Spektrum DX7 Programming Notes

Mike Shellim, 21 Jan 2007
Updated 05 Feb 2007 - crow brakes

Updated 03 Jun 2007 - introduction

Updated 25 July 2008 - crow brakes

Updated 28 July 2011 - flapperon example

Introduction and brief review

Tx, Rx and ServosThe Spektrum DX7 is the first full-range 2.4 GHz radio control system for R/C modellers. I reviewed it in the April issue of RCMW, and found it to be a well featured system (if not quite as flexible as some other 7-channel radios). It is nicely constructed, and provides a rock solid radio link.

Downsides are the poor balance of the box, rather archaic programming (though by no means worse than some other Far Eastern radios), and the need to install two receivers in the model.

The spread spectrum data link is very robust however, and as a result the set has become very popular here in the UK.

Here are some programming notes which owners may find useful.

Programming 4-Servo Wings

The increasing popularity of 3D electric models and F3x sailplanes has made support for four-servo wings almost essential in mid-range radios. While the stock mixers on the DX-7 only support two servos, the other two servos can be driven by means of a couple of PMIX's and a bit of effort.

So, let's see how to implement a basic 4-servo setup to support the following:

STEP 1: Configure Flapperons

in the System menu, set the model type to ACRO, and the wing type to Flapperon (and optionally enable the V-tail mixer).

STEP 2: Connect the Servos

The table below shows the servo assignments. The wing servos are shown in purple - chs 2 and 6 are assigned to the inner surfaces ('flaps'), while chs 5 and 7 are assigned to the outer surfaces ('ailerons').

Rx Channel Connect this Servo
1 THRO Throttle
2 AILE Right Inner
3 ELE Elevator
4 RUDD Rudder
5 GEAR Left Outer
6 FLAP Left Inner
7 AUX2 Right Outer

STEP 3: Disable the 'Gear' switch

The Gear function must be disabled, so it doesn't affect channel 5.

  1. Enter the SERVO TRAVEL menu
  2. Set the servo travel for 'Gear' to zero/zero.

STEP 4: Configure Flap system

The DX-7 provides a choice of activation method for flaps. The 'System' option will suit most sailplane pilots. It provides a choice of three flap presets, selectable via the flight-mode switch.

  1. Enter the SYSTEM->INPUT SELECT menu.
  2. Select 'system'.

STEP 5: Configure the mixers

In the final step, we'll set up the stock mixers for the inner surfaces, with PMIX's for the outer.

Function Mixer Menu Rate Offset
Inner Snapflap [ELEV->FLAP] D=0/U=+10 n/a
Inner Camber [FLAP SYS] NORM=0
Inner Aileron [D/R] (to set travel)    
Outer Snapflap [PMIX3]: Elev->Gear 0/+10 0
  [PMIX4]: Elev->Aux2 0/+10 0
Outer Camber [PMIX1]: Flap->Gear +100/+100 0
  [PMIX2]: Flap->Aux2 +100/+100 0
Outer Aileron [PMIX5]: Aile->Gear +100/+100 0
  [PMIX6]: Aile->Aux2 -100/-100 0


Crow Mixing

Owners of F3B/F/J sailplanes will be wondering whether the DX-7 can be used for crow brakes. Well yes it can, but the programming is tricky. Actually it's actually pretty horrible especially if you're used to the flexibility of a Multiplex radio as I am.

The main problems are:

  1. insufficient PMIX's to support crow mixing in addition to the basic mixing described above. As a result, two of the mixes above have to be dropped.
  2. the unpredictable effect of the Offset parameter in the Throttle mixes. This makes it tricky to set the crow travel accurately on the outer surfaces (it's easy enough on the inner flaps).

Nevertheless, here's my solution for crow. In order to free up PMIX's for crow, I've dropped two of the mixes, namely snapflap and camber on the outer surfaces.

Function Ch Mixer Menu Rate Offset
Inner Snapflap   [ELEV->FLAP] D=0/U=+10 n/a
Inner Camber   [FLAP SYS] NORM=0
Inner Aileron   [D/R] (to set travel)    
Inner Crow   [PMIX3]: Thro->Flap +50/+50 +100
Outer Crow   [PMIX1]: Thro->Gear +50/+50 -100
    [PMIX2]: Thro->Aux2 +50/+50 -100
Crow/Elev Compensation   [PMIX4]: Thro->Elev 0/+25 0
Outer Aileron   [PMIX5]: Aile->Aux2 +100/+100 0
    [PMIX6]: Aile->Gear -100/-100 0


We've seen how it is possible to program a basic 4-servo wing up quite easily if you don't need crow brakes.

Crow brakes are possible but with some compromises and difficulties.

It's worth remembering that the DX-7 also lacks flight modes and curves, and there may be installation issues arising from the twin receivers. While some modellers get around the latter by using the AR6100 park-fly receiver, this practice is not recommended for full-range applications.

Implementing a Throttle Kill function

There is no throttle kill function provided with the DX-7, instead there's 'Throttle Idle Recovery'.

For those who prefer a traditional Kill function, here's a solution which works off the MIX switch:

Note 1: use only PMIX-5 or 6, otherwise the kill setting will vary with idle trim.

Note 2: The kill position is adjusted via Offset.

Channel Assignments

Table below shows channel assignments of the DX-7.

  Wing Type  
Ch # Normal Flapperon V-tail Delta Notes
1 Throttle        
2 Aileron Flapperon   Elevon  
3 Elevator   Vtail Elevon  
4 Rudder   Vtail    
5 Gear        
6 FLAP Flapperon     labeled as 'AUX1' on AR7000
7 AUX2        

Mixing Architecture

The mixing architecture is quite simple.

Mixers: Simple case

Consider a mixer C1 -> C2, e.g. THRO -> ELEV

In the simple case (e.g. for Normal wing type), moving stick C1 will effect just the channel corresponding to C2. So with THRO -> ELEV mixer, moving the throttle stick will modify the signal to servo #3 .

Mixer: special cases

Things are only a little more complicated for the special wing types. Again, consider a mixer C1->C2. The table below shows the effect of different C2 and wing types:

Wing Type C2 Output Channels Relative sense
Flapperon AILE #5, #6 opposite sense
  FLAP #5, #6 same sense
Delta AILE #2, #3 opposite
  ELEV #2, #3 same
V-tail RUDD #3, #4 opposite
  ELEV #3, #4 same

Examples for wing type = Flapperon

Rudd -> Aileron
Here, C2= Aileron, so the mix drivers flapperons in opposite directions (i.e. as ailerons). Possible applications include:
1. Coupling of ailerons to rudder, where the rudder is the primary turn control.
2. Roll correction during knife edge.
Elev -> Flap
Here, C2=Flap. Displacing the elevator stick causes flapperons to move in the same direction. Use this to implement 'snapflap'.

Pulse Widths

Measured on aileron channel, trim at centre, all settings at default values (measured using servosim).

Servo Travel Min (mS) Max (mS)
+50/+50 1.3 1.7
+100/+100 1.1 1.9
+125/+125 1.0 2.0
+150/+150 0.9 2.1

Interestingly, the DX-7 outputs eight (not seven) channels at the DSC interface.