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Re: [Paparazzi-devel] Futaba S.Bus for Paparazzi

From: Eric Parsonage
Subject: Re: [Paparazzi-devel] Futaba S.Bus for Paparazzi
Date: Thu, 25 Aug 2011 20:32:58 +0930


On 25/08/2011, at 7:16 PM, Ensslin Oliver (enso) wrote:

Hi Eric and Martin
I am thinking of giving this assignment to a student at our university. They maybe can program a module for Lisa and Tiny. Is it feasible by the Hardware? (I am not a electronic engineer)

This is very feasible. He will need a transmitter and receiver to test it but it should definitely not be too difficult for a student. They can use an existing subsystem as the basis for this project.


Here is the text from the German link translated with Google. It’s not perfect but I guess you will understand it:
The Futaba S-BUS protocol 

For anyone interested I present the technical description of the S-BUS Protocol. The description makes no claim to completeness or accuracy, if necessary, are to make their own measurements to verify the accuracy. The information is sufficient to e.g. RSAT JetiDuplex one - to develop S-BUS converter, or other proprietary developments to operate directly on the S-BUS. The PSS2018 will e.g. Emcotec of developed and allows to connect up to four satellite spectrum. 

In normal mode, the receiver every 14 ms, a frame is sent in the high-speed mode, all 7ms. 
A frame (25 bytes) lasts 3ms. 

Data Format 
The baud rate is 100.000Bit / s (not compatible with 115.200Bit / s) - look for in a development that is on the appropriate processor Quartz and prescaler 
The format is 8E2 (1 start bit, 8 data bits, 1 even parity bit and 2 stop bits) 

The bits are transmitted inverted (0V = High) 

The data 
The first byte is always 11110000 

The next 22 bytes contain 16 servo position without channel number 
A record consists of 11 bits per channel, LSB first (least significant bit first), range 0-2047, 1024, the center 
The data here jump on the byte boundaries, ie: 
Channel 1 requires the second Byte of the sequence to the first 3 bits of the third 
Channel 2 uses the remaining 5 bits of the 3rd Bytes, plus the first 6 bits of the fourth Byte. 

The last byte contains 4 flags (and 4 with unknown importance): 
Flag1: DG1 (channel 17, digital switching channel 1) 
Flag2: DG2 (channel 18, digital switching channel 2) 
Flag3: Lost frame 
Flag4: failsafe indicator 

In conclusion, there's still a 00000000 

There is no checksum, error correction codes or encryption. 

The analysis is extremely simple. All servos must be programmed. Here they are assigned a channel, i.e. is a data packet, it will be read and considered the position of the servo data from the saturated part of the rich data packet out. 

As you can see, the S-BUS is a very simple structured serial data transmission without checking whether the data was correctly transferred. Apparently here is placed on the repetition of the data, but can vary in nature and response components already connected the wrong lead or disturbed by external factors in the model frame to unwanted events. Here's engineers have been able to give Futaba little more effort and donate to the protocol error detection and error correction can. 

Basically imply that in any arbitrary information to a receiver Futaba S-BUS recipients expand by taking the appropriate PWM signals evaluates and implements described in the digital protocol. So it would be possible with little effort to turn a Jeti satellite receiver has an S-BUS compatible receiver. 

All data without guarantee. The author assumes no liability for damages caused by the use of the information.
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