FMS Serial Interface v1.01

Introduction
Hardware
Protocol description
History
Credits

Introduction

As far as you know, when FMS migrated to Windows, it had some problems with sencing transmiter signal. It's not evident, how to measure TX signal accurely under Windows operating system. There can be several solution for that problem.

One solution is to use GamePort controller, to attach transmitter to computer. But that's not the best way, because only 4 proportional chanels are allowed. Futhermore, Gameport interface cable has relatively complicated construction.

Another solution is to write a good driver for FMS. But that's very difficult, and require different drivers for each Windows type. We even can't guarantee, that once been written, drivers will work in the future, when Microsoft will reinvent it's Windows weel next time.

So, the third solution was selected, as the most acceptable. We've developped interface cable with open protocol, to maintain all time-critical tasks. This cable has the following advantages:

1. It converts PPM signal from transmitter into serial data, and send that data to computer via COM-port.
2. No kernel-mode drivers required to process data. COM-port programming is very easy under any kind of operating system.
3. Cable contains microcontroller, but it has minimum of details, and requires no tuning.
4. Cable materials are cheap. All details cost less than 5$.

Archive files:

/readme_en.htm	this file
/readme_ru.htm	russian description
/images/*.*	schematics and other pictures, used in readme
/firmware/	original firmware files
/firmware/12C508.hex	binary for PIC12C508
/firmware/16C84.hex	binary for PIC16C84(PIC16F84)
/firmware/sources/*.*	firmware sources
/firmware.new/	new firmware by Rasmus Geidnert
/firmware.new/16C84.hex	binary for PIC16F84(PIC16F84)
/firmware.new/sources/*.*	new firmware sources (with MPLAB project and stimulus files)

Hardware

FMS cable contains Microchip PIC microcontroller, that makes all work. There're 2 modifications of the cable, with PIC16F84 (PIC16C84) and with PIC12C508 microcontroller. I'd recomment to use PIC12C508, because it is more cheap, and it doesn't requires crystal. But if you are more familiar with PIC16F84 - no problems. You can chose any modification. Those work both well.

You can see here all available schematics, for both PIC16F84/PIC12C508 and all types of COM-ports (with 25-pin and 9-pin connectors).

Cable with PIC12C508, 9-pin connector (recommended).

 

Cable with PIC12C508, 25-pin connector.

 

Cable with PIC16F84 (PIC16C84), 9-pin connector.

 

Cable with PIC16F84 (PIC16C84), 25-pin connector.

 

Select any schematic, that is most suitable for you. The type of details is not critical. You can use any PNP and NPN transistors (Q1 & Q2) with beta >100. I've prefered to use surface-mounted conponents, and here you can see picture of cable (from first schematics) in assembled state and without case. Device is very small and it was fited into 9-pin connector hood.

  

After you selected cable type and got all details, you need to programm PIC. Select an appropriate firmware and burn it into your PIC chip. That's really easy. Even if you don't have a PIC programmer, you can build your own one. Schematic and software for home-built programmer are provided here: http://www.jdm.homepage.dk/newpic.htm.

(!) Note, that cable schematics above have Hitec connector pinout at another end. If you have another type of transmitter, you should connect it to C1 (signal) and ground in different way. Here are wiring for some types of transmitters that I know:

	- Hitec (all types) - Futaba 8 - Futaba 9Z
	- Futaba SkySport - Futaba AM - Futaba 6

Protocol description

FMS cable uses the serial interface, to transmit data from TX to computer. For proper work, COM-port must be configured in following way:

- 19200 baud
- 81N (8-bit data, 1 stop bit, no parity)
- RTS set to HI (+10v)
- DTR set to LO (-10v)

TX channel data been combined into packets and transmitted to computer. Data is combined into packets for each PPM cycle:

- Every packet starts with 0xFF
- For each channel, 1 byte been added to packet (after 0xFF).

Channel byte value vary from 0 to 0xFE. Table below describe dependency between pulse length and channel data.

pulse length	converted to
760 uS	0x00
1528 us	0x80
2284 us	0xFE

each 1 in data is equal to 6 uS (+/- 6 uS in signal => +/- 1 in data).

EXAMPLE:

pause 0xFF 0x80 0x80 0x80 0x80 pause 0xFF 0x80 0x80 0x80 0x80 ...

That means, we have 4-channel transmitter, and all sticks are now in the midle position.

(!) Note, when transmitter is off, cable sends to computer 0xFF bytes constantly. That helps to determine, that cable itself is ok.

History

25.11.2000, version 1.00
First version of interface. PIC16F84(PIC16C84) and PIC12C508 are supported. Firmware sources have comments in russian (I'm sorry, guys :) ).

28.02.2001, version 1.01
Added alternate firmware, written by Rasmus Geidnert.
New firmware transfers data after each PPM pulse, instead of PPM pause. That's for sure, no collisions will happen if you use 8-channel TX. I don't know, if an old firmware has ever had such problem, but anyway, Rasmus's idea is nice, and his firmware should be more reliable. Firmware sources are now commented in english.

Credits

This interface was designed specially for FMS team, that provide the best free flying models simulator. Information about FMS serial interface cable is freeware. You can freely use it for non-commercial purpuses and distribute it to anyone, provided that all files are intact. You must distribute FMS serial cable documentation only in original package. Commercial usage of this information without author's permission is prohibited.

WWW:
http://www.rcdesign.ru - FMS serial interface homepage. Original archive placed there.
http://simulator.home.pages.de - FMS homepage.

Email:
Vitaly Puzrin - author of FMS serial interface.
Rasmus Geidnert - author of the alternate firmware for FMS serial cable.

Copyright © Vitaly Puzrin, RC Design 2000