Is there a PPM signal in an Attack 2ER radio ?

[The Touch]

Hi,

Hi I have a SC8000 servo controller which outputs a PPM signal. I was hoping I'd be able to feed a Futaba Attack 2ER system this signal for it to transmit but I'm not having any luck.

I asked here http://www.rcgroups.com/forums/showthread.php?t=724477 which made me think I could do it. I've been probing around the circuit board with a soundcard based oscilloscope and I havent been able to find one.

Is this possible or am I out of luck ?

[Bruce Abbott]

The FP2108T IC emits a PPM signal on pin 17. This modulates the collector voltage of Q2.

[The Touch]

Ok I looked at that pin again. I've attached the ppm signal I receive from my servo controller. The second is off pin 17 and the third is from pin 17 but with the oscilloscope settings changed.

Is this right ?

[Bruce Abbott]

Oh sorry, I mis-remembered. Although there is a small effect on pin 17, I believe the modulation is actually applied to the internal crystal oscillator (pins 13~16).

I have now found the notes I made when I had a T2ER transmitter. There is a PPM signal present on pin 18. The voltage range is +3.2V to +5.6V, the pulses are 450uS long and inverted, and the rise and fall times are about 200uS. Unfortunately this signal is internal to the chip and connot be disconnected. However it is possible to override it with a stronger signal.

To keep the bandwidth within limits you should duplicate the waveform of the internal signal, otherwise the RF output may splatter across adjacent channels! Your signal could be shaped with a level shifter and low pass filter, followed by a buffer amp to provide a low impedance output. The circuit below should work.

[The Touch]

Here's a picture of pin 18's output for those of you playing at home.

I actually had pin 18 listed as being a potential candtidate in my own notes. Do you know what pin 8 does Bruce ? Also without trying to sound like a jerk how confident are you of your circuit design working ?

[Bruce Abbott]

I suspect that pins 8~10 are for additional voltage indicator LEDs.

I haven't built the circuit so I can't guarantee that it works. I have tried connecting various resistors from pin 18 to +12v or Ground to see whether the internal signal can be overridden - which it can. Unfortunately I no longer have a T2ER so I cannot do any more tests.

The FP2108T is a proprietry chip made (for?) Futaba to use in their tranmitters only, and AFAIK they have not released any documentation for it. However the T2ER and T2PG FCC test reports give a few clues:-

IC1 – Encoder, Voltage stabilizer, Waveform shaper, Oscillator

The type of oscillator utilized by the transmitter is a Crystal controlled modified Colpitts oscillator.

The following components are used for suppression of spurious radiation
Audio Low Pass Filter
- IC1 (FP2108T)
- C13 (15nF)

Method of stop modulation
Attach a resistor 4.7KOHM between no.18 and no.19 of FP2108T

[The Touch]

Yeah there doesn't seem to be any info around at all for this chip. I'll give it a crack but probably wont be able to get the parts until next week. If I can't get this to work then I'll just have to save and buy a modular based radio. Thanks again for you help Bruce.

Will pin 18's original PPM output be enough to support a smartpropoplus interface ?

[The Touch]

Can you confirm the 7k6 value of R4 in your circuit Bruce ?

[Bruce Abbott]

R4=7k6 is correct. However R6 should be 3k6, not 1k8 (the goal is to get 5V at the cathode of D2 and 3.8V at the anode of D1). I hope there aren't any more errors!

The Zener should be a low wattage type (100~300mW) for accurate voltage at ~2mA shunt current. High wattage Zeners will drop less than their rated voltage at this low current.

[The Touch]

I hope there aren't any more errors!

Here's hoping :)
Is the 7k6 an uncommon resistor or am I missing something as I can't find any on jaycar's or DSE's site.

Also are there any other requirements for the resistors etc or can I use the standard 1/4W variety ?

