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)