Reverse Engineering Temperatures (RS485 Bus)

Question

I have the following hex byte data. Can someone help me reverse engineer the temperatures calculation?

ID
||
81 00 8c 00 
82 00 aa 00 
83 00 04 01 
84 00 ff 00 
85 00 21 04 

The first byte is the id! 81-84=1-4

One of the data is 14,0 C° and another one is 17,0 C°.

My tries:
- (Second Byte * 10) + (Third Byte *10/256)
- (Second Byte - 0x80) / 2
- (Second Byte / 2) - 0x80
- Second Byte * 100
- (Second Byte / 2) - 20

Answer 1

I would guess that this is raw (unencoded) data and every row is consisted of two 16 bit integers stored in little-endian order. This means that

temperature = (float)(b[3] << 8 + b[2]) / 10;

where b[2] is the third byte and b[3] is the fourth.

So using the above formula, the data would yield the following temperatures:

81 00 8c 00; #Measurement 0x0081 - 14.0
82 00 aa 00; #Measurement 0x0082 - 17.0
83 00 04 01; #Measurement 0x0083 - 26.0
84 00 ff 00; #Measurement 0x0084 - 25.5
85 00 21 04; #Measurement 0x0085 - 105.7

Update:

Here is a Python snippet which does the conversion given a string with space-separated hex values:

input = "81 00 8c 00"
byte_arr = input.split(" ")

for i in range(len(byte_arr)):
    byte_arr[i] = int(byte_arr[i], 16)

measurement_id = byte_arr[1] << 8 | byte_arr[0]
temperature = (byte_arr[3] << 8 | byte_arr[2]) / 10.0

print("Measurement: %d" % measurement_id)
print("Temperature: %.2f C°" % temperature)