Last element of a struct disappearing on write/read

Question

I'm building a weather station with two UNOs, using NRF24l01+ radios. Communications are fine. I'm sending a struct from one to the other. The struct has three elements:

 struct weather {
 float tempData;
 float humData;
 float pressData;
 };
 weather wData = {0, 0, 0};

I then populate the struct with values from my DHT22 Temperature Sensor and my BMP085 pressure sensor.

 float c = dht.readTemperature();
 wData.tempData = (c * 9/5) + 32;
 wData.humData = dht.readHumidity();
 sensors_event_t event;
 bmp.getEvent(&event);
 wData.pressData = (0.0295 * event.pressure);

Now I send it via the NRF24L01+.

 radio.write(&wData, sizeof(wData);

On the receiving Uno I have this.

 struct weather {
 float tempData;
 float humData;
 float pressData;
 };
 weather wData = {0, 0, 0};
 radio.read( &wData, sizeof(wData) );
 Serial.println(wData.tempData);
 Serial.println(wData.humData);
 Serial.println(wData.pressData);

I get results similar to this:

 Temp = 75.43
 Hum = 35.76
 Press = 0.00

The first two are correct. The final one is not. I can change the order and the first two will always be correct, but the last element is always 0.00. For example:

 Hum = 35.76
 Press = 30.14
 Temp = 0.00

I know I'm missing something here with my code but I can't find it. Anyone have some suggestions?

• jfpoilpret: read() and write() return a bool, did you try to get it and print it to Serial, just to check both functions consider everything's OK? That might help.
• jfpoilpret: Also, did you check that the weather on the emiiter contained a non-0 pressure before write()?
• jfpoilpret: The line with radio.write(... is missing a closing parenthesis.
• brorobw: read() and write() return 1. Missed the closing parenthesis while moving code here but it's there in my code.
• jfpoilpret: After taking a look at the RF24 library source code, I wonder if you have changed the payload size before read or write. To know about it, you should Serial.println() of radio.getPayloadSize() in both sketches; normally that should be 32 by default, but I wonder about it...

Answer 1

Modified my struct by changing the first two elements from float to int.

struct weather {
 int tempData;
 int humData;
 float pressData;
}
weather wData;

Everything transmits fine now. I don't really need the precision of a float for temperature and humidity. However, I would still like to find the problem. Could there be some problem with the size of three floats versus two ints and a float? Serial.print(sizeof(wData)) is 12 when all elements are floats and 8 when using two int and a float. My understanding is the NRF24L01+ has a transmit and receive buffer of 32 bytes.

• jfpoilpret: Have you tried the opposite, ie putting 4 floats instead of just 2? Maybe transmission is done by pack of 8 bytes?
• brorobw: Have not tried that. Will try soon and report back.
• Craig: Did you try testing with a known strings of different lengths instead of your struct.
• brorobw: Have note tried anything else as of yet. Work and life getting in the way!