Wired HART CommunicationsThe Hart Communications Protocol utilizes the OSIcommunications model. As a matter of fact, most of the communication systemsfollow suit.
The HART protocol uses only three of the seven layers of the OSImodel. 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.
Figure1 Wired HART DiagramLayer OneThe physical layer works on the Frequency Shift Keyprinciple, established by the Bell 202 Communication Standard where we have adata transfer rate of 1200 bits per second, logic 0 frequency of 2200 Hz, and logic1 frequency of 1200 Hz. For long distances, (up to 1,500 m), single, shieldedbundles of 0.2mm2 twisted pairs can be used. Beyond this, distancesof up to 3,000 m can be covered using single, shielded, twisted 0.5 mm2pairs. For shorter distances, unshielded 0.
2 mm2, two-wire lines aresuitable. A total resistance between 230 ohms to 1100 ohms must be available inthe communication circuit. (Boyes, 2009)Layer TwoThe data link layer establishes the configuration fora HART message. HART is based on the master/slave protocol.
All of the commandmessages 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 linklayer also improves transmission reliability by adding the parity bit for errordetection. (Boyes, 2009)Layer SevenThe application layer brings the HART instruction setinto the equation. The master sends messages with requests for specific values,real values, and any other data or parameters that are available from thedevice. The field translates these instructions as defined in the HARTprotocol. The response message provides the master with status information anddata from the slave. To make interaction between HART-compatible devices asefficient as possible, classes of commands have been established for slaves.
Forslave devices, logical, uniform communication is provided by the followingcommand sets. There are universal commands that are understood by all fielddevices. Common practice commands that provide functions that can be carriedout by many field devices, and device-specific commands that provide functionsthat are exclusive to that particular device, permitting incorporation ofspecial features that are accessible by all users. A field device normallyoperates with all three command sets on board. (Boyes, 2009) WirelessHART CommunicationsWirelessHART is a commonly used communication systemwithin industrial networks. It limits the amount of physical cabling we need toestablish a strong network.
WirelessHart runs along the same OSI layers asnormal 4-20mA HART does; 1, 2, and 7 or commonly known as the Physical,Datalink and Application layers respectively. WirelessHART uses a mesh topologyto communicate between the master and all if its correlated nodes. (FieldCommGroup, Date Unknown)Figure2 Wireless HART DiagramLayer OneThe physical layer, which is backboned by IEEE STD802.15.4-2006 which is the standard for short range frequency communication.
Ithas a frequency range between 2400MHz and 2483.5MHz, which has data speedsaround 250KBPS with a maximum data payload of 127 bytes. For a standardantenna, the maximum distance is around 225m or around 750ft. The networkconsists of routers, receivers, and gateways.
(Rfwireless-world.com,Date Unknown)Layer TwoThe datalink layer of the OSI stack consists ofWirelessHART’s capabilities, and how it communicates on a node to node basis.WirelessHART uses a strict 10ms time slot within TDMA (Time Division MultipleAccess) technology. This means it uses distinct time slots to allow differentusers to share the same frequencies without collisions. The transmission ofdata 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 TDMAmedia access. This directs the message to the correct time frame the data isneeded at. It then includes the usual arguments such as address, function codeand data. (Rfwireless-world.
com, Date Unknown)Layer 7The application layer of WirelessHART is the closestthis communication system will get to contact with the user. It provides abasis of which the user can search and retrieve any data they may require outin the network. It identifies any communication on the network, searches forresources, and synchronizes the communications between everything. The applicationlayer also provides a high encryption and decryption system ensuring data andinformation can not be accessed. It consists of a robust, multi-tiered industrystandard 128 bits of AES security encryption. Restricting access to anypersonal without several encryption keys. (Rfwireless-world.com,Date Unknown)Data TransmissionIn HART protocol data can betransmitted in two modes:• Poll (AKA: Response) mode• Burst (AKA: broadcast)mode (liPták, Date Unknown)Poll ModeIn the poll/response mode, the master polls each ofthe field devices in the network and requests for required information.
(liPták,Date Unknown)Burst ModeIn burst mode, the field device uninterruptedly transmitsprocess data without any request message. (liPták, Date Unknown)The HART Message Structure and FormatThere are 9 fields in a HART message format. The messagestructure of HART communication is shown in the figure below:Figure3 :HART Structure FSK (Mishra, 2011)Field 1: PreambleThe preamble field consist of 5 to 20 bytes. In hex itis FF i.e. all are 1. Synchronization with character stream is initiatedthrough this field.
As mentioned above all the 5 to 20 bytes are only 1 hereand it offers delay between the synchronization and the starting of start bitas well. (Mishra, 2011)Field 2: Start CharacterSecond field is Start Character which consist of 1byte (8 bits). Start character can have various values and its function are:- Specify the message type. – Indicates, is it slave to master ormaster to slave?- Designate the mode of thetransmission, is it in burst mode or poll mode? (Mishra, 2011)Field 3: AddressBoth the master address and slave address isincorporated in this field.
If it is 1 that indicates it is a primary masterand 0 indicates for a secondary master. There are two frame formats of messagelike short format and long format. For short format, the polling address of theslave is 4 bits. On the other hand, it is 38 bits in long format for the same. (Mishra,2011)Field 4: ExpansionExpansion field purpose is to make a space betweencommand and address field. It allows extra 3 bytes (24 bits) between thosefields.
The number of bytes (max 3 byte) is specified by the 6 and 5 bits ofstart delimiter. (Mishra, 2011)Field 5: CommandCommon field are two types:Universal Command: Range 0 to 30Common Practice Command: Range 32 to 126.Sometimes there are device specific commands alsowhich range is 128 to 253. All type of command field consist of 1 byte (8bits).
The command field is the instruction for field device to what to do.(Mishra, 2011)Field 6: Byte CountSince HART message format doesn’t has any “end ofmessage” character, Byte count field is used to confirm the receiver about themessage completion. How many bytes of status and data bytes should be followedby the receiver is confirmed by this field. The byte count field contains thenumber of bytes to follow in the status and data bytes. And this way, it helpsthe receiver to know when the message is going to complete. (Mishra, 2011)Field 7: StatusThe other name of status field is “response code”. Itconsists of two bytes (16 bits). Only the response message from the slavecontain this field.
The purpose of the field is:- To confirm about any error in theoutgoing message.- To declare the status of received commandform master.- To inform about its own (fielddevice) status as well. (Mishra, 2011) Field 8: DataAs mentioned earlier, there are several type ofcommand in HART communication protocol. And all messages don’t have the datafiled. It depends on the type of command. In universal and common practicecommand, they use 33 bytes for reasonable message duration.
Whereas some devicespecific command uses longer data field up to 253 bytes. (Mishra, 2011)Field 9: ChecksumThe last field is checksum field which contains byte 1(8 bits). This field is used to detect any communication error. Starting fromstart character to end, checksum field work as a longitudinal parity for allthe 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=falseAccessed Jan. 2018.2. FieldCommGroup. (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. onlineAvailable at: http://www.
rfwireless-world.com/Articles/wirelessHART.htmlAccessed Jan. 2018. 4. Chen, D., Nixon, M.
and Mok, A. (DateUnknown). Application Layer. 5. “Process Software and DigitalNetworks” volume 3, 4th edition by Béla G. liPták6. Mishra, R. (2011).
HART PROTOCOL.online Automate process industry. Available at:https://learnprotocols.wordpress.com/2011/07/28/hart-protocol/ Accessed Jan.2018.