Published on Feb 12, 2016
For many years, the field communication standard for process automation equipment has been a milliamp (mA) analog current signal. The milliamp current signal varies within a range of 4-2OmA in proportion to the process variable being represented.
Li typical applications a signal of 4mA will correspond to the lower limit (0%) of the calibrated range and 2OmA will correspond to the upper limit (100%) of the calibrated range. Virtually all installed systems use this international standard for communicating process variable information between process automation equipment.
HART Field Communications Protocol extends this 4- 2OmA standard to enhance communication with smart field instruments. The HART protocol was designed specifically for use with intelligent measurement and control instruments which traditionally communicate using 4-2OmA analog signals. HART preserves the 4- signal and enables two way digital communications to occur without disturbing the integrity of the 4-2OmA signal. Unlike other digital communication technologies, the HART protocol maintains compatibility with existing 4-2OmA systems, and in doing so, provides users with a uniquely backward compatible solution. HART Communication Protocol is well-established as the existing industry standard for digitally enhanced 4- 2OmA field communication.
THE HART PROTOCOL - AN OVERVIEW
HART is an acronym for "Highway Addressable Remote Transducer". The HART protocol makes use of the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4-2OmA. This enables two-way field communication to take place and makes it possible for additional information beyond just the normal process variable to be communicated to/from a smart field instrument. The HART protocol communicates at 1200 bps without interrupting the 4-2OmA signal and allows a host application (master) to get two or more digital updates per second from a field device. As the digital FSK signal is phase continuous, there is no interference with the 4- 2OrnA signal.
HART is a master/slave protocol which means that a field (slave) device only speaks when spoken to by a master. The HART protocol can be used in various modes for communicating information to/from smart field in3truments and central control or monitor systems. HART provides for up to two masters (primary and secondary). This allows secondary masters such as handheld communicators to be used without interfering with communications to/from the primary master, i.e. control/monitoring system. The most commonly employed HART communication mode is master/slave communication of digital information simultaneous with transmission of the 4-2OmA signal. The HART protocol permits all digital communication with field devices in either point-to-point or multidrop network configuration. There is an optional "burst" communication mode where single slave device can continuously broadcast a standard HART reply message.
HART COMMUNICATION LAYERS
The HART protocol utilizes the OSI reference model. As is the case for most of the communication systems on the field level, the HART protocol implements only the Layers 1, 2 and 7 of the OSI model. The layers 3 to 6 remain empty since their services are either not required or provided by the application layer 7
Data transmission between the masters and the field devices is physically realized by superimposing an encoded digital signal on the 4 to 20 mA current loop. Since the coding has no mean values, an analog signal transmission taking place at the same time is not affected. This enables the HART protocol to include the existing simplex channel transmitting the current signal (analog control device ®field device) and an additional half-duplex channel for communication in both directions.
The bit transmission layer defines an asynchronous half- duplex interface which operates on the analog current signal line. To encode the bits, the FSK method (Frequency Shift Keying) based on the Bell 202 communication standard is used. The two digital values 0 and 1 are assigned to the following frequencies as shown in the above fig.
Logical 0: 2200Hz
Logical 1: 1200Hz
Each individual byte of the layer-2 telegram is transmitted as eleven-bit UART character at a data rate of 1200 bits/s.
The HART specification defines that master devices send voltage signals,, while the field devices (slaves) convey their messages using load-independent currents. The current signals are converted to voltage signals at the internal resistance of the receiver (at its load).
To ensure a reliable signal reception, the HART protocol specifies the total load of the current loop including the cable resistance to be between minimum 230 ohms and maximum 1100 ohms. Usually, the upper limit is riot defined by this specification, but results from the limited power output of the power supply unit. The HART masters are simply connected in parallel to the field devices, so the devices can be connected and disconnected during operation because the current loop need not be interrupted.
HART wiring in the field usually consists of twisted pair cables. If very thin and/or long cables are used, the cable resistance increases and, hence, the total load. As a result, the signal attenuation and distortion increases while the critical frequency of the transmission network decreases.
If interference signals are a problem, long lines must be shielded. The signal loop and the cable shield should be grounded at one common point only. According to the specification, the following configurations work reliably:
• For short distances, simple unshielded 0.2 mm 2 two-wire lines are sufficient.
• For distances of up to 1,500 m, individually twisted 0.2 mm wire pairs with a common shield over the cable should be used.
• For distances of up to 3,000 m, individually twisted 0.5 mm 2 two-wire lines shielded in pairs are required.
Most of the wiring in the field meets these requirements and can therefore be used for digital communication.
Services of layer 2
• Access control
The HART protocol operates according to the master-slave method. Any communication activity is initiated by the master, which is either a control station or an operating device. HART accepts two masters, the primary master usually the control system and the secondary master a PC laptop or and held terminal used in the field.
HART field devices the slaves never send without being requested to do so. They respond only when they have received a command message from the master. Once a transaction, i.e. a data exchange between the control station and the field device, is complete, the master will pause for a fixed time period before sending another command, allowing the other master to break in. The two masters observe a fixed time frame when taking turns communicating with the slave devices.
• Communication services
The HART protocol provides standard and broadcast commands:
Standard command: master/slave data exchange
broadcast command: HART command received by all devices
The simplest form of a transaction is a master telegram which is directly followed by a response or acknowledgement telegram from the slave
This communication mode is used for the normal data exchange. When connection is established, the HART command 11 can be used to send a broadcast message to all devices to check the system configuration.
Some HART devices support the optional burst communication mode. A single field device cyclically sends message telegrams with short 75-ms breaks, which can alternately be read by the primary as well as the secondary master. While usually only two transactions per second are possible, the field device can send up to four telegrams using this method.
• Telegram structure
The structure of a HART telegram is shown below. Each individual byte is send as 11-bit UART character equipped with a start, parity and a stop bit. In the revision 5 and later, the HART protocol provides two telegram formats which use different forms of addressing. In addition to the short frame slave address format containing four bits, a long frame address format has been introduced as an alternative. This allows more participants to be integrated, while achieving more safety in case of incorrect addressing during transmission failures.
The elements of the HART telegram perform the following tasks:
The preamble consisting of three or more hexadecimal FF characters synchronizes the signals of the participants. The start byte indicates which participant is sending (master, slave, and slave in burst mode) and whether the short frame or the long frame format issued.
The address field of the short frame format contains one byte with one bit serving to distinguish the two masters and one bit to indicate burst-mode telegrams. For the addressing of the field devices, 4 bits are used (addresses 0 to 15)
The address field of the long frame format contains five bytes; hence, the field device is identified using 38 bits.
The command byte encodes the master commands of the three categories, Universal, Common-practice and Device- specific commands. The significance of these commands depends on the definitions in the application layer 7.
The byte count character indicates the message length, which is necessary since the number of data bytes per telegram can vary from 0 to 25. This is the only way to enable the recipient to clearly identify the telegram and the checksum. The number of bytes depends on the sum of the status and the data bytes
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