Unpack Dec From Hex - Via Bit Offsets
I have a block of hex data which inicludes settings of a sensor, I will include the beginning snippet of the hex (LSB first):
F501517C 8150D4DE 04010200 70010101 05F32A04 F4467000 00000AFF 0502D402
This comes straight from the documentation to decode this hex to dec:
3.1. Full identifier and settings record (0x7C)
Offset Length (bytes) Field description 0x00 6 Full identifier 0x06 40 Settings
3.1.1 Full identifier
Offset Field description 0x00 Product Type 0x01 Device Type 0x02 Software Major Version 0x03 Software Minor Version 0x04 Hardware Major Version 0x05 Hardware Minor Version
Offset Length(bit) Offset(bit) Default value Min Max Field Description 0x00 8 0 0 0 255 Country number 0x01 8 0 0 0 255 District number 0x02 16 0 0 0 9999 Sensor number ... 0x27
This being the only information I have to decode this. The offset column must be the trick to understanding this.
- What are the hex values offset from?
- I see 7C in the first hex string.
- The Settings section goes to 0x27 = 39 in decimal which is stated in the 3.1 section as the length being 40.
The given hex bytes are byte offset from the beginning of the data.
Assuming that your given dump is little endian 32-bit, let's have a look:
Value in dump - separated in bytes - bytes in memory F501517C - F5 01 51 7C - 7C 51 01 F5 8150D4DE - 81 50 D4 DE - DE D4 50 81 04010200 - 04 01 02 00 - 00 02 01 04
Now let's assign them to the fields. The next list has both records concatenated.
Byte Offset Field description 7C 0x00 Product Type 51 0x01 Device Type 01 0x02 Software Major Version F5 0x03 Software Minor Version DE 0x04 Hardware Major Version D4 0x05 Hardware Minor Version Byte Offset Length(bit) Offset(bit) Default value Min Max Field Description 50 0x00 8 0 0 0 255 Country number 81 0x01 8 0 0 0 255 District number 00,02 0x02 16 0 0 0 9999 Sensor number
Whether the result makes sense, is your decision:
- Product Type = 0x7C
- Device Type = 0x51 = 81 decimal (could also be ASCII 'Q')
- Software Major.Minor Version = 0x01.0xF5 = 1.245 decimal
- Hardware Major.Minor Version = 0xDE.0xD4 = 222.212
- Country number = 0x50 = 80 decimal (could also be ASCII 'P')
- District number = 0x81 = 129 decimal (perhaps 0x01 = 1 with bit 7 set?)
- Sensor number = 0x0002 = 2 decimal (big endian assumed)
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