in reply to: HART physical layer posted by Christian Kolonko on 06 January 2003 at 10:41:05.
Yes, logical 1 for idle time means a 1200 Hz carrier, just as for "1" bits in the data and the "stop bit". (Personally, I don't regard the stop bit as idle time, although it is at the same logical "1" level. The one stop bit, of course, is a vital part of the character structure.)
It seems that the HART specs say two different things about the allowed idle time between characters. It says (i) that all bytes must be transmitted as a contiguous stream, without any gap (beyond the single "stop bit"), and (ii) that the inter-character gap must be less than 1 character time (which would be 11 bits!).
I think the reasoning is that you must design to not add extra idle time between characters, but that if a hardware or software fault results in a gap, the receiver is required to detect a "gap error" if the gap is more than 1 character time. (I think this sloppiness in detecting the error might be just to simplify the design of the data link timers.)
You must keep the f.s.k. carrier going without loss of amplitude, throughout the message frame, otherwise the receiver modem and carrier detect will surely get confused!
Hope this helps.