Character Time is based on Baud Rate. HART characters consist of 11 bits. For 1200 Baud networks, the character time is as follows:
1200 Baud = 1200 bit/Sec (11 bit/Character) / (1200 bit/Sec) = 9.167E-3 Sec/Character = 9.167 ms/Character
The timers are listed and defined in the Data Link Layer Specification. Some of the important timers are listed below:
Parameter Requirement Value BAUD Rate FSK 1200 Baud Preamble 5 each 0xFF bytes (strongly recommended) Start of Message (SOM) Detect Two consecutive 0xFF’s followed by a valid Delimiter, no communication error or gap error. Character Format Asynchronous, 11 Bit characters (1 Start bit, 8 data bits, odd parity, 1 Stop bit) Slave Time-Out (STO) 28 character-times 256.667ms HOLD 2 character-times 18.333ms Link Grant Time (RT2) 8 character-times 73.333ms Link Quiet Time (RT1)Primary 33 character-times 302.5ms Link Quiet Time (RT1) Secondary 41 character-times 375.833ms Gap Error Must be less then 1 character-time between any two characters. 9.1667ms