Wired shielded, twisted 0.5 mm2 pairs. For shorter distances,

Wired HART Communications

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.

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Figure
1 Wired HART Diagram

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. 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)

 

WirelessHART Communications

WirelessHART is a commonly used communication system
within industrial networks. It limits the amount of physical cabling we need to
establish a strong network. WirelessHart runs along the same OSI layers as
normal 4-20mA HART does; 1, 2, and 7 or commonly known as the Physical,
Datalink and Application layers respectively. WirelessHART uses a mesh topology
to communicate between the master and all if its correlated nodes. (FieldComm
Group, Date Unknown)

Figure
2 Wireless HART Diagram

Layer One

The physical layer, which is backboned by IEEE STD
802.15.4-2006 which is the standard for short range frequency communication. It
has a frequency range between 2400MHz and 2483.5MHz, which has data speeds
around 250KBPS with a maximum data payload of 127 bytes. For a standard
antenna, the maximum distance is around 225m or around 750ft. The network
consists of routers, receivers, and gateways. (Rfwireless-world.com,
Date Unknown)

Layer Two

The datalink layer of the OSI stack consists of
WirelessHART’s capabilities, and how it communicates on a node to node basis.
WirelessHART uses a strict 10ms time slot within TDMA (Time Division Multiple
Access) technology. This means it uses distinct time slots to allow different
users to share the same frequencies without collisions. The transmission of
data uses few different topics in which is sorts and directs the messages,
first we have frame ID, which is exclusive to network protocols that use TDMA
media access. This directs the message to the correct time frame the data is
needed at. It then includes the usual arguments such as address, function code
and data. (Rfwireless-world.com, Date Unknown)

Layer 7

The application layer of WirelessHART is the closest
this communication system will get to contact with the user. It provides a
basis of which the user can search and retrieve any data they may require out
in the network. It identifies any communication on the network, searches for
resources, and synchronizes the communications between everything. The application
layer also provides a high encryption and decryption system ensuring data and
information can not be accessed. It consists of a robust, multi-tiered industry
standard 128 bits of AES security encryption. Restricting access to any
personal without several encryption keys. (Rfwireless-world.com,
Date Unknown)

Data Transmission

In HART protocol data can be
transmitted in two modes:

• Poll (AKA: Response) mode

• Burst (AKA: broadcast)
mode (liPták, Date Unknown)

Poll Mode

In the poll/response mode, the master polls each of
the field devices in the network and requests for required information. (liPták,
Date Unknown)

Burst Mode

In burst mode, the field device uninterruptedly transmits
process data without any request message. (liPták, Date Unknown)

The HART Message Structure and Format

There are 9 fields in a HART message format. The message
structure of HART communication is shown in the figure below:

Figure
3 :HART Structure FSK (Mishra, 2011)

Field 1: Preamble

The preamble field consist of 5 to 20 bytes. In hex it
is FF i.e. all are 1. Synchronization with character stream is initiated
through this field. As mentioned above all the 5 to 20 bytes are only 1 here
and it offers delay between the synchronization and the starting of start bit
as well. (Mishra, 2011)

Field 2: Start Character

Second field is Start Character which consist of 1
byte (8 bits). Start character can have various values and its function are:

–         
Specify the message type.  

–         
Indicates, is it slave to master or
master to slave?

–         
Designate the mode of the
transmission, is it in burst mode or poll mode?             (Mishra, 2011)

Field 3: Address

Both the master address and slave address is
incorporated in this field. If it is 1 that indicates it is a primary master
and 0 indicates for a secondary master. There are two frame formats of message
like short format and long format. For short format, the polling address of the
slave is 4 bits. On the other hand, it is 38 bits in long format for the same. (Mishra,
2011)

Field 4: Expansion

Expansion field purpose is to make a space between
command and address field. It allows extra 3 bytes (24 bits) between those
fields. The number of bytes (max 3 byte) is specified by the 6 and 5 bits of
start delimiter. (Mishra, 2011)

Field 5: Command

Common field are two types:

Universal Command: Range 0 to 30

Common Practice Command: Range 32 to 126.

Sometimes there are device specific commands also
which range is 128 to 253. All type of command field consist of 1 byte (8
bits). The command field is the instruction for field device to what to do.
(Mishra, 2011)

Field 6: Byte Count

Since HART message format doesn’t has any “end of
message” character, Byte count field is used to confirm the receiver about the
message completion. How many bytes of status and data bytes should be followed
by the receiver is confirmed by this field. The byte count field contains the
number of bytes to follow in the status and data bytes. And this way, it helps
the receiver to know when the message is going to complete. (Mishra, 2011)

Field 7: Status

The other name of status field is “response code”. It
consists of two bytes (16 bits). Only the response message from the slave
contain this field. The purpose of the field is:

–         
To confirm about any error in the
outgoing message.

–         
To declare the status of received command
form master.

–         
To inform about its own (field
device) status as well. (Mishra, 2011)

 

Field 8: Data

As mentioned earlier, there are several type of
command in HART communication protocol. And all messages don’t have the data
filed. It depends on the type of command. In universal and common practice
command, they use 33 bytes for reasonable message duration. Whereas some device
specific command uses longer data field up to 253 bytes. (Mishra, 2011)

Field 9: Checksum

The last field is checksum field which contains byte 1
(8 bits). This field is used to detect any communication error. Starting from
start character to end, checksum field work as a longitudinal parity for all
the bytes and thus check the error. (Mishra, 2011)

 

 

 

 

References:

1.     
Boyes, W.
(2009). Instrumentation Reference Book. online Google Books. Available at:
https://books.google.ca/books?id=ZvscLzOlkNgC=PA244=PA244=hart+protocol+osi+model+reference+book=bl=HYpPJT4g54=bYMQ1FqvIE1TB76hWJ0ivUmEpwg=en=X=2ahUKEwiT5ojk_TYAhUE9GMKHTHaDhMQ6AEwAXoECAsQAQ#v=onepage=false
Accessed Jan. 2018.

2.     
FieldComm
Group. (Date Unknown). HART. online Available at:
https://fieldcommgroup.org/technologies/hart Accessed Jan. 2018.

3.     
Rfwireless-world.com. (Date Unknown).
WirelessHART protocol stack Basics | WirelessHART physical layer. online
Available at: http://www.rfwireless-world.com/Articles/wirelessHART.html
Accessed Jan. 2018.

4.     
Chen, D., Nixon, M. and Mok, A. (Date
Unknown). Application Layer.

5.     
“Process Software and Digital
Networks” volume 3, 4th edition by Béla G. liPták

6.     
Mishra, R. (2011). HART PROTOCOL.
online Automate process industry. Available at:
https://learnprotocols.wordpress.com/2011/07/28/hart-protocol/ Accessed Jan.
2018.