Repost: Using a Spektrum DM9 (and Futaba TM-7)

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Mike_K
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Repost: Using a Spektrum DM9 (and Futaba TM-7)

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Postby Mike_K » Wed Dec 13, 2017 5:15 pm

I've been offered a Spektrum DM9 module for a JR set by a clubmate at a favourable price, but I seem to remember reading somewhere that they don't work with a standard ppm stream. I want to test my propo encoder with as many 2.4GHz modules as I can, but don't want to waste my money buying it, if it definitely won't work. I know JR uses the ppm stream inverted to Futaba, but are there any other differences?

I've already tested a Spektrum DM8 (for Futaba sets) and also a FrSky, OrangeRx, Futaba TM-7, Futaba TM-8, Futaba P-FTM/FH and an old Futaba 35MHz modules OK.

Has anybody used a DM9 successfully?

Thanks Mike

Re: Using a Spektrum DM9
Postby Phil_G » Wed Dec 13, 2017 7:06 pm


Hi Mike, the DM9 is fine, straightforward PPM driven module but with one proviso
If you want to maintain standard timing the PPM pulse width MUST be 400uS. It will work with more or less,
but the channel timing will be out by the difference - ie a 1500uS neutral into the DM9 module will come out
of the receiver at 1600uS if the PPM pulse is 300uS. Shorter PPM pulse = longer channel pulse.
I've spoken at length about the DM9 with Andy Kunz and he told me that they had a different team of
developers for the DM9 who interpreted the spec 'differently' and included the PPM pulse width in the timing
calculations, where conventionally as you know the actual pulse width is excluded and timing is taken edge to edge.
Its happy from 8 down to 4 channels (not tried any more or any fewer) and with 400uS pulses can be
considered a very 'ordinary' module. It needs negative-going PPM, ie standing high pulsing to 0v, wont work
at all with positive. Its not level critical, and will happily take 3v3, TTL or full battery levels.
Remember its DSM2 only, no DSMX.
My Gem that I use for the Sharkface has a DM9, with an Orangerx R610 receiver (£3 jobbie).
It was converted back in 2010 so has done 7 years in S/C guise, no problems whatsoever once the timing peculiarity
was identified. Strangely, this has only just resurfaced with Taranis lads wanting DSM2 and fitting a DM9 - they found
all the channels were 100uS off until the PPM pulse width was changed to 400uS.
Gem (Spektrum DM9 / OrangeRx R610)
Gem (Spektrum DM9 / OrangeRx R610)
Incidentally, you mentioned the Futaba modules Mike, you're aware that whilst Futaba channel timing is edge to edge, they actually use the PPM pulse width to control RF output power and failsafe. I have some notes somewhere, I'll dig them out.

Cheers
Phil


Re: Using a Spektrum DM9
Postby Phil_G » Wed Dec 13, 2017 8:13 pm

Heres some code comments:

Code: Select all

;
; The Futaba FASST pcb needs a special 8 channel ppm input. The length of the 1st positive
; pulse determines if the module transmits on normal or reduced power level.
; 1st pulse: 
;      420 usec: normal power
;      440 usec: reduced power, range check mode
; 2nd to 9th pulse:
;      400 usec
; The value of the 8th channel determinies the fail-safe value and mode.
; The failsafe mode means that in case of connection break the 3rd channel output gets the last
; value of the 8th channel.
; If the 8th channel value is 0,9 ms = -110% (on a -100% to +100% scale) then fail-safe is off, the 3rd channel just keeps the 
; last valid value.
...and some text translated from German:

Code: Select all

<400us     Transmitter remains in the operational error flashing mode
400-420uS  operating mode with no fail-safe
425-440uS  secure Failsafe mode with adjusted Failsafe value on channel 8 
445uS      Transmitter diodes flash, when like if them in the power down mode would be. Whether that the case is real could not because of to large housing ranges despite walls not be verified. And also not yet whether that switches off automatically power down. The second COUNT down bleeper does not function anyhow
Its 7 years since I played with a few FASST modules and I cant remember if this refers to all Futabas or just 'some'. Worth checking though, you dont want your encoder to accidentally invoke low-power!
I think I ran mine at 410uS, no failsafe, but my own conclusion was - avoid them, too much faff and doubt, plus they're too expensive anyway!
[Edit: I checked, yes mine were 410uS)
Cheers
Phil


