Casio EA-200 Technical Reference page 19

Checksum (1 byte)
Checksum is added to the end of every returned header data proper.
"Checksum" is the code (hexadecimal number) to check if data transfer is
successful. The method to calculate the code is as follows:
1 Except the star code, add the data code to send (hexadecimal number)
one by one byte.
2 As a result, one byte of the code that excludes the overflowed digit is
set to "SUM".
3 Calculate the complementary number for 2 of SUM'. The answer is
used as the checksum code.
The specific example is shown below.
Data (Example) : 1 2 3 X
Middle code 3A 31 32 33 39 34 35 36
31 Calculate the complementary number for 2 of SUM'.
+32
+33
+39 The complementary number for 2 of &HBD
+34 (=10111101) is &H43 (=01000011).
+35 Therefore, put &H43 in the checksum.
+36
+FF
2BD
=SUM'
4 5 6 EOF SUM
← Hexadecimal
FF 43
number
↓
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