|Jul 31, 2007, 12:38 AM|
Build your own DSM2 transmitter module (its working!)
I opened up my E-Flite LP5DSM transmitter this evening to have a peak inside. This is the 1mw transmitter that is included with the Blade CX2 Helicopter.
I see the DSM2 transmitter is the size of a postage stamp and has 4 connections. Vcc(3.3volts), ground, serial data in and LED out(I believe)
I did a quick check of the serial data, running it through a level shifter it appears each frame is 14 bytes @ 115200.
EDIT - 115200 is incorrect - see newer posts for updated information
Assuming the data values for each channel can be determined and any checksums they may be adding we could easily take that module and connect it to a PPM decoder from another radio. I already have a PPM decoder that outputs serial data I have been using with an xbee radio system. If I can figure out the data I will adapt the code to drive the 1mw DSM2 module.
Not sure how useful this would really be but if it worked you could buy the LP5DSM, remove the tx module, connect it to a PPM decoder board and use it with your favorite radio for anything you intend to fly within 100 feet.
I'm going to try to do it just for fun so I can fly my Blade CX2 with my Futaba 9CAP.
If anyone else is interested crack open a LP5DSM and check out the serial data - perhaps someone else has already figured it out?
|Jul 31, 2007, 01:11 AM|
Is the data encrypted inside the 2.4Ghz transceiver chip? If you are going to reverse engineer the data set, you could probably buy the full power chip.
|Jul 31, 2007, 01:29 AM|
Not sure if the data from the radio controller(Atmel Mega8) to the radio module is encrypted or not. Thats something I'll have to look for when checking the serial data. If it is then I'll give up pretty quick. I have no real interest in the higher power version, not to mention its fully integrated with the microcontroller, this doesn't appear to be.
I'm not a big fan of Spektrum but I got this blade cx2 really cheap so what the heck, it could be a fun project.
|Jul 31, 2007, 02:27 AM|
Looking at the data further it doesn't appear encrypted.
EDIT - see newer posts for updated information
|Aug 02, 2007, 03:20 PM|
Joined Sep 2004
Here's more info on the Spektrum system:
The link between Rx and Tx is all done by some commercially available transceiver chips on the 2.4GHz. Unless I-m mistaken the link between Rx chips and microprocessor is and I2C link, otherwise a simple direct one wire serial link. The processor must contain firmware to compare the two sets of data received and determine which is best. There is NO PPM output out of the receiver at any point as far as I have been able to determine. I-m actually trying to regenerate a PPM signal by recombining the separate channel PWM outputs (http://www.rc-cam.com/forum/index.ph...ic=2064&st=20). And special thanks here to Mr. RC-Cam for providing a circuit which hopefully will solve my problem.
Instead of fully copying the Spektrum DX system you could also go complete homebrew, e.g. http://webx.dk/rc/uhf-link2/uhf-link2.htm
|Aug 02, 2007, 03:32 PM|
Thanks for the links - I'm pretty familiar with the Spektrum system and the Cypress chips they use. The only reason I thought this might be fun is because they appear to use a simple serial connection to the DSM2 module in the transmitter. I already have a homemade full working 2.4Ghz system using Maxstream xbee and xbee pro modules. It works well enough I've sold off my other Spektrum equipment.
I use an Atmel tiny2313 to convert the PPM data to serial data to feed the xbee modules, so it seems with a little rework to the protocol I could directly feed the DSM2 module. This would allow me to use my Futaba 9CAP with my Blade CX2 heli and other small(short range) parkflyers using the really small Spektrum receivers. It would also mean I could stuff an xbee into the Spektrum transmitter if I felt the need.(this is unlikely)
Not sure how much of this I would really use, but I like building stuff so its a fun project. I haven't analyzed the serial data any further since my original post as other projects have taken priority. It does seem like one more evening though and I'll have the data figured out enough to use it.
|Aug 04, 2007, 03:26 AM|
Decided to play with this a little more this evening and everything fell into place.
I tried a serial port rate of 125K instead of 115200. The data is now super easy to see and follow.
The radio transmits 14 bytes per frame, 6 channels of information, 2 header bytes. Data rate is 125K 8,N,1
Each frame begins with 00 00
Each pair of bytes after that represent the channel and its value. Each channel has a valid range(1024 steps - 10 bit), the channel number is embedded in each pair of bytes.
