How to connect to FrSky DFT Tx module using FTDI - RC Groups
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Jul 28, 2011, 12:37 AM
Sudden Freak
c2po's Avatar

How to connect to FrSky DFT Tx module using FTDI

Interesting... I didn't think it would be that easy.

It is a well known fact that FrSky Tx modules (and receivers too) use RS232 as interface, therefore connecting them to a computer might be kinda problematic.
Basically there are three options:
- find a PC with a real RS232 COM port, usually old PC and laptops have it, and use a RS232 cable that came with the module;
- buy a RS232 to USB adapter - proves to be the most problematic since most of the adapters are not properly working under newer versions of Windows, like Windows 7 x64;
- since RS232 uses inverse voltage levels comparing the FTDI TTL levels - build a signal inverter using two transistors which is kinda messy, but then you can use any FTDI adapter, usually no drivers needed, works with every version of Windows, just plug it in.

Last option is the most preferrable because probably anyone who messes with microcontrollers should already have FDTI cable or adapter anyway.
I decided to look inside the FrSky Tx module to see if I can find an easier way than finding four resistors, two transistors, and then solder all that stuff on some board, and then putting it somewhere... Hmm, there's a Sipex SP3232E chip that provides the RS232 output levels and actually does quite opposite from what I need - it converts TTL levels into RS232 levels. So why don't I just circumvent those two inverters... then I would get normal TTL levels!

Tiny soldering iron - check, helping hands with magnifying glass - check, steady hands - oh shoot, where are they... oh well, have to go with those that I have. In ten minutes I desoldered and lifted legs 13 and 14, soldered tiny pieces of wire to 11 Out(Tx) and 12 In(Rx), and voila! - it's working through the normal FTDI adapter streaming bytes of telemetry data at the 9600bps speed... sweet.

But mind you - that mod is not for everyone, distance between the chip legs is 0.65mm (1/40 of an inch)...
Use that mod at your own risk, I have NO RESPONSIBILITY WHATSOEVER for anything that may occur as a result of your actions if your board is destroyed and warranty is lost.

Update: added some macro shots to show what I'm talking about.
Last edited by c2po; Jul 29, 2011 at 02:54 AM.
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Aug 29, 2011, 10:51 PM
Old retired Electronics Fart
Charles B's Avatar
Thanks for showing me the MOD.

It took me 35 minutes to do it but no shorts . LOL
we will be using the Arduino also but with the Nokia3310 display. Just because we have three of them at our shack.

I'll let you know my progress. I'm 65 and a little slow now and then.

Your doing the shame way we wanted to do it is feed the current sensor into A2 and use the A1 for the Lipo voltage.

Aug 30, 2011, 01:19 AM
Sudden Freak
c2po's Avatar
Sure, I'm glad that it was useful. Funny that taking good macro pictures of the mod took me much longer than to do the mod itself...
Sep 01, 2011, 01:47 PM
Old retired Electronics Fart
Charles B's Avatar
I got my FrSky Tx and 8 receivers on Monday and installed on Wed..
Your Mod works with no problems.
We got the interface for the receivers built and working ( LiPo voltage/ current ) reading.

I did a range check with one of my planes and we got 150 Ft. 50 meters.

We have our mini Pro Meg328 on order.

Thanks again.
Sep 02, 2011, 08:31 PM
Sudden Freak
c2po's Avatar
No problem! I've also ordered another set of Tx/Rx to install in my other radio, I really like that FrSky two way series.
And range seems to be really good, considering that DFT module consumes only 75mA, comparing to 72MHz module that is eating up almost 1 Amp and having probably third of a range!
Sep 06, 2011, 11:40 AM
Old retired Electronics Fart
Charles B's Avatar

my update

Got my minPro atmeg 328P

Well here is some things I have added to the program.

Auto sense the number Lipo cells.

Set the min voltage for each cell in the program.

Set the capacity of the Lipo's in the program.

Have an alarm that sound off when capacity gets to 80% . That sends a Morse code "M" = da da to the speaker.

Have an alarm that sound off when the voltage for each cell drops below 3.4 volts. That should give time to land the plane.
That sends a Morse code "V" = dit dit dit da to the speaker pin 9.

We have a bar graft displaying current used so far.

If the weather is good tomorrow we will do a test flight.
Sep 06, 2011, 07:12 PM
Sudden Freak
c2po's Avatar
Cool, nice additions!
Sep 08, 2011, 03:17 PM
Old retired Electronics Fart
Charles B's Avatar
Have one more update.
I added a Menu to the LCD.

It allows you to increase or decrease the Max battery current.
It can increase/decrease by 50ma each press.
When you leave the Menu it stores that value into EEprom memory address 10
At the top of the PDE file will tell you the three switchs wiring.

You know that you can use any or all the changes that I give you.
code for code Or just the Idea or not at all.

We took our plane out yesterday for the first time with the FrSky and with the
Tele. data hookup . It worked real good but I did have to add some filtering to the current sense it was to noisy when the motor was running. LOL

Thanks Charles
PS If you want me send you my updates via a Private message I will.
Sep 08, 2011, 08:16 PM
Sudden Freak
c2po's Avatar
Thanks, Charles, those are very interesting additions.
I also had to add 22uF capacitor to a current sensor to smooth the current readings (that gives about 4Hz filter cutoff frequency), and now it seems to be quite accurate, at least the amount my charger puts back in the battery is within 5% of what I see on telemetry screen, which I consider to be a very good result.

I was also thinking about adding some menu with configurable parameters written to EEPROM, but I'm too lazy, and so far I'm happy with what I have right now, so I'll leave that for the winter when I hope I will have more spare time.
Too many toys, too little time...
Sep 09, 2011, 11:20 AM
Old retired Electronics Fart
Charles B's Avatar
My filter on the current sensor is 10K to a 2ufd to gnd and that junction to a 27k with a 10ufd tantalum cap to ground. I've only flown around 1000 ft away but still look like a strong signal coming back.

Today we just go my display that we will be used as the final display on the control box. It from SparkFun a Nokia 5110 newer version of 3310 display.

I'm retired so have lots of time to play around.

Feb 05, 2013, 06:36 PM
Registered User

No Need to ruin the board

Please view the picture for the location of RX / TX on TTL level on the board, Just solder there and you dont need to mess with the chip, I do not recall which is which since I have closed the unit.
But in order to find out just beep them with the legs on the TTL to RS232 (Sipex SP3232E) chip on legs 11 & 12 (the one that show that the legs are raised up)

Good Luck
Oct 13, 2015, 03:22 PM
Registered User
I did the mod on the PCB, also I soldered Pin header for the connection to the sender and for the TTL Port

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