers with PPM output.