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    27Mhz - Radio Ideas

    C-3PO
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    27Mhz - Radio Ideas - Page 5 Empty Re: 27Mhz - Radio Ideas

    Post  C-3PO Mon Jan 16, 2023 10:06 pm

    Hi David,

    I will read your thread and code in more detail to come up to speed with your topic.

    It does look like you will have to negotiate a tightrope with these modules. I think you will need to create your own packet protocol.

    I would go back to grass root basics and construct a hard coded message, remove all code not associated with basic message delivery, and work on receiving that successfully and then work back up to the whistles and bells.

    Code:

    E.g.
    //preamble - send preamble 0x51 for minimum of 30ms
    starttime = millis();
    endtime = starttime;
    While ((endtime - starttime) <=40) // do this loop for min 30mS
    {
    Serial.write(0x51);
    endtime = millis();
    }


    Serial.write(0xFF); // FF start framing bit
    Serial.write(0x00); //

    byte message[] = {0x93, 0x95, 0x8E, 0x8E, 0x99 }; // msg=SUNNY
    Serial.write(message, 5));



    A couple of initial observations

    - Have you initialised the modules? Are they on the same channel/frequency?

    - Have you spotted that you can only TX a subset of 66 useable values. Values from 0-255 where the binary equivalent has a balanced number of 1's and 0's (4 of each)

    - I see you send the preamble of 55h 6 times - does the meet the required duration of 30ms from TXE transition

    - You cannot use message start  0x21 ascii ! (b100001) or messager end 0x23  ascii # (b100011) - see valid 60 binary patterns (4 zero's and 4 one's)


    I spotted several other things but cannot recall them now as I type this - will update you later


    I am tempted to buy a couple of units... I have a call out for them to ring me back...


    Update: - Have you seen the Radiometrix Narrow band Evaluation Kit (NBEK) IC. Looks like this is a chip that handles the preamble & framing - from what I can see you can use one both ends and then you end up with a traditional 1200 Baud Modem radio link that you can plug into. Not sure this is compatible with 4kbps LMT0 as it states > >5kbps data rate. I guess a call to the Radiometrix mothership would confirm ...

    http://www.radiometrix.com/content/nbek-ic

    I have scanned the net and not found any code Arduino code examples or libraries that have been written for this module - maybe Radiometrix have a code shell - looks likely when you read the PDF file "Your data over a simple radio link" as it has everything apart from the code :(

    http://www.radiometrix.com/files/additional/your-data-over-a-simple-radio-link.pdf


    Regards
    Jonathan

    Lucky 66 :) - binary codes with "balanced" 4 x Zero & 4 x One digits

    Extract from Your data over a simple radio link

    Then, to maintain mark:space balance, data can be sent by using only those characters with an equal number of ones and zeros in them.

    Of the 256 possible 8 bit codes, 70 contain 4 ones & 4 zeros.

    Omitting 0Fh, F0h, 3Ch and C3h (worst case ‘four ones in a row’ bit sequences) still leaves 66 usable codes per byte, which allows six bits of actual data to be coded into each transmitted byte.


    (A full explanation of this method can be found here: http://www.radiometrix.co.uk/products/bimsheet.htm#rs232) - invalid URL

    27Mhz - Radio Ideas - Page 5 Image

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    27Mhz - Radio Ideas - Page 5 Empty Re: 27Mhz - Radio Ideas

    Post  david f Tue Jan 17, 2023 12:19 pm

    Thanks for all the very useful suggestions C-3PO and geofrancis!
    Working through them in time order:

    I have again tried sending direct ppm data through and it now works! Thanks!
    I had the data voltage level at the TX too low.

    This shows the trace at the RX data output (range about 30 feet indoors- too cold to head into the village!) and it looks like a perfectly respectable ppm stream to me.

    27Mhz - Radio Ideas - Page 5 2023-012

    This tells me a lot:

    - The modules can send data.
    - It may provide a very simple setup for 27 MHz r/c .

    On to the other suggestions - it would be good to go digital.

    David

    * The ppm stream measured at the TX for reference.

    27Mhz - Radio Ideas - Page 5 2023-013

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    C-3PO
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    27Mhz - Radio Ideas - Page 5 Empty Re: 27Mhz - Radio Ideas

    Post  C-3PO Wed Jan 18, 2023 10:18 am

    Hi David,

    That looks very encouraging.

    If you plug the received ppm stream into a decoder I wonder what the individual channel values are compared to the original ppm stream?

