checksum calculation

[wanglm]

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~

[srikrishnaNF]

Hi Wanglm,

 Please find the attached VI with proper comments inside.

Regards,

SrikrishnaNF

[RavensFan · Accepted solution]

Srikrishna,

I don't think your message is what the original poster is looking for.  All you are doing is extracting the checksum from the string of messages, which is actually pretty trivial and can be done more easily than what you have posted.

I'd be willing to bet they are actually asking how to CALCULATE the checksum based on the previous bytes.

That can't be answered with the limited information since there are many different checksum calculation schemes out there.

WangIM, you need to read the communication manual for the instrument.  It should tell you how to calculate the checksum.

Though strangely enough, with the limited sample you provided, if you add up all the bytes except for the checksum, (i.e. the first 8 bytes, plus the final byte of x88) you get the same value as the checksum byte.  This is done if all the bytes are stored in a U8 array and the array elements are summed.

[wanglm]

Hi SrikrishnaNF:

Thanks!

I'm afraid I haven’t make it clear.

I want a command, but don't know how to calculate the checksum.

for example: 7e 09 01 00 ff 00 ff 00 ?? 88, I need to get the ?? first.  That's my problem.

[wanglm]

RavensFanThanks a lot!

I get a wrong message from the manufacturer, checksum should contain the final byte ox88. 

Now the calculation is clear.