in reply to: charater gap time HART communication posted by Anirudh on 23 November 2009 at 09:08:48.
The spec says: Characters forming a message must be transmitted in a single contiguous stream. There should not be a single idle bit (i.e. additional stop bits) between consecutive characters. Furthermore, consecutive characters in a frame must have intervening inter-character gap of less than 1 character time.
I agree this is confusing! The third sentence is already implied by the second. Perhaps the reason for the third sentence is that detection for a Gap Error is specified as a gap of more than 1 character time.
Anyway, I'm sure the signal level should stay at 1 (1200Hz) following the stop bit, until the (immediate!) 0 start bit of the next character. If the system fails to supply the next character, the UART will surely keep the signal at 1 (idle). Using a buffered UART, this should never be a problem. The bus will not be inactive, until the whole message is completed.