Can't believe how stupid I have been, indeed it was an extract from a much larger program which I was having a problem with. In the extract I was skipping over loading the array so it was never getting to operate on correct data.

I will now have to go back to my original code and see where it is going wrong as I still have a problem with the real code, it just not with the CRC calc as I h ad originally thought.

Sorry to have bothered you.

JohnB