Spectrum Controls 1756sc-OF8H Series A - Manual (Page 109)

Chapter 7: HART Channel Configuration, Data, and Status 95
from the manufacturer code, the device type code, and the device
identification number. The long frame format provides extra security
against acceptance of commands meant for other devices, due to external
interference or excessive crosstalk. The IF8H and the OF8H support only
the long frame format.
Preamble
The preamble consists of three or more hexadecimal FF characters (all
1s) allowing the receiving modem to get its frequency-detection circuits
synchronized to the signal after any pause in transmission.
Note: The preamble does not need to be included in the HART message
when using the module specific Pass-through command. The Pass-
through command already includes the preamble.
Start Character
The start character in a HART message has various values, indicating
which frame format is being used, the source of the message, and whether
a field device is in burst mode. The possible definitions are shown in the
table below.
Table 7.17 (Start Character Definition)
Short Frame Long Frame
Master to slave 02 82
Slave to master 06 86
Burst mode from slave 01 81
Address
The address field contains both the host and field device addresses for the
message. These may be contained in a single byte (short frame format)
or in five bytes (long frame format). Since the module presently only
supports the long frame form, we will omit the discussion of the short
frame form. In either format, the single-bit address of the master is the
most significant. Only two masters are allowed for example, a control
system and a hand-held communicator. The most significant bit of the
address field differentiates these two hosts. Primary masters, such as the
IF8H and OF8H, use address 1, and secondary masters, such as
handhelds, use address 0. Please see figure below.