Page 244 - Instrumentation Reference Book 3E

228 Microprocessor-based and intelligent transmitters

Table 13.12 Layers 1-7 of the OS1 reference model-the HART protocol implements layers 1, 2, and 7
Layer Function HART
7. Application Provides formatted data HART instructions
6. Presentation Converts data
5. Session Handles the dialogue
4. Transport Secures the transport connection
3. Network Establishes network connections
2. Link Establishes the data link connection HART urotocol regulations
1. Physical Connects the equipment Bell 202’

communication is provided by the following com-
mand sets:
mmMe SD AD CD BC SWua Data Parity
(1) Universal commands: understood by all
field devices.
(2) Common practice commands: provide func-
tions which can be carried out by many, though
not all, field devices. Together, these commands
comprise a library of the most common field
device functions.
(3) Device-specific commands: provide func-
tions which are restricted to an individual device,
permitting special features to be incorporated
The structure of these messages can be seen in that are accessible by all users.
Figure 13.36. In multidrop mode this can accom- Examples of all three command sets can
modate the addresses for several field devices and usually be found in a field device, including all
terminals. universal commands, some common-practice
A specific size of operand is required to enable commands and any necessary device-specific
the field device to carry out the HART instruc- commands.
tion. The byte count indicates the number of
subsequent status and data bytes.
Layer 2 improves transmission reliability by 13.10.4.3 Operating conditions
adding the parity character derived from all the The HART Standard (HART Communication
preceding characters; each character also receives Foundation) requires level-3 resistance to inter-
a bit for odd parity. ference in the lines in accordance with IEC 801-3
The individual characters are: and 8014. This satisfies the general requirement
for noise resistance.
1 start bit Connecting or disconnecting a user, or even a
8 data bits breakdown of communication does not interfere
1 bit for odd parity with transmission between the other units.
1 stop bit. Intrinsically safe applications deserve special
Layer 7, the application layer, brings the mention. Barriers or isolators must be able to
HART instruction set into play. The master sends transmit the Bell 202 frequencies in both direc-
messages with requests for specified values, actual tions (see Figure 13.34). As can be seen in Figure
values, and any other data or parameters available 13.35, in multi-drop mode it is also possible to
from the device. The field interprets these instruc- interconnect field devices in accordance with
tions as defined in the HART protocol. The DIN VDE 0165.
response message provides the master with status
information and data from the slave. 13.10.4.4 Technical data
To make interaction between HART-compat-
ible devices as efficient as possible, classes of Data transmission
conformity have been established for masters, Type of data transmission: frequency shift key-
and classes of commands for slaves. There are ing (FSK) in accordance with Bell 202, relating to
six classes of conformity for a master as seen in the transfer rate and the frequency for bit infor-
Figure 13.37. For slave devices, logical, uniform mation 0 and 1.