Futaba FP-T7UAF transmitter conversion

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solar
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Joined: 12 Feb 2021, 17:25

Re: Futaba FP-T7UAF transmitter conversion

Post by solar »

MJF wrote: 13 Feb 2021, 18:07
I do not know where you are located in the world but I would bet if in Europe your rules would be different than ours. I know for instance that in England they used 35MHz (and some others on HAM bands) . Hopefully someone from your part of the world can clarify what you can use and what not to use.


Mike F
MAAC Safety Document (MSD) 17 - Radio Spectrum.pdf
I should have said that I am in UK. I looked at whether there might be a market for my old 72 MHz Rx and module and it seems the Kazakhstan is the most active seller of such on eBay!
solar
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Joined: 12 Feb 2021, 17:25

Re: Futaba FP-T7UAF transmitter conversion

Post by solar »

Wayne_H wrote: 20 Jan 2021, 23:39 If you can't pick up a genuine Futaba compatible Frsky module, then depending on how much you really want Frsky functionality in a Futaba-compatible module, another option could be to use a Corona Futaba-compatible module such as the Corona CT8F - e.g. https://www.aliexpress.com/item/1005001 ... 6062%23774
Is there somewhere that explains all these PCM, PPM, manufacturer's protocols, telemetry, etc issues ? I stopped buying proportional RC sets many years ago (the FP-T7UAF was given to me and I never did really understand it) when life was simpler and it was only about FM versus AM and 27 versus 35 MHz. But am interested in getting some of this old equipment going again. I have some 35Mhz FM Futaba kit too with old style plugs. Main problem with that is corrosion in the sockets and on the Tx battery contacts.
solar
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Joined: 12 Feb 2021, 17:25

Re: Futaba FP-T7UAF transmitter conversion

Post by solar »

Wayne_H wrote: 13 Feb 2021, 20:29
My reference to reprogramming using Martin's procedure was to offer an alternative if you HAD to have the Frsky protocol but couldn't source a Futaba compatible Frsky tx module.

If you’re not tied to using Frsky, then much simpler & easier to just "plug n play" using Corona's Futaba compatible Tx module & Rx's.
Certainly is a fascinating project and I, as a Arduino (and PIC) capable programmer, could take it on. But, as I’d have to buy a 2.4GHz Rx whichever route I use, it seems simpler to just go down the ready built and programmed route as long as the Corona kit is at least average to good.
tedbmoss
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Joined: 15 Oct 2020, 07:00

Re: Futaba FP-T7UAF transmitter conversion

Post by tedbmoss »

There are a slew of possibilities for replacing the module. (I have the FP-T5UAF.) Such as https://www.multi-module.org/. Some might need to be put inside the TX, but mostly need only three wires to connect without telemetry. PPM, Positive battery and Earth. I have both (actually there are three) sizes of modules. The newer Futaba (as well as the older) is a different shape than most modules (Frsky) but the inside is compatible. Frsky made my Futaba modules. The antenna is separate on the 2.4 GHz modules obviously. You would have to possibly program the module and connect the switch for the protocol you wanted and set up the bind button on the TX. There might be 3D printed module cases to fit the TX. Or you could make your own. There is also a smaller module for the Jumper TX. I hope this helps a little.
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Wayne_H
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Re: Futaba FP-T7UAF transmitter conversion

Post by Wayne_H »

solar wrote: 14 Feb 2021, 12:31 Is there somewhere that explains all these PCM, PPM, manufacturer's protocols, telemetry, etc issues ? I stopped buying proportional RC sets many years ago (the FP-T7UAF was given to me and I never did really understand it) when life was simpler and it was only about FM versus AM and 27 versus 35 MHz. ....
There are others on this forum far more technically competent than me - I'm sure they'll chime in if I inadvertently give you any duff gen :?

For a simple start to understanding the different types of information transfer using pulse modulation, have a look at this YouTube clip

Being simplistic (i.e. ignoring AM & FM differences):
Our 'old' 27/29/35/36/40/72/etc Mhz radios all used a crystal (or electronically synthesized) locked single frequency to operate on. If you had multiple Rx's on the same spot frequency as a single Tx, they would all receive and attempt to decode the signal from that same Tx. That's why if another transmitter switched on nearby on the same spot frequency, you got interference.

