Check Sum Question - for you math people

JohnMichnr

Hey All - I have a Hitachi LCOS video wall cube that says it's check sum is a "twos complement" check sum. Looking Two's compliment up on Wikipedia I find that it is a inversion of the bit in a byte plus one. So I am thinking that I can calculate the checksum by doing this:

sum (addition of the Bytes in question)

CheckSum = !Sum + 1

Will that work ? or do I have to go into the bits in the byte and invert each one? before I add one.

Anybody got any suggestions?

DHawthorne

JohnMichnr wrote:

Hey All - I have a Hitachi LCOS video wall cube that says it's check sum is a "twos complement" check sum. Looking Two's compliment up on Wikipedia I find that it is a inversion of the bit in a byte plus one. So I am thinking that I can calculate the checksum by doing this:

sum (addition of the Bytes in question)

CheckSum = !Sum + 1

Will that work ? or do I have to go into the bits in the byte and invert each one? before I add one.

Anybody got any suggestions?

!Sum will make Sum = 1 if it's currently 0, otherwise it will make it 0. So, no matter what's it's value, you are going to wind up with 0 or 1. You need to do a bitwise operation.

JohnMichnr

Yeah now that I go back and re-read the exact defintion of Not, I gues all it does is produce a true/false result.
So it is back to the bitwise not on each bit.

mpullin

CheckSum = ~Sum + 1 is what you're looking for, I believe.

JohnMichnr

Yeah - that is what I am going to try - the other thing that works in teh Windows calculator is taking sum and subtracting it from 0. Cause the twos compliment is just the inverse of the number (negative. I think I'll try both and see what happens.

yuri

isnt it BNOT? binary not?

edit:
damnit... BNOT == ~ :P

JohnMichnr

I was asked to post the description of the command protocol for this issue. I am going to try to do this.

I don't know how to do the little quote boxes

FORMAT of COMMANDS TO THE PROJECTOR
(A) When individually specifying a projector
STX + BC + CMD + PARA + CS + SN
STX - Start command (fixed to 02H) 1 Byte
BC - Byte count from CMD to the last byte of PARA 1 Byte
CMD - Command code 1 Byte
PARA* Parameter Variable
CS - Value when the sum of the lower 8 bits from BC to CS is 0. Two?s complement. 1 Byte
SN - Projector No.. FFH corresponds to all projectors. 1 Byte

Obviously the area I am looking at is the CS. And I think the BNOT (~) is the way to go about it. I'll be trying that this morning with some strings.

Chip Moody

Make "count" or somesuch equal to the sum of all the bytes from BC to the last PARA byte. BAND it with $FF to make sure it's 255 or less. Then...

CS = 256 - count.

- Chip