Trimble FM750 Interface

Hi all!
Our spray vehicles run FM750 gps units.
I’ve worked out how to get the unit to output NMEA data but the pc fails to receive it.

There is one section on the GPS setup where I can stipulate “NMEA GGA output decimal places”
This is at 8 by default but will accept 6 as a minimum.

Maybe I change that to a lower number?
The USB to serial interface I’m using is set to 38400 baud rate with Data Parity set at None

The GPS output settings are the same.
AgOpen GPS program is also set to the correct COM Port and baud rate.

Funny thing is I can plug in my Garmin GPS 60CS and AgOpen GPS is able to talk to it.

Would anyone be able to help me out.


You may need a null modem cable or maybe not be using a null modem cable - they reverse the pins #2 and #3.

Actually I made my own cable as per wiring schematic on one of the farming forums… but good point I guess I could try switching 2 & 3 and see if that works?

The way I’m thinking I can get data out of the unit I have to be able to, everyone else seems to be able to :joy:

I’ll try switching pins and yeah see what happens I guess. Thanks :slight_smile:

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Got it working! Awesome. Thanks very much! More questions to come as I continue my Autosteer build but so far so good!

Good to hear!

Tags: GPS not working. No RS232 output. Trimble serial output. Null modem cable. Swap pins

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Regarding the number of decimal places in the NMEA latitude and longtitude, for cm precision, don’t go below 7 decimal places.

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Field test of my Kogan tablet PC and AgOpen GPS with the Trimble CFX 750. Super keen to eventually look at Section control :slight_smile: but I’ll get AutoSteer happening first.image

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How can I unlock my console Trimble 750 for rtk correction ?

You can unlock RTK paying your local dealer about $3500CDN if you are already unlocked to Omnistar level, and you still will have to pay extra for Glonass.

Or you can buy an Emlid M2 for $600, and everything is unlocked, and you can send corrections from your base through your cellphone.

For AGOPENGPS, your best bet will be EMLID M2 or RS2, or any other UBLOX F9P variant.

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Sorry if I’m impolite but shouldn’t you ask Trimble. I’m not asking AgOpenGPS questions from Trimble.

You need an unlock code and I’m not aware of any hacks. So the Trimble dealer is the only one that can help I’m afraid.

EDIT: seems I completely misunderstood the question.

I understand this guy. Its hard to let go.

Trimble makes some nice stuff. I have written a converter to give my Ez150 GNSS RTK over TSIP. Now the 150 can live out its days on the lawnmower with cm precision and no RTK fees.
Emlid to Ez 150 sucsess

I am currently working on a converter for the EZ 250, i really like that unit.
252 Reciever emulator

But eventually I am sure it will be to complex for it to be worth making a converter for, thats why i am jumping on AGOPENGPS whole heartedly.

Trimble made some very nice, very robust equipment. But they go obsolete so darn fast.


i misspoke, is it possible to put AGOpenGPS with RTK correction on a trimble screen ?

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No it is not.

You would first have to write custom firmware for the 750 to accept windows as an operating system.
Or gut it and interface a windows computer with the screen. Both are a lot of work.

I am also in search of the best dollar per value tablet for AGOPEN.

But even if the tablet is $2000, AGOPEN will still be cheaper than just the unlocks to get RTK GNSS on the 750.

Plus my hope is once everything is running AOG, I will just move one screen per operator, VERY AWESOME.

That’s awesome, @PotatoFarmer! This was on my list of things to do but you’ve already done the hard work! I will be deploying this on my EZ Steer! Many thanks.

Also I want to do some sniffing of the TSIP that my 372 sends to the Nav II controller. Looks like you’ve already got a good way there. If handshaking is problematic, was thinking of just rewriting the data on serial and CAN from the 372 with RTK-fixed positions from the F9P. I’ve already used that technique with another GPS steering system (CAN bus).

Anyway looking forward too seeing some code. Or reading a complete description of what you did. I read your emlid forum thread.

Hello @torriem I have read your post as well, Mr. 1008 himself.