The 2.4Ghz band is used by everything & everyone from garage door openers to wifi routers to paging systems to communications to our modelling applications - its crowded! To overcome this, our RC devices - transmitters & receivers - use transmissions on frequencies spread across the 2.4Ghz band, known as spread frequency or spread spectrum transmissions. The Tx uses an algorithm to sample other frequencies for transmissions in the 2.4Ghz band then switches/jumps to a clear part of the band. It does this continuously, many times a second. The Rx uses an embedded signal from the Tx to jump to the next frequency told to it by the Tx. That is why the Rx has to be "bound" to the Tx - they exchange & store each other's unique ID's - so that they can stay "in sync" with each other for every frequency step or jump, while ignoring all other transmissions/noise. The algorithm is proprietary to each manufacturer - Flysky, DSM, Frsky, Futaba, etc - and (generally) unique to each manufacturer. However, some algorithms have been made public and others have been reverse engineered or cracked, so that after-market manufacturers can sell compatible gear. A variation of this is Phil's software encoded unique ID, as used in his clever home-brewed Frequency Hopping experiments - viewtopic.php?p=7664#p7664
solar wrote: 14 Feb 2021, 12:31 .....But am interested in getting some of this old equipment going again. I have some 35Mhz FM Futaba kit too with old style plugs. Main problem with that is corrosion in the sockets and on the Tx battery contacts.
Some things to be very wary of:
  • if you have corrosion in the sockets and battery contacts, get rid of them - don't waste your time cleaning them 'cause the corrosion will be in the wiring as well. Better to replace all the wiring with the now virtually universal 3 pin 2.54mm connectors. You can use 3-row pin headers in the Rxs and the readily available "servo" plugs on the servos.
    e.g. leads - https://www.banggood.com/10X-22AWG-60-C ... mds=search These are long enough to cut in half to retrofit 2 servos.
    pin headers - https://www.banggood.com/2Pcs-2_54mm-3x ... mds=search These are right angle headers, but you can get straight pins as well
Hope I haven't confused you :shock:
Cheers,

Wayne
Once a Retrobate, always a Retrobate............ ;)
solar
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Joined: 12 Feb 2021, 17:25

Re: Futaba FP-T7UAF transmitter conversion

Post by solar »

20210218-0003_10.jpg
Thanks. With an oscilloscope as per diagram I can see that what goes into the RF module is basically a series of 8 to 10 1.5ms( +/- 0.5) pulses then start/ stop pulses to define the frame. i.e. just the 8 servos’ standard frame. Then it’s RF modulated using probably FM modulation. If true then PCM is just a fancy name for what proportional sets have been doing since the 1960s. But possibly Futaba insert a few different start and stop pulses that make it different from other manufacturer’s similar protocol. The protocol that encodes the 2.4GHz signal is a separate issue as have to cope there with spread spectrum, telemetry etc.

I assume the hash on the signal in this diagram is just RF pickup from the 72Mhz getting back into my scope probe and not a digital code added on top of the 1.5ms frames.
Martin
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Re: Futaba FP-T7UAF transmitter conversion

Post by Martin »

PCM is different though. It uses high/low pulses in a frame, just like normal PPM, but the information is different.

With PPM, it's the time difference between the leading edges of successive pulses that gives the position of each channel. Some brands use the trailing edge of the pulses instead - but that doesn't usually make any difference because the narrow pulses are usually all the same fixed width.

With PCM a pulse is either wide or narrow, and this contains "one bit" of digital information. The exact timing doesn't matter: Simplifying, we could say that if a pulse is less than (say) thirty microseconds, it's a "0", and if it's more than thirty microseconds, it's a "1".

The Receiver assembles those "0" and "1" bits to give a number, and the number specifies the position of a channel. The number of bits in the number sets the resolution - and that's why you see names like "Futaba PCM 1024" which uses ten-bit numbers that resolve 1024 discreet positions.

The PCM encoding uses other tricks - Futaba used to send the "main" channels 1-4 more often than the "auxilliary" channels 5 and up - and it would occasionally also send the fail-safe positions for each channel, which the receiver would remember in case the signal was lost.

Another trick was to add a checksum. Imagine I'm sending you 8 numbers all in the range 0 to 1023. To check for noise, I could send you a ninth number that you're not actually going to use other than as a check. I would work out that ninth number so that when you add all nine numbers together, the answer is some exact multiple of 1024 with no remainder. You can then add all nine numbers together and if the sum isn't an exact multiple of 1024, then you know that the packet has been affected by noise, and you can carry on using the previous good values until you either get a good, error-free, packet, or give up and go to fail-safe.
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