Programmable transmitters with a classic 4...20 mA signal transmission are available with a digital communication interface. These interfaces are used primarily for diagno- sis or for selecting the required transmitter functions for the application while continuing to use the analog output for fast measured value transmission. There are different programmable transmitters interfaces, suitable for local as well as for remote program- ming. Transmitters with fieldbus interfaces usually no longer include an analog output. The measured signal, diagnosis and parameters are transmitted digitally over the field- bus.
Local Programming
The transmitters with local communication interfaces (LCI) often have, in addition to the connections for the 4...20 mA signal, a separate, manufacturer specific programming connection. An adapter is used to connect the instrument directly to the programming device. A requirement is that the distance between the instrument and the program- ming device is only a few meters (yards).
This type of local programming is found primarily in transmitters designed for installa- tion in control rooms and for the economical sensor head transmitters. The program- ming is usually a one time event, made prior to the start-up of the transmitter, e. g. in the work shop. Continuous monitoring of the transmitter, because it only has a locally accessible interface, is not possible. Changes to the parameters or inspections of the transmitter by service can only be accomplished using portable programming devices.
Fig. 5-15: Local Communication Interface
Remote Programming
When the transmitter is to be programmed or monitored from large distances, transmit- ters with FSK-communication are used (FSK = Frequency Shift Keying). In this design, a frequency of 1200 Hz or 2200 Hz is superimposed on the analog 4...20 mA signal. This type of data transmission is based on the Bell 202 Communication Standard.
LCI
Transmitter LCI Adapter
Fig. 5-16: Bell 202 Communication Standard
The two frequencies contain bit information 1 or 0. A real simultaneous communication with a response time of approx. 500 ms per measured value can be achieved. Because the average value of the frequency is zero, the FSK-communication does not affect the analog signal. To program the transmitter a FSK-modem is required.
The HART-Protocol
The HART-Protocol (Highway Addressable Remote Transducer, i.e. a protocol for bus addressable field instruments) operates using the above named technology. The HART-Protocol is an industry tested digital communication method available for field instruments. There is a worldwide HART-User Group. All well known companies in the measurement and control fields are members. HART conforms to the Open Systems Interconnection basic reference model (OSI) for open system communications, devel- oped by the International Standards Organization (ISO).
Point-to-point operation is used for simple programming of HART-instruments. When programming, it is always necessary that the connected HART-instrument is powered. There are suitable programming adapters or transmitter power supplies available for this purpose. The following figure shows the various point-to-point operating modes.
The Bell 202 Communication Standard + 0.5 mA - 0.5 mA 0 "1" = 1200 Hz "0" = 2200 Hz Analog Signal (4...20 mA) -
Fig. 5-17: FSK-programming
The manufacturer specific programming adapters accept the HART-temperature trans- mitter and provide its power supply. The FSK-modem is used to convert the FSK-infor- mation into a PC compatible format. Using this design, the transmitter, prior to start-up in the field, can be programmed from the control room without any large wiring expenses. If the temperature transmitter is already installed in the field, it is possible to program it using a handheld terminal (HHT) without any effect on the 4...20 mA output signal. The FSK-modem is integrated in the HHT. The power supply is provided by the transmitter power supply in the control room.
According to the HART-specifications, a load of at least 250Ω must always be installed in the 4...20 mA loop. This assures that the low internal resistance of the power supply cannot short out the HART-signal. When using older or simple power supply instru- ments, the connection wire must be opened and a resistor installed. In modern HART transmitter power supply instruments this load is integrated. In addition, they are trans- parent to the FSK-signals. A simple connection of a handheld terminal or FSK-modem can be made either in the field or in the control room. Many of the transmitter power supplies contain sockets, for connecting terminals or modems so that the current out- put or supply circuit need not be opened. If power supplies are installed instead, which do not have the ability to transmit FSK-signals, then an FSK-modem must be installed between the transmitter power supply and temperature transmitter. In every HART-in- terconnection two indicating/operating instruments are allowed. A primary one, usually in the process control system, and a secondary one, e. g. a handheld terminal or a laptop. 4...20 mA 4...20 mA FSK Transmitter FSK Modem Field Control Room Isolator FSK Modem FSK Modem Configuration Adapter FSK Transmitter FSK Transmitter Isolator
HART Multi-Drop-Mode
In the Multi-Drop-Mode the transmitter with a FSK-interface is also bus capable. The two connection wires for the 4...20 mA signal is also used for the bus communication. This operating mode requires only a single pair of wires and a power supply to commu- nicate with up to 15 field instruments. When the connected instruments are configured for this operating mode, their output current is frozen at 4 mA. The instruments only communicate digitally. Their analog output signal is no longer used to transmit temper- ature values. The connection of a recorder or an analog indicator is no longer possible.
Fig. 5-18: Bus operating mode Multi-Drop
In this operating mode the transmission of parameters and diagnosis data is in the fore- ground. Since only about 2 measured values can be transmitted digitally over the HART-Protocol per second, this communication method is only used for slower pro- cesses, e. g. the monitoring of very distant systems such as pipelines or tank farms.
