in reply to: Tolerence on received bit time posted by Anirudh on 12 January 2010 at 12:17:06.
The bit time in Hart is exactly 1/1200 second. However, because the bits are coded with 1200 Hz and 2200 Hz it is not easy to detect exactly the start of a bit. Because the two frequencies are not harmonic to each other the receiver always has a bit time error of 60° minimum which means an uncertanty of about 30 % related to the start of a bit.
This explains your observation with 700 us and the 1000 us while the bit time itself is 833 us.
The Hart spec is defining the bit time and the two frequencies and there are precisely defined tolerances.
The shift you are observing is introduced by the design of the electronics.