User Manual

Compact IO™ Isolated HART Analog Input Module
U ser 's M a n u a l 0 300 21 5- 03 R ev. A
7-44
HART revision 5 introduced the long frame format. In this format, the address of a slave
device is a worldwide, unique 38-bit number derived 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 IF4IH supports only the long frame format.
Each item of the message structure shown above is explained as follo ws.
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 m e ssage 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 7-19 (Start Character Definition)
Short Frame Long Frame
Master to slave 02 (Hex) 82 (Hex)
Slave to master 06 (Hex) 86 (Hex)
Burst mode from slave 01 (Hex) 81 (Hex)
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 IF4IH use address 1, and secondary masters
such as handhelds use address 0. Please see figure below.
Figure 7-12 (Long Frame Address)
Note: The IF4IH does not support burst mode.