Re: Using a Spektrum DM9
Postby Phil_G » Wed Dec 13, 2017 9:19 pm


Original German article with timings that helped me back in 2010:

http://www.drivecalc.de/circuits/GoingFASST.html


Re: Using a Spektrum DM9
Postby Mike_K » Thu Dec 14, 2017 10:30 am


Thanks for the quick and detailed reply Phil. You learn something every day! I'll buy the DM9 on that basis.

As far as the FASST test, they say that ignorance is bliss, and I just used my standard 300uS timing. Last week I test flew the FASST module in my Micron PL7D flying my long suffering Acrowot that I always use for testing new radios, without problem. If I had know what you just told me I would have stayed on the ground.

But I think I'm OK with the pulse timings for the TM7 module as I checked the output from my old FF7 or FF8 (I can't remember which) and it was a typical 350us pulse for all the channels and I've flown the FF8 with the FASST module hundreds of times. From what you've said it appears that you can control the range check and binding from a suitable transmitter, but if you use timings of <400uS it just runs in normal mode and you bind and range check with the switches on the back of the module?

But it has made me think. When I read the first bit about the DM9 needing 400uS timing I thought I'd just make my encoders default timing 400uS as I assumed it would work with all other radios, but it looks like it could mess up the FASST timings. But as they need different ppm polarity I could make one 400uS and leave the other polarity at 300uS.

And yes I agree with you that all new Rx I buy are FrSky, but I've a cupboard full of Spektrum, OrangeRx and Lemon Rx so I still want some transmitters on DSM2. I've given up with the OrangeRx DIY modules as I've a mixture of DSM2 and DSMX Rx and changing mode between them is a pain. As I've never had a problem with DSM2 I'm using a DM8 in my Skyleader and plan to use the DM9 in my still to be converted OS Cougar. The reason for the FASST test is a friend of a friend wants one of my conversions but he ideally wants FASST Rx as he has a shed load of them. Before I said "yes or no" I thought I ought to test it. As I've already said yes I hope I've got the timing bit OK. He's having a Futaba M6 FM converted as it's got open gimbal sticks that feel as good as modern sets.

Cheers Mike


Re: Using a Spektrum DM9 (and Futaba TM-7)
Postby Phil_G » Wed Dec 14, 2017 8:13 pm


For a long time I've mistakenly believed that the Futaba TM7 module used the same RF power and failsafe control as the internal 6EX FASST board, ie by varying specific PPM pulse widths. It has also been reported that they need specifically seven channels, no more, no less. Consequently, though I'd no personal experience of it, I've always steered people away from the TM7, saying that without Futaba-specific timing in the encoder software, its not readily usable.

It transpires that whilst the power/failsafe thing is true of the internal boards, the TM7 does not use PPM timing in the same way, and in fact can be used as a conventional RF module on any of our projects - and the channel count stipulation appears to be tosh.

To find out once & for all I bought a new TM7 and linked to an R617FS receiver its absolutely fine, green leds both ends and no problems with less than 7 channels, in fact it runs the piggy-backed S/C encoder perfectly - which is only four PPM channels (rudder, kickup, throttle & switch) with positive-going 250μS PPM pulses.

They are still available (though etsi ruled out fasst on excessive MU) though more expensive than the usual Frsky, Flysky, Orange etc but the Futaba is a super quality setup with a supremely reliable link if you have a 'special' model. Frsky FASST receivers appear to be just as good, I use several of their TFR6's and 8's.

https://www.ebay.co.uk/itm/FUTABA-TM7-2 ... 3497906418

https://www.ebay.co.uk/sch/i.html?_osac ... e&_sacat=0

Cheers
Phil
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