checksum calculation

example:(hexadecimal format)

7e 09 68 00 00 00 00 00 77 88 
7e 09 69 00 00 00 00 00 78 88 
7e 09 02 00 00 00 00 00 11 88 
7e 09 10 00 00 00 00 00 1f 88
7e 09 01 00 00 00 00 00 10 88

7e 09  --  header

68 00  --  command type

00 00 00 00  --  data

77 (blue text)  --  checksum (sum of the 8 bytes before it)

88  --  end

These are some data packet send to an instrument  via a COM port.

I've tried some methods but can't get the right checksum in the example. Please give me a hand!

Thanks~