Beaten to death: Serial DSM Module - RC Groups
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Jan 27, 2014, 03:55 PM
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Beaten to death: Serial DSM Module

Big thanks to John Prikkel for designing this board:

His site contains all the instructions with parts and prices.

Deprecated content:

I'm sure there are a ton of resources online that describe how to build your own DSM2/DSMX Module for the 9X/9XR/Taranis, but I didn't find an all-in-one thread that wasn't 5000 posts long, so for anyone that's as impatient and lazy as I am, here's my build.

I started with the replacement JR-Module from Spektrum (SPM6817).

The popular Spektrum donor-radios seem to be the MLP4DSM (DSM2 only) and the DX4e (DSM2/DSMX). The MLP4DSM radio is 10mW and considered short range. The DX4e actually contains a DX5e radio that is 100mW and full range.
There are other donor radios that can be used, but these tend to be the least expensive and most widely available options.

All of them essentially function the same, with Ground, 3.3v, and the Serial TTL RX pin which everyone simply refers to as Signal.

Here's the MLP4DSM pinout, notice that the outside pins are reversed depending on where you choose to wire it up:

And the DX5e pinout, the pin with the square pad is almost always ground:

The OpenTX firmware for the Taranis, and the various firmwares for the 9X[R] include support for these radios. Though this mod uses a JR-Style case, the module won't work in most other radios, as most other radios will only output PPM on it's signal line.

The following schematic seems to be the recommended/accepted layout, but after testing out the mlp4dsm on an arduino by wiring it up directly, I decided to skip the diode, resistor, and incoming capacitor. I'm no electrical engineer, and I'm sure someone could school me, but it seems to work just fine without any issues. So until it fails, I'll probably leave it as is.

Here's a shot of the front:

I'm not a huge fan of soldering, and definitely not going to make a living at it, so I kind of cheated and used a prototyping board that had multiple pins in a row that were already connected.

When using in the Taranis, simply disable the Internal RF and configure the External RF to the appropriate DSM radio of your choosing. Binding on the Taranis is as easy as putting the receiver in bind mode and then clicking Bind from the model menu, same as an FrSky receiver.
Last edited by ModulusX; Jan 07, 2015 at 03:17 PM.
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Jul 10, 2014, 11:49 AM
WZ9V, AMA 409852
Nice write-up. I have a FlySky module on the way to provide a donor case and antenna hookup. I just have to find my old DX5 to scavenge for the donor RF section. The 1N914 is a reverse voltage protection for your TX and the resistor appears to be a pullup for that logic line and the incoming cap is just a filter cap. I would leave them in.

Here is an OpenSource PCB design I found on OSH Park if someone is looking for a PCB to diy things up.

I just ordered some, they come in a Qty of 3 so unless I screw up I'll have two extra if someone is interested.
Last edited by dfruehwald; Jul 10, 2014 at 12:39 PM.
Jul 10, 2014, 03:31 PM
Registered User
That's awesome! I wish I had seen that when I made mine, I'm pretty bad with a soldering iron.
How far from me are you? I have to build one of these for my friend now, I was going to solder it tonight..
Jul 10, 2014, 07:54 PM
WZ9V, AMA 409852
I'm a ways away but I am headed to Detroit the 26th/27th. Don't know if I'll have them by then but it would be about $6 for the board alone. I don't have a 3D printer operational yet so no case and I have not ordered the other parts so no idea what they would run.
Jul 11, 2014, 09:36 AM
Registered User
I ended up ordering a set on I wanted to design something like it, but have basically no experience at PCB design.
You should be able to get a DX4e on ebay for $20. I believe the other parts are all less than $10 in total. I got most of them from digikey. The case I got on amazon, but you could even use standoffs on the stock module bay cover plate and just install an antenna adapter right on the plate.
Jul 11, 2014, 09:55 AM
WZ9V, AMA 409852
Cool. I'm trying to get a parts list from the guy who did teh PCB design over on the OpenTX forum for the Taranis. I'm going to buy parts for all three boards and solder them. I'll figure out how to test them and offer them and offer the other two for cost + postage.
Jul 11, 2014, 12:32 PM
Registered User
ringo42's Avatar
Anybody have one of these PCB's for sale? I'd rather not wait on OSHPark. I'll take one with or without parts. I probably have all the parts already.
Last edited by ringo42; Jul 11, 2014 at 12:40 PM.
Jul 11, 2014, 01:13 PM
WZ9V, AMA 409852
I had to order 3 so I'll have two extra. The estimate is 7/22 for delivery to me though.
Jul 11, 2014, 01:15 PM
Registered User
ringo42's Avatar
If ModulusX does not have any then put me down for one of yours please.
Jul 11, 2014, 01:20 PM
Registered User
I don't have any either, but I ordered some.. I haven't received an estimated ship date yet.
Jul 11, 2014, 01:52 PM
Registered User
ringo42's Avatar
ok. I'll take one of whoever gets one in first. If anyone needs help soldering just let me know. Can someone post a link to the parts needed please
Jul 19, 2014, 04:56 PM
Team Warpsquad
NoShoes's Avatar
hey man, I'm planning to do the same thing with my blade nano and the MLP4DSM i have coming in with my taranis. So you just wired the PPM out directly to the Rx? and how are you binding it to models with the Taranis?


Just saw the part about binding in your post. Can't wait to try it.
Jul 19, 2014, 10:15 PM
Registered User
ringo42's Avatar
Yes, you use the ppm line. Set the taranis external module to dsm2 though in the menu, not ppm. Then in the menu you get a bind option.
Jul 19, 2014, 10:50 PM
Team Warpsquad
NoShoes's Avatar
okay, now i notice theres a few secondary options after DSM2. Like LP45, DSM2 again, and DSMX. Which one of those should I use? It defaults to LP45.
Jul 20, 2014, 08:26 AM
Registered User
ringo42's Avatar
I don't think I changed anything else. Don't have my radio right now. It just worked.