The radio transmits a 6th channel which is a copy of channel 1 with its endpoints limited. This appears to simply be a function of the radio when in airplane mode, channel 6 is still a full 10 bit channel.
Here is a breakdown of the min and max values for each channel and the valid 10 bit range for each channel.
ch1(left stick, up/down)
0056-03AA -measured min/max
10 bit range - 000-3FF
ch2(right stick, left/right)
0455-07A9 - measured min/max
10 bit range - 400-7FF
ch3(right stick, up/down)
0855-0BA9 - measured min/max
10 bit range - 800-BFF
ch4(left stick, right/left)
0C56-0FAA - measured min/max
10 bit range - C00-FFF
ch5(knob, upper right)
1056-13AA - measured min/max
10 bit range - 1000-13FF
ch6(copy of throttle channel) - travel limited
152A-162A - measured min/max
10 bit range - 1400-17FF
First two bits are 00
Next four bits indicate channel #
remaining 10 bits are serial values for the PPM data
00 00 00 xx xx xx xx xx - ch1 0000-03FF
00 00 01 xx xx xx xx xx - ch2 0400-07FF
00 00 10 xx xx xx xx xx - ch3 0800-0BFF
00 00 11 xx xx xx xx xx - ch4 0C00-0FFF
00 01 00 xx xx xx xx xx - ch5 1000-13FF
00 01 01 xx xx xx xx xx - ch6 1400-17FF
If supported by DSM2 low power module:
00 01 10 xx xx xx xx xx - ch7 1800-1BFF
00 01 11 xx xx xx xx xx - ch8 1C00-1FFF
So there it is, using the above data its now possible to build a 6 channel PPM to serial encoder to interface to the DSM2 module from the LP5DSM radio.
Questions or comments let me know!
|Aug 04, 2007, 09:56 AM|
Do you think there is any encryption going on in the Cypress chip so that we wouldn't be able to make our own receivers? Would be nice if they "forgot" about this because then people like Castle will start making receivers and get the price wars going!
Also you may be able to turn out a Futaba compatible (PPM) module before Spektrum manages to ship. That module has been pushed back again! I wonder what the problem is?
|Aug 04, 2007, 10:38 AM|
OK, here's an off topic question.... What is a good, cheap, easy to work with RF link that will fall under FCC part 15 regulations (900Mhz or 2.4Ghz)? The Maxstream boards are too big for my needs, are these Cypress chips worth using? I would need a PPM to serial encoder (there are a few examples posted in these forums), and then a serial to servo PWM on the receiver side (plus the glue to hold things together).
|Aug 06, 2007, 04:11 AM|
Hard to say if they are using any encryption. I know the ground DSM system was trivial to reverse engineer as a third party transmitter can drive the Spektrum ground receivers.
I wouldn't doubt they have added a little something to prevent other companies from building DSM2 compatible receivers. The way to check is to grab a compatible Cypress chip on an eval board and connect the SPI up to a microcontroller and see whats being transmitted over the air.
I have no idea whats taking Spektrum so long, we know they have been doing PPM to serial data for a few years now.
Perhaps its nothing more then waiting for FCC authorization. I know one of the Futaba modules was finally approved last month.
|Aug 06, 2007, 04:18 AM|
There are many options out there, I haven't tried them myself though.
If you're into micro flight then perhaps using the DSM2 module out of LP5DSM with your favorite transmitter and the new AR6300(I think thats the model) micro receiver would be a nice option.
I pondered using the DSM2 module with a 3 channel car radio so that I could use the insanely small/light Spektrum receiver but my two 1/36th scale micro RC trucks already have xbee based receivers in them.(receivers are down to 7 grams or so and could be a little lighter still)
I am running my xbee based system every weekend along side other 2.4Ghz radios with flawless performance.
Anyway it will probably take a week or two but the PPM to serial converter to use my Futaba 9CAP with the DSM2 module is going to be written.
Now I need another empty Futaba transmitter module case...
|Aug 06, 2007, 08:45 AM|
|Aug 06, 2007, 02:00 PM|
Joined May 2004
Anyways, I have a unused LP5DSM sitting here, so it will be interesting what you come up with.
|Aug 10, 2007, 12:13 AM|
USA, CA, San Diego
Joined Jun 2007
There are apparently some pads inside that might switch to idle-up, but I'll let that wait.
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