Exercise - Thinking like a computer

It's time to put your newfound CAN bus knowledge to use. In this exercise, you'll manually decode a few CAN frames in preparation for later automating this first stage of the telemetry processing pipeline with Python.

For this exercise, assume we are working on a CAN bus containing the following four devices:

1. An Elmar Solar MPPT with base address 0x600.
2. An Elmar Solar MPPT with base address 0x610.
3. An Elmar Solar MPPT with base address 0x620.
4. A Tritium WaveSculptor 22 Motor Controller with base address 0x400.

To decode the following CAN frames observed on this network, you will need to refer to the relevant datasheets for each device, which can be found in the corresponding sections of the Hardware Reference in this handbook (see the sidebar on the left).

A note on types

In the CAN frame specifications for each device, you will find information about the "type" of a variable inside a message. For example, the "Identification Information" frame for a motor controller is specified as containing two variables of type Uint32. Similarly, you will also find references to Uint8, Uint16, etc. These types signify that the bit pattern in the data field should be parsed as an unsigned (i.e. nonnegative) integer, with bit-widths of 32, 8, 16, etc. Unsigned integers are not too hard to parse by hand; it's analogous to the discussion in Section 3.1.

You will also see messages where variables are given the type float (explicitly in the MPPT datasheet; implicitly in the motor controller specs). A float is a representation of fractional numbers, following the IEEE 754 Standard. 1.5, -2.0, 10000000.0, and 0.30000000000000004 can all be represented with floating point numbers. Although most signals are naturally of type float (temperature, velocity, voltage, etc. are not going to be integers), parsing floats by hand is entirely different than for integers and is very tedious. You can use an online converter like this one to decode floats from their hex representation (you can omit the "0x" in the converter). The result will be in the "little-endian" cell.

Frames to decode

{
    "CAN_RX_error_counter": 0,
    "CAN_TX_error_counter": 0,
    "CAN_TX_overflow_counter": 0,
    "HW_overvoltage": 1,
    "HW_overcurrent": 0,
    "Twelve_V_undervoltage": 0,
    "Batter_full": 0,
    "Battery_low": 0,
    "Mosfet_overheat": 0,
    "Low_array_power": 0,
    "Global_MPPT": 0,
    "Local_MPPT": 0,
    "Duty_cycle_max": 0,
    "Duty_cycle_min": 0,
    "Mosfet_temperature_error": 0,
    "Output_voltage_max": 0,
    "Input_current_max": 0,
    "Input_current_min": 0,
    "Mode": 1,
    "Test_counter": 40
}

{
    "Supply1V9": 1.868830919265747,
    "Supply3V3": 3.3266239166259766
}

{
    "Controller_temperature": 22.182621002197266,
    "Mosfet_temperature": 15.401199340820312
}