Hello NevadaMark,
I have some egg on my face, the problem was with a two byte fifo on the micro causing a one byte delay in reception of the byte. What I thought was happening was simply not the case, it helps to sleep on the problem sometimes doesn't it?
The information contained in the ADIS16210 was very helpful and including the information in revision D of the ADIS16209 datasheet would certainly help others, I hope I am not the only one silly enough to have missed this in the first hour ;-).
Thank you for your help.
