LoPy connects to TheThingsNetwork but payload is empty

[philwilkinson]

Sorry, beginner question…
I am connecting a LoPy to TheThingsNetwork via OTAA. I am sending a integer of the temperature in the payload (which is converted to bytes using the Pycom lora socket object’s send method). The sensor and node all appear to work correctly, however I don't actually know what is being sent by the LoRa radio module.

The node’s application on the TTN console shows the device activating, then the uplink occurs a few seconds later.

The Payload in the uplink however shows nothing (actually is reads: 00000000000000000000000000).

Have I forgotten something really obvious? I have already posted this question on TheThingsNetwork forum but I am not sure if this problem may be a LoPy issue.

[robert-hh]

@philwilkinson Can you post the code that creates the payload. Pleas enclose code in lines of three backticks (```)

[philwilkinson]

I didn't include the whole script as it would get too long...!

s = socket.socket(socket.AF_LORA, socket.SOCK_RAW)

# set the LoRaWAN data rate
s.setsockopt(socket.SOL_LORA, socket.SO_DR, 5)

# make the socket blocking
# (waits for the data to be sent and for the 2 receive windows to expire)
s.setblocking(True)

from pysense import Pysense
from SI7006A20 import SI7006A20
py = Pysense()
si = SI7006A20(py)
data = int(si.temperature())  #converted float to integer

s.send(bytes(data))
print('sent data to TTN')

# make the socket non-blocking
# (because if there's no data received it will block forever...)
s.setblocking(False)

[philwilkinson]

results in

[robert-hh]

@philwilkinson Yes. That's why you get e straing ouf zeroes:
when you convert the float number to an int, you may receive a number like 8. bytes(8) however creates a bytearray of 8 bytes length, each with the value 0.
To convert a number into a bytearray containing this number you can either usr str(number) or similar to convert it to an ascii string, or struct.pack(), like:
struct.pack("<i", number)
See https://docs.python.org/3/library/struct.html

[philwilkinson]

Brilliant! thanks @robert-hh for the guidance. I need to study my data types a lot more!
As I thought I should keep a float temperature reading for reasonable accuracy; my code now reads:

s = socket.socket(socket.AF_LORA, socket.SOCK_RAW)
s.setsockopt(socket.SOL_LORA, socket.SO_DR, 5)
s.setblocking(True)

raw_data = round((si.temperature()), 2)
data = struct.pack('f', raw_data)
s.send(data)
print('sent ', raw_data, ' to TTN')

I now have to work out how to get the payload back into a readable format using something like

struct.unpack('f', (bytes.fromhex('HEX value in payload')))

[robert-hh]

@philwilkinson Since you did no use tohex while sending, fromhex is not needed, unles the tohex is done somewhere else.

[robert-hh]

@philwilkinson For readable data you could also use:

data = "%.2f" % si.temperature()

and use float(recv_data) for back-conversion.