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I started building a radio stack from spare parts. An old 19" 3U token-ring hub was emptied from all content and now gives a nice chassis for the "radios". All elements communicate via the I2C TWI interface of the used AVR microcontroller and are bridged with an [http://www.harbaum.org/till/i2c_tiny_usb/ i2c_tiny_usb] I2C/USB interface to the FlightGear computer.
 
I started building a radio stack from spare parts. An old 19" 3U token-ring hub was emptied from all content and now gives a nice chassis for the "radios". All elements communicate via the I2C TWI interface of the used AVR microcontroller and are bridged with an [http://www.harbaum.org/till/i2c_tiny_usb/ i2c_tiny_usb] I2C/USB interface to the FlightGear computer.
 
Schematics, firmware and userspace programs are at [http://gitorious.org/flightgear-pmpt/ Gitorious].
 
Schematics, firmware and userspace programs are at [http://gitorious.org/flightgear-pmpt/ Gitorious].
[[File:Pmpt-Radiostack-detail.jpg|thumb|250px|NAV/COMM radio and DME indicator|left]]
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[[File:Pmpt-Radiostack-detail.jpg|250px| [NAV/COMM radio and DME indicator|left]]
[[File:Pmpt-Radiostack-dme-front.jpg|thumb|250px|left|DME indicator, front view]]
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[[File:Pmpt-Radiostack-dme-front.jpg|250px| [DME indicator, front view]]
[[File:Pmpt-Radiostack-dme-back.jpg|thumb|250px|left|DME indicator, electronics "half-solid state"]]
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[[File:Pmpt-Radiostack-dme-back.jpg|250px| [DME indicator, electronics "half-solid state"]]
 
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The DME is made from an old real-life DME indicator with all electronics removed. The original PANAPLEX(R) gas discharge display has been removed and replaced by 5 7-segment red LED indicators to avoid a high voltage driver circuit. The two 12V light bulbs for front panel illumination were replaced by two ulra-bright white 5mm LED, so the entire circuit now runs on 5V drawing less than 200mA. The front panel is made of 4mm plywood. The design was printed with a laser printer on a sheet of paper and than glued to the wood.
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Using the I2C/SMBus interface, the properties for power, ground-speed and distance are sent from FlightGear to the indicator. The ET and FT timers are computed internally, so is the TTS (time to station). FlightGear reads the position of the source switch to select the active NAV receiver.
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More details, source code, schematics and the panel design is at [http://gitorious.org/flightgear-pmpt/dm http://gitorious.org/flightgear-pmpt/dme].
    
== The Basic Hardware Interface ==
 
== The Basic Hardware Interface ==

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