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2 way communication from ground to ardupilot using XBee 900

Posted by Antonio on August 6, 2010 at 3:58pm
Hi,

Cant seem to get my head around this one.

I have an ArduPilot board and this is connected to the ArduIMU+ V2 (flat).  Tx from the IMU is connected to Rx on the Ardupilot and everything is working as it should.

However i would like to investigate using the groundstation (software on windows) to perform 2-way communicate with the Autopilot but if Rx on the ArduPilot is already used (to talk to IMU) how do i connect the Xbee to the Ardupilot on the aircraft side.

Any help / diagrams would be greatly appreciated.

Regards,
Tony Brock

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Replies

  • Greg Fletcher August 6, 2010 at 8:31pm
    I have developed a method for 2 way coms with the 328 ardupilot with thermopiles. I can change the PID gains while in auto mode. It works by time sharing between the GPS and the Xbee. It requires a simple multiplexer to electrically isolate the two TX lines that both go to the Rx on the Ardupilot. When a serial line is idle, it holds the line high. When the other Tx starts sending by pulling the line low the idle Tx would short out the sending one. Short circuit protection must be built in to the chips because mine is not fried ;) It also requires a digital output to control the mux and trigger the RTS line on the Xbee. The Xbee holds incoming data in a 256 byte buffer until the Ardupilot asks for it. This is done right after The GPS is done sending. Uplink data must be less than 256 bytes.

    With an IMU unit you can't send data directly to the main board because it's busy receiving IMU data at 50 Hz. Receiving GPS data at 4 Hz (Ublox) leaves a lot of time in between to recieve the Xbee data(if the is any). So you need to send the Xbee data to the IMU shared with the GPS and then send it from the IMU to the Ardupilot. I haven't had time to modify the IMU code yet to make this happen, but I plan to. This is a learning exercise for me plus it's good to have if you using the 328 Ardupilot. I will post a link to this if/when I do it. Getting a mega system would be a lot easier with the 4 ports.
  • 3D Robotics
    Chris Anderson August 6, 2010 at 4:58pm
    If you want 2-way communications, you should use ArduPilot Mega, which is designed for that (it has four serial ports). Regular ArduPilot only has one serial port, and the RX side is used by the GPS, so it is designed only to send telemetry downstream

    There is a hack that will allow you to kinda get around this, but it's not officially supported and we can't help you with it if it doesn't work.
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