Operate in API mode API frame format
Digi XBee3® 802.15.4 RF Module User Guide
132
To verify the checksum of an API frame:
1. Add all bytes including the checksum; do not include the delimiter and length.
2. If the checksum is correct, the last two digits on the far right of the sum equal 0xFF.
Example
Consider the following sample data packet: 7E 00 0A 01 01 50 01 00 48 65 6C 6C 6F B8
Byte(s) Description
7E Start delimiter
00 0A Length bytes
01 API identifier
01 API frame ID
50 01 Destination address low
00 Option byte
48 65 6C 6C 6F Data packet
B8 Checksum
To calculate the check sum you add all bytes of the packet, excluding the frame delimiter 7E and the
length (the second and third bytes):
7E 00 0A 01 01 50 01 00 48 65 6C 6C 6F B8
Add these hex bytes:
01 + 01 + 50 + 01 + 00 + 48 + 65 + 6C + 6C + 6F = 247
Now take the result of 0x247 and keep only the lowest 8 bits which, in this example, is 0x47 (the two
far right digits). Subtract 0x47 from 0xFF and you get 0xB8 (0xFF - 0x47 = 0xB8). 0xB8 is the checksum
for this data packet.
If an API data packet is composed with an incorrect checksum, the XBee3 802.15.4 RF Module will
consider the packet invalid and will ignore the data.
To verify the check sum of an API packet add all bytes including the checksum (do not include the
delimiter and length) and if correct, the last two far right digits of the sum will equal FF.
01 + 01 + 50 + 01 + 00 + 48 + 65 + 6C + 6C + 6F + B8 = 2FF
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