The 1100 ohms must be available in the

The Hart Communications Protocol utilizes the OSI
communications model. As a matter of fact, most of the communication systems
follow suit. The HART protocol uses only three of the seven layers of the OSI
model. Namely: Layer 1 – physical. Layer 2 – data link. And layer 7 –
application. Layers 3 to 6 are empty because they are simply not required here.

Layer One – the physical layer works on the Frequency Shift
Key principle, established by the Bell 202 Communication Standard where we have
a data transfer rate of 1200 bits per second, logic 0 frequency of 2200 Hz, and
logic 1 frequency of 1200 Hz. (Boyes, 2009)

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For long distances, (up to 1,500 m), single, shielded bundles
of 0.2mm2 twisted pairs can be used. Beyond this, distances of up to
3,000 m can be covered using single, shielded, twisted 0.5 mm2
pairs. For shorter distances, unshielded 0.2 mm2, two-wire lines are
suitable. A total resistance between 230 ohms to 1100 ohms must be available in
the communication circuit. (Boyes, 2009)

Layer Two – the data link layer establishes the configuration
for a HART message. HART is based on the master/slave protocol. All of the command
messages are sent from a master, this master locates a field device (slave),
which interprets the command message and sends a response. In multidrop mode,
this can contain the addresses for several unique field devices. The data link
layer also improves transmission reliability by adding the parity bit for error
detection.      (Boyes, 2009)

Layer Seven – the application layer brings the HART
instruction set into the equation. The master sends messages with requests for
specific values, real values, and any other data or parameters that are available
from the device. The field translates these instructions as defined in the HART
protocol. The response message provides the master with status information and
data from the slave. To make interaction between HART-compatible devices as
efficient as possible, classes of commands have been established for slaves. For
slave devices, logical, uniform communication is provided by the following
command sets. There are universal commands that are understood by all field
devices. Common practice commands that provide functions that can be carried
out by many field devices, and device-specific commands that provide functions
that are exclusive to that particular device, permitting incorporation of
special features that are accessible by all users. A field device normally
operates with all three command sets on board. (Boyes, 2009)