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Understanding the MUXs

Posted by James on March 25, 2014 at 7:51am

Hello,

I am trying to build a UML model of the Ardupilot system using the Topcase framework for hazard analysis. I am building both a software and hardware model of the APM 2.5 board. I have been looking at the board schematics and source code and I have idea of most of the component behaviour. 

There are couple of  things  which I  would like to check with the community, first is the "Radio Passthrough Mode" for the AruPlane firmware. I want to confirm that this requires a separate 8 channel mux which can switch the radio PWM channels directly on to the control surface servos lines.

I would also like to try understand  the role of the mux between the PPM encoder and the main controller a bit better. If I have interpreted the schematics correctly the PPM encoder seems to be the component switching the MUX. It seems to switch a UART connection on the main controller between a UART on the PPM encoder and another UART on the  main controller. I am assuming the PPM encoder UART  is the USB signal used to update the main controller configuration settings, firmware and return readouts. What is not clear is the purpose  of  switching the main controller UART signal and feeding back to the main controller... I am little lost with that one.

Any help would be appreciated!

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Replies

  • Developer
    Bill Bonney March 25, 2014 at 8:01am
    The PPM encoder does 'Pass through mode' when its fed with a PPM signal from capable RC receiver. Its not related to the PWM failsafe pass through mode used on APM1. I.e an output mechanism. Vs an input one.

    The PPM encoder is also used as the USB interface, and it switches serial input from it to the 2560 chip and the telemetry radio. uart0 and uart2
    • James > Bill Bonney March 25, 2014 at 8:26am

      Thanks for your reply.

      The PPM encoder does 'Pass through mode' when its fed with a PPM signal from capable RC receiver. Its not related to the PWM failsafe pass through mode used on APM1. I.e an output mechanism. Vs an input one.

      So is this document out of date? http://ardupilot.com/wp-content/uploads/downloads/2013/04/PPM-Encod...

      In the Radio Passthrough Mode section it say this:


      This feature is only available, when the PPM Encoder for ArduPlane firmware is used:
      →  e. g. ArduPPM_v2.3.16_ATMega328p_for_ArduPlane.hex
      This mode is described as hardware failsafe in ArduPlane terminology.
      Radio Passthrough mode is trigged when servo channel 8 > 1800 μs.
      When the passthrough mode is active, the Mux output on JP5 is enabled, so that an external bypass circuit can
      be switched on.
      The blue LED has different behaviors in passthrough mode:
      • If throttle position < 1200 μs, status LED is off
      • If throttle position > 1200 μs, status LED is on

      The PPM encoder is also used as the USB interface, and it switches serial input from it to the 2560 chip and the telemetry radio. uart0 and uart2

      So just to confirm the PPM encoder is switching signals between the main controller UART0 and telemetry radio for the USB interface? Does that mean the UART2 on the main controller is just used for reading the same telemetry data that would be sent to the USB?

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