    It will also be interesting to see if there is any jitter...

    Regards
    Jonathan
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    Post  david f Wed Jan 18, 2023 10:28 am

    Thanks Jonathan. No sooner said than done!

    https://youtu.be/M31LBSBRkPw

    Geofrancis suggestion works well on the bench and it is so simple. Just the RX module and an old friend the JHMCU convertor.

    Wiring chaos is down to temporary removal of Arduino. I will now move onto trying your suggestions.

    David

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    Post  david f Sat Jan 21, 2023 2:17 pm

    Still a bit puzzled why I am unable to transmit the digital data. (i am not very knowledgeable about sending serial data)

    Using  a twin trace (TX and RX) on the scope (RX is the red trace) starts to give clues.

    This is the direct ppm stream. A pretty exact copy.


    27Mhz - Radio Ideas - Page 5 2023-014

    This is a simplified digital stream the software is on:

    https://github.com/rdforrest/27MHz--Radiometrix-module

    27Mhz - Radio Ideas - Page 5 2023-015

    Not an exact copy by any means but has got much "cleaner" over the last few days. I have given the TX module a separate 5 volt supply rather than tap off the Arduino.
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    27Mhz - Radio Ideas - Page 5 Empty Re: 27Mhz - Radio Ideas

    Post  geofrancis Sat Jan 21, 2023 8:26 pm

    Just curious what happens when you disconnect the ppm signal to emulate loss of RC, does the pwm outputs from the ppm decoder stop or do they hold the last PWM value? If it stops then that will work as a motor failsafe and any ESC connected should shut off immediately. For servos they will essentially hold position if the signal just stops so in order to have a functioning failsafe you would just need to add a nitro engine failsafe device to whatever channels you need to move when it fail safes, this basically just sets the servo to a preset position when there is a loss of signal. so for a sub you need one for the ballast tank and possibly the dive planes.

    https://www.ebay.co.uk/itm/401482463111

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    Post  david f Mon Jan 23, 2023 2:07 pm

    Interesting question. I will connect the decoder again and report back tomorrow.

    Well I have actually got the modules to send digital data. It is all a bit wobbly and on only one channel but you have to start somewhere. Range about 1 metre!

    This shows the TX (top) and RX streams displayed with the, much clearer, Picoscope Version 7 software (just updated it).

    27Mhz - Radio Ideas - Page 5 2023-016

    (95 is the stick position data in hex.)


    https://www.youtube.com/shorts/0n0mM4qhMKs?

    RX 2.7 & TX 2.6 versions of the software on:

    https://github.com/rdforrest/27MHz--Radiometrix-module


    Last edited by david f on Thu Jan 26, 2023 4:00 pm; edited 1 time in total

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    Post  david f Tue Jan 24, 2023 2:05 pm

    Yes, i can confirm that with the direct ppm setup the servos simply freeze when the TX is turned off i.e no failsafe. (The failsafes you indicate look pretty nifty. My only worry may be that "pressing a little button"  on the unit may be difficult to do on a submarine! Hopefully it saves the value even on power-off.)

    I am still puzzled by how much better the direct ppm setup seems to work than the digital setup???

    I have still only got the digital version to work on one channel with a wobbly servo on the bench. (Though I am not very expert on WiFi software!)

    The direct ppm version is now working on the bench with 3 (very steady) servos.

    I think I may go on to range testing the direct ppm version.

    The ultimate goal is to try it out in a model sub. (Once the ice thaws!)

    And then try immersed antenna (much smaller) with help from Rob in Holland.

    David


    Last edited by david f on Thu Jan 26, 2023 9:22 am; edited 1 time in total

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    Post  C-3PO Tue Jan 24, 2023 2:25 pm

    Hi David,

    Interested to see your journey progressing.

    Radiometrix sell a - RF modem chip – for Narrow Band FM wireless data link

    M48A-000-DIL @£9.75 each ex  VAT (you would need a pair)

    http://www.radiometrix.com/content/m48a

    Datasheet
    http://www.radiometrix.com/files/additional/m48a.pdf

    This does all the protocol handling, message heading, hand shaking, preamble etc

    The M48A is a half-duplex serial modem controller suitable for use with narrow band receivers, transmitters and transceivers.
    It supports a maximum transparent data throughput of 4800baud while baseband modulation frequencies do not exceed 3.2KHz
    The device supports (programmable) long startup/preamble durations sometimes required for synthesized narrow band devices.


