Operate in API mode API serial exchanges
XBee® SX 868 RF Module User Guide
164
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 XBee SX 868 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
API serial exchanges
You can use the Frame ID field to assign an identifier to each outgoing API frame. This Frame ID, if
non-zero, can correlate between the outgoing frames and the associated responses.
AT command frames
The following image shows the API frame exchange that takes place at the interface when sending an
AT command request to read or set an XBee parameter. To disable the response, set the frame ID to 0
in the request.
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