Turns out I caught quite the bug and was laid up for over a week. Just
getting back on tops of things now.
I have attached the manual for the AR7700. It may help explain some
things. Note that the AR7700 has the ability to use one Remote Receiver
(or satellite) so that user has the ability to separate the two
receivers and have the antennae orientated in different directions to
maximise the RF link quality. This is why I chose this receiver over a
single Remote Receiver. It also has the ability to have Preset Failsafe
positions on all channels, so with a very low throttle fails safe
condition the autopilot can detect this. I don’t believe a single Remote
Receiver on its own can provide this. This is another reason I chose it.
Note: You can also use a Spektrum AR9020 which has two additional Remote
Receivers for even greater installation options on large vehicles, or
those where many metal and carbon components may interfere with
reception. The AR9020 has SRXL output. It also has Preset Failsafe on
all channels. I have used htis on a Pixhawk previously as well.
The AR7700 has several output types which could be used with Pixhawk.
SRXL - This is the one I chose at is recommended as the highest
performance. I believe it is also an open standard. It just worked out
of the box with the Pixhawk. Was I just lucky ?
Remote Receiver Output - also known to most of us as a “Satellite”
but this really pisses the Spektrum guys off !! This would replicate
the output you would get from a single satellite and may be what the
code on the Pixhawk is actually expecting. I believe it is a proprietary
Spektrum format though a lot of people have worked (successfully) on
reverse engineering it. Back in April 2016 they published the
specification for this interface for the first time.
PPM Output - The original stream of PPM values
So, I used SRXL because it had worked for me before with the AR9020 and
it was recommended as the best option for communication with flight
controllers. Hopefully you can continue to support this interface so
users can continue to use the AR9020 as this is the ONLY output option
it has which is directly compatible with the Pixhawk. For a large
multirotor it is the only receiver I would choose due to it having the
two Remote Receivers.
As far as my use of the AR7700 goes I could possibly just change the
connection type to Remote Receiver. However if it were possible to keep
the SRXL compatibility that would be great.
In terms of your questions, as the AR7700 is a “normal” receiver I just
put the bind plug in and bind as normal, as described in the manuals. I
select the Preset Failsafe option so that my throttle failsafe position
is low enough (925us I think) to trigger a loss of RC link failsafe
condition on the Pixhawk.
I am not actually sure how many channels it is presenting as I don’t
know where to check this in Mission Planner, but if I had to guess I
would say 18 as according to its manual it can support up to 18 in the
SRXL and Remote Receiver formats, being only limited by the number of
channels transmitted to it by the transmitter. I am using a DX19
transmitter with 18 channels.
Yes, I have an AR7700 I could send you if necessary.
Finally, note that although I have been powering the AR7700 directly
from the Spektrum Input port (which should not have worked as it is only
3.3V and the AR7700 has a specified minimum of 3.5V !!) I will be
correcting this by taking the approx 5V from the RC Servo In port. I
checked this with Phil Rowse this morning as being appropriate.
Please don’t hesitate to contact me with any further questions when you
get around to looking at this.
Senior Research Engineer
Australian Centre for Field Robotics
University of Sydney
Mob: 0418 390 446
Fax: 61-2-9036 7474
AR7700 Manual.pdf (234 KB)