HART-Multiplexer
It is also possible using a FSK-multiplexer to connect multiple instruments to a single programming instrument. Several hundred HART-field instruments can be accessed from a central location. This simplifies the start-up and maintenance since they can be performed while the system is operational. It is possible to set a HART-transmitter in the simulation-mode, so that the 4...20 mA signal can be set to a fixed, user program- mable current value. In this manner, the current loop can be tested without using the measured value. The measuring location parameter values can be stored in the pro- gramming instrument. This is a practical function for accessing the diagnosis and asset management data. This allows a quick response when service is required. This func- tionality can only be viewed as an intermediate step for fieldbus systems with open fieldbus protocols. Transmitter 1 4 mA 4 mA 4 mA Transmitter 2 Transmitter15 FSK Modem Power supply
Fieldbus Systems
The art of instrumentation was dramatically changed by the introduction of fieldbus technology. In the past, a two conductor wire had to be connected from each instrument to the control room for the analog 4...20 mA signals. In the fieldbus only a single two connection wire cable is required to connect up to 32 temperature transmitters.
Fig. 5-19: PROFIBUS PA installation using a PROFIBUS PA-profile
This figure shows an example of a PROFIBUS PA installation of 32 temperature trans- mitters. Since this concerns a fieldbus, it is necessary to install a bus termination at the end of the cable. The transmission medium is a twisted two wire copper cable with a shield. Instruments can be exchanged or added during operation. With a common transfer rate of 31.25 KBaud distances up to 1900 m (6200 ft.) can be spanned.
The temperature transmitters can easily be integrated into PROFIBUS DP-Networks using a segment coupler. The segment couplers have a simple baudrate conversion factor of 1:3. Therefore the transmission speed of the PROFIBUS DP when using these segment couplers is fixed at 93.75 kbit per second (93.75 KBaud). If one wants to cir- cumvent this fixed transmission ratio between the PA and DP, a DP/PA-Link can be used instead of the segment coupler. This allows, dependent on the transmission length of the PROFIBUS DP, the total transmission speed to be realized.
Segment - koppler PROFIBUS PA PROFIBUS DP 850°C 100°C 0°C -200°C 1 3 32 2 Temperature transmitter Temperature transmitter Temperature transmitter Temperature transmitter Bus connection Segment coupler
PROFIBUS PA Profile (Pt100-Temp.)
Physical Limitation Measurement Limit Upper Alarm Limit Upper Warning Limit
Measurement Value Lower Warning Limit Lower Alarm Limit Measurement Limit Physical Limitation
What has been accomplished in the European markets through the activities of the PNO (Profibus-Nutzer(User)-Organization), is accomplished in the American market place by the FF (Fieldbus Foundation). Each organization supports a non-compatible bus protocol. Only the bus supplied transmission technology per IEC 1158-2 and the data transmission speed of 31.25 kbit per second are identical for PROFIBUS PA and FOUNDATION Fieldbus.
Fieldbus Profiles
The PROFIBUS PA-Profile enables the exchangeability and interoperability of field instruments from different manufacturers. It is an integral component of PROFIBUS PA and can be obtained from the PROFIBUS-User Organization. The PA-Profile consists of a framework specification, which contains valid definitions for all instrument types, and instrument specification sheets which include the specific agreements which were reached for each instrument type.
The profiles use standardized function blocks. The description of the instrument behav- ior is accomplished by defining the standard variables, which describe the properties of the transmitter in detail. Every instrument must have a GSD (Generic Slave Data) file, which contains the specific instrument data. These files are necessary in order to connect the instrument described therein into the bus. The procedure is supported by the software tools from the different manufacturers. Every instrument must make avail- able the parameters defined in the PROFIBUS PA-Profiles.
Measured values are calculated in a Transducer-Block (TR) and transmitted over an AI-Block to the PROFIBUS-Master. The following table lists the most important param- eters of an AI-Block. For actuators, AO-Blocks are used.
Parameter Read Write Function
Out ● Actual measured value of the process variables
PV_SCALE ● ● Scaling of the process variables
PV_FTIME ● ● Rise time of the function block- output in s
ALARM_HYS ● ● Hysteresis of the alarm function in % or range
HI_HI_LIM ● ● Upper alarm limit
HI_LIM ● ● Upper warning limit
LO_LIM ● ● Lower warning limit
LO_LO_LIM ● ● Lower alarm limit
HI_HI_ALM ● Status the upper alarm limit with time stamp
HI_ALM ● Status the upper warning limit with time stamp
LO_ALM ● Status the lower warning limit with time stamp
For the various parameters it can be seen that not only the measured value, but also the alarm and warning information is transmitted. The digital transmission of the mea- sured values allows a higher accuracy to be achieved, because the conversion of the measuring range to a span of 4...20 mA is no longer necessary. Wider measuring ranges can be defined, without sacrificing any accuracy.
Programming Software
For the different instruments from the various manufacturers, special programming software is available. A number of firms have developed a common programming soft- ware for their entire instrument palette. It can be used, from a common user interface (GMA-Standard), to program the parameters and read the measured values and diag- nosis information from different instrument types.