This is the module that actually talks to the weather station base unit. I don't have much understanding of USB, so copied a lot from Michael Pendec's C program wwsr. This module only reads data from the station (in 32 byte chunks). It does not attempt to write to the station, e.g. to set alarms or reset min/max values.
The weather station memory has two parts: a "fixed block" of 256 bytes and a circular buffer of 65280 bytes. As each weather reading takes 16 bytes the station can store 4080 readings, or 14 days of 5-minute interval readings. As data is read in 32-byte chunks, but each weather reading is 16 bytes, a small cache is used to reduce USB traffic. The caching behaviour can be over-ridden with the unbuffered parameter to get_data and get_raw_data.
Decoding the data is controlled by the static dictionaries reading_format, lo_fix_format and fixed_format. The keys are names of data items and the values can be an (offset, type and multiplier) tuple or another dictionary. So, for example, the reading_format dictionary entry 'rain' : (13, 'us', 0.3) means that the rain value is an unsigned short (two bytes), 13 bytes from the start of the block, and should be multiplied by 0.3 to get a useful value.
The use of nested dictionaries in the fixed_format dictionary allows useful subsets of data to be decoded. For example, to decode the entire block get_fixed_block is called with no parameters:
ws = WeatherStation.weather_station() print ws.get_fixed_block()
To get the stored minimum external temperature, get_fixed_block is called with a sequence of keys:
ws = WeatherStation.weather_station() print ws.get_fixed_block(['min', 'temp_out', 'val'])
Often there is no requirement to read and decode the entire fixed block, as its first 64 bytes contain the most useful data: the interval between stored readings, the buffer address where the current reading is stored, and the current date & time. The get_lo_fix_block provides easy access to these.
For more examples of using the WeatherStation module, see the TestWeatherStation program.
Python software for USB Wireless WeatherStations © Jim Easterbrook.