    I love the word transparent - if you take them at their word you simply squirt serial data in @TX and catch serial data out @RX :)

    Regards
    Jonathan

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    Post  geofrancis Tue Jan 24, 2023 4:05 pm

    david f wrote:Yes, i can confirm that with the direct ppm setup the servos simply freeze when the TX is turned off i.e no failsafe. (The failsafes you indicate look pretty nifty. My only worry may be that "pressing a little button"  on the unit may be difficult to do on a submarine! Hopefully it saves the value even on power-off.)


    the failsafe devices should only need set once and should hold that value, they are originally designed to cut off nitro engines on rc cars since regular 27mhz and 40mhz systems work exactly the same as your test setup, cutting off pwm when signal is lost.
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    Post  david f Fri Jan 27, 2023 4:30 pm

    I have just been range testing the 27Mhz direct ppm version and it performs well - past the church in our village, so 100m+.  I think it may just make it to the pond at Barrow for a submerged test.

    (Antennas used are flexible wire dipoles, 2m long so could be better!)

    David

    31st Jan - Later edit

    The 2 fail safes arrived today and they are working fine. I can confirm that they DO hold their value after power off.


    Last edited by david f on Tue Jan 31, 2023 12:18 pm; edited 1 time in total

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    Post  geofrancis Fri Jan 27, 2023 6:31 pm

    NICE! your probably the owner of the first new 27mhz RC system with more than 4 channels since the 80s.

    once you get a digital version working the biggest advantage it would have is interference rejection, because it would be bound and each packet would have an identifier so if its not addressed to the receiver it would just ignore it. even if someone switched on another old 27mhz transmitter on the same channel near you it should just reduce your range but you should still keep some control. that's the main weakness of the analogue system is it cannot reject interference, so it shows up as erratic servo movement.

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    Post  david f Mon Jan 30, 2023 11:22 am

    NICE! your probably the owner of the first new 27mhz RC system with more than 4 channels since the 80s.

    Yes! I hope this is not an exercise in creative nostalgia!

    I tried out some underwater trials yesterday and it seems to work well and better than I expected.

    This is the prototype TX and RX. Really very simple and practical (Not too much soldering). Note the dip switches to change frequency. The antenna for both was a a stiff wire dipole, 2 m total length. To fit in the tube for the RX the wire was loosely  wound around the dowel which formed a chassis. The antenna was later cut shorter - see later comments on range.

    27Mhz - Radio Ideas - Page 5 20230115

    The basic setup, same as the LoRa trials really. A float supports the tube which is suspended about 0.5m below. The float has a cluster of bright LEDs which are switched by the rudder stick on the TX.

    27Mhz - Radio Ideas - Page 5 20230114

    27Mhz - Radio Ideas - Page 5 20230110

    The float moored by the side of the lake. (Note healthy milk bottle rather than bad tonic water bottles in LoRa trials! Lockdown!)

    27Mhz - Radio Ideas - Page 5 20230111

    The map of the lake showing the range which was at least 50m transmitting down to 0.5m. (Points 2 to 6)
    The cut down antenna gave about half this range.

    27Mhz - Radio Ideas - Page 5 Barrow12

    The surprise was how well the crude antenna functioned.
    The next step will be to try it out in a working submarine. (Fail safes on order - thanks!) Interference from drive motors etc. may be a new problem.
    I will continue work on the digital version also - this has to be the way to go, as you say.

    One useful feature of the RX is that it has an audio output. The first thing I did when I arrived was plug an amplifier and earpiece in  and click through the 6 frequencies to check for interference. Nothing. No " Breakers" in Barrow!

    What the range looks like. You wouldn't want to control a sub at more than this. My thanks to the lonely figure on the shore - Adrian who worked as an "LED repeater". (I couldn't see them at that range.) (Adrian tells me that he has a very old 27Mhz setup in his garage - 4 channel!)
    27Mhz - Radio Ideas - Page 5 20230116

    Jonathan - Thank you very much for the notes on the M48A. I will try to get software working first - to save space and possibly cost.


    Last edited by david f on Wed Feb 01, 2023 7:58 am; edited 2 times in total

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    Post  tsenecal Mon Jan 30, 2023 11:02 pm

    excellent work on all these different tests of the differing technologies.

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    Post  david f Tue Jan 31, 2023 8:39 am

    Very kind words, Tim. Thank you for "setting the ball running."

    David

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