[Bruce Abbott]

Seems 7k6 is a 0.5% value and not that common. Jaycar have 7k5 which is close enough. Best to use 1/2 Watt metal film, not for power handling but because they are more stable than carbon.

[The Touch]

Will pin 18's original PPM output be enough to support a smartpropoplus interface ?

For those wondering I've just tested it this morning and it works, theres a little jitter in the centre posotion but besides that it's great.

[The Touch]

Just an update, the parts have been ordered just waiting for them to arrive.

The lowest value Zener Diode I can find is 400mW will this be alright ?

[The Touch]

Well my order finally arrived. I've designed a layout for veroboard/stripboard. Still could only find a 1N5235B Zener Diode, which seems to be a 6.8v 500mW so I hope that'll do.

Let me know if you find any errors. I was height limited which is why resistors are horizontal etc.

[The Touch]

Well I made some minor changes and here is the result.

Bruce is there a way to test this prior to connecting it to the transmitter ?

[Bruce Abbott]

Connect ground to the input and the output should be approximately +3.2V, then connect +5V to the input and the output should be +5.6V.

[The Touch]

Thanks for repying Bruce, I thought I'd lost you there :) . Unfortunately I did not get the expected results. I got a reading of 4.42v when connecting to ground and as I didnt have a 5v source except from the transmitter I did not proceed. I also got a reading of 7.68v when nothing was connected to the input, these are all based on an 11.92v source. This is the final board design I went with. The Gnd was fed to the line marked PPM in which would normally be coming from the servo controller. The voltage was measured from the point marked signal out.

[Bruce Abbott]

There is a mistake in your board layout. The top of the 2k4 resistor should connect to the right of the 3k6 resistor, not to the left. The easiest way to fix it is move the left-hand lead of the 3k6 resistor down one track, remove the link between tracks, and put in a link to join the top track's left and right halves back together.

You can use the top of the 6.8V zener diode as 'close enough' to 5V for testing.

[The Touch]

I can't believe I missed that.

The new default voltage is 5.66v and I got 3.02V when connected to gnd. Voltage at the top of the diode was 5.66v (no change) bottom was 3.05v.

[The Touch]

Were these the results we were chasing Bruce ?

[Bruce Abbott]

Close enough. Just try it!

[The Touch]

Will try it tonight and give you an update :)

[The Touch]

We'll I finally got to give it a shot tonight. I was a bit to busy over the weekend and I had to wire up a new harness.

When I first fired it up the receiver was beeping as it could find a signal. Disconnecting the power plug of the board which also has the connection to Pin 18 stopped that and the transmitter could then operate normally. The board didn't have power at that time as the switch in my harness was in the off position. Switching it to the on position with no incoming signal made no change, the receiver still couldn't find a signal.

Adding a signal from the servo controller with the harness switch in the off position also made no change as expected. The receiver could still not find a signal. Switching the board to the on postion cause the car to have full throttle. I had steering control of the car on channel 2 of the servo controller demo but I had no throttle control. Do you know why this would be ? When transmitting a signal from the board the transitter controls had no effect as expected. When turning off the board using the harness switch the car continued to operate for about half a second. Is this normal ?

[The Touch]

I thought about it last night. Changinging the switch position from between the incomping servo controller signal and the board to the wire between the board and pin 18 should fix the first problem.

[The Touch]

Well I swapped the switch over and it has sorted out the first problem. I also made a pass through cable for the trainer port which incldes a lead with a 3.5mm audio jack and internal voltage divider for use with ppjoy or similar.

Just need to work on the throttle problem at this stage.

[The Touch]

Well I replaced the ESC with a servo on ch 1 and that also works. It would seem that the throttle control on the servo controller is very sensitive. I did notice that there was a bit of crosstalk when using channel 2 though, when using the steering i would get some small movement on the other servo.

I am also getting some jitter when any metallic object is touching the ground of the trainer port regardless of the switch position (adaptor board or regular signal) do you know why that would be ?