I spent much time reading on Combine Forum when I was first learning about RTK. Started a couple of years ago trying to get a Emlid M+ to work as a base station for the 750. In the process learned how to make Emlid Interface with EZ 150 in both TSIP and NMEA. My current converter for 150 works better than the original shipmodul does.

But you can also configure the NMEA of the F9P to work directly with the EZ150 as well. Anyone who kept their 150 is lucky.

But TSIP is not the same for all devices. I have recordings of 252 serial connection data. But to attempt to replicate it had me make the move to Ublox F9P to directly access the full variety ublox messages. But the 252 sends considerably different data than the EZ150 TSIP uses. But I am chipping away at the 252 one message at a time.

If I can get the 232 side working, Canbus data is next. Good to hear you have had some success at sending position data over CAN. Also want to see what Deere positions look like. So many things to try its hard to stay on task.

I have tried your 1008 injector, but winter has put a hold on a field test.

Though I have programmed extensively in Ladder Logic on PLC systems, I only started with Arduino’s and coding C at the end of October. But its much easier to learn it with an interesting project.

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I used an Arduino Due to do my CAN bus work initially. It’s perfect for the job. I use this little shield: The Due also supports super fast serial over USB (1 Mbit/s), which makes dumping the data to a computer easy. Now I’m using a Teensy 4.0 since it’s smaller, but the Due is definitely a great platform for doing analysis.

When analyzing CAN traffic I find it helpful to physically break the bus and then use the Due to bridge the traffic across the break, recording the direction as well as the raw data. Really helps to identify what messages are important based on where they are originating. I posted my bridging and logging code here.

John Deere uses industry-standard CAN PGNs for GPS position, at least on the old brown box systems. 65267 for GPS lat and lon, 65256 for heading and altitude. Other pieces of information including TCM data and the accuracy of the GPS fix are in proprietary, undocumented PGNs. I can make sense of a small fraction of the traffic. From what I can tell the receiver itself adjusts the lat and lon to compensate for the terrain, but the roll information is still sent to autotrac for whatever reason. The overall system is fairly complex and obviously undocumented publicly. It’s a bit odd but one message I found contains the ground speed as reported by the tractor in big endian. Normally CAN SPNs (the individual fields of data in a message) are little endian. That’s the CAN standard. Kind of hints that although John Deere develops slick technology, they are no Apple or Google! Plus I’m sure they didn’t think it mattered since it was all internal to their system.


I am a big fan of the DUE, will be looking into getting this CAN adapter for sure. On aliexpress ordered a due variant “DUE core” version which is half the original DUE board size. But recently blew all my fun money on parts for AG OPEN!

Been working with a Teensy 4.0 as well, its specs are insane for its size.

The Arduino Nano Every is actually really handy, If you run the MEGAcoreX board manager in Arduino IDE it gives you a second hardware UART. As well as unlocks much more functionality out of the rest of the pins.

Looks like our paths will be crossing a lot working on machine data integration. The Corporations make farmers out to be clueless about their equipment and how it works and using that as a reason to prevent us from making repairs due to greed. AOG project and forums like these have shown me that farmers are a lot smarter than they are portrayed.

I really wish I could become a full time Potato Farmer, but mostly help two other farmers with grain operations.

Once you use RTK in the field you cant go back to WAAS, Deere knows this. But a $15000 SF6000 + $1200/year subscription seems a tad like robbery, especially when a ublox F9P is $300.

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I gave my 250 away on a trade in deal for a 750, have hardly used the 750 at all and I am sure they just threw the 250 away, would like to have it back to send a f9p into it.

I have used a Microsoft surface tablet for AOG and it has worked well, probably overkill but I use it at home rest of year. Saw today that best buy had $300 off some versions of the surface

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I am sure a lot would like to continue with the EZ250 or 500. they are well made and take up very little cab space.
A sales person told me that the trade in units are shipped back to the manufacturer, so i can almost guarantee they are getting crushed.

Thank you for the tablet recommendation. I have also been eyeing up the PIPO X4, its IP67 with a rJ45 and DB9 serial port in that $500CDN area. My tractor is open station so the poor tablet will have to be able to take some rain.