The data sources of the field devices at a remote site are the sensors/transducers. The sensor/transducer output is converted to digital data through the RTU. In the case of radio telemetry, the output of the RTU is entered into a radio modem.
The modem converts the digital data into an analog signal that can be transmitted over the air. If telephone lines are used as a means of communication, then the output of the RTU is fed to a modem (or the RTU may have a built-in modem) from where the signal is placed on the 2-wire telephone line (twisted pair).
At the receiving end, the previous process is reversed. The data recovery present at the receiving end will process the required compatible data to the master computer (or to the field device).
In a typical application, the master computer requests data from remote field devices (polling), one RTU at a time. Send a request to the RTU to send your data. Then, it returns to the standby reception mode of the data coming from the remote site.
After the RTU sends the requested data, it returns to the receiving mode waiting to receive further instructions from the master station. Once the master station receives the data from the remote site, it enters transmission mode, it can send additional instructions to that site or go to the next one to be polled.
The polling process continues until all remote sites in the system have sent their data. This process is repeated indefinitely if a continuous survey is needed. The RTU can return to the sending if an alarm or a limit is exceeded (report by exception) and, under the control of the prevailing communication protocol, sends the data to the main station.
Functions of a SCADA system:
The main tasks that a SCADA system must provide are: controlling the field devices of the plant/system (obviously), handling alarms, changing limits, providing more than 1 mode of operation, data file arrangement, event log and the production of graphical trends reports and graphs).
Controlling field devices:
The remote components of the system are controlled from the master computer. The means for controlling the system can be any or all of the following: positioning of the cursor, function keys, personalized menus and alphanumeric keys.
Alarm handling:
When an unexpected event generates an alarm, several alarm indicators are provided to the operator (through the software). These indicators can be all or any of the following: audible alarm, the alarm light of the station flashes in the mimic panel, the last alarm line is updated in all the images, the alarm is inserted in the alarm list corresponding, the object symbol (element) or the value flashes in the station diagram (process image). An automatic dialer / voice unit can be included.
Limits changing:
The upper and lower limits for a remote measured value (which was previously established) can be changed from the master station. There is a waiting period for the operator to make the change. If the waiting time period expires, the operator must cancel the sequence that he has already entered and restarted the change sequence from the beginning.
Operation Modes:
The system must provide the following modes of operation: operator mode (normal operation of supervision and data acquisition), programming mode (it is used by programmers/designers to change data, extend the system, maintain the program, generate reports; network control is disabled) Training mode (self-explanatory) and special mode.
Data archiving:
It regularly samples the values of the measurands and the states of the monitored points in the database and stores them in historical files for a predefined retention period. This task is useful for monitoring networks and generating reports, as well as for providing data to trend facilities.
Events logging:
it is a list of all events that occur in the system and that are printed chronologically in the recorder. The types of events can be: changes of status of alarms and indications, waiting times, limit violation, operator input and system alarms (loss of communication with remote stations, hardware errors and failures in remote stations).
Production of reports & trend charts:
A report consists of data that is presented to the user in a predetermined tabular format. It can be produced cyclically from the historical database. The trend function allows the operator to generate graphic screens that show the historical trend of the measured data.
