When we talk about telecommunication we refer primarily to the communications equipment needed to establish communication between the different components of the SCADA system.
As such this unit will be known what communications equipment are required and what are your selection criteria.
COMMUNICATIONS MADE POSSIBLE SCADA SYSTEM
Whereas a SCADA system consists of one or more master stations sending instructions to the remote stations and receiving information from these it is clear that communication plays a vital role.
Figure 1
The rationale for installing a SCADA system is based on the ability to communicate stations located at considerable distances and the cost of accessing information from this location. The major rationale is the avoidance of having a person at the remote station running the operations that can be replaced by an RTU.
To carry out the implementation of a Scada system, you must perform a conversion from analog to digital. This is because the entire flow of information between the master station and remote station are binary data.
Figure 2 shows the waveform of the output of a limit switch that is used to indicate the status of a control valve. At first the valve is open and is sending a signal to +5 V. In a second stage valve is closed and is sending a signal of 0 V.
Figure 2
Figure 3
The state of the valve is led to a search of a bit or a flip flop as shown in Figure 3. The state of the valve is brought to the enable signal (Enable) of the record. This record has an additional channel for the system clock (Clock). The updating of the output register is updated with every change of state from 0 to 1 o'clock.
Figure 4
As you can see the update status to the log output is a function of updating the system clock. In our case, updating the status of the valve occurs in the fourth rising clock pulse. This causes you to lose the information about a life cycle of the clock. That is why the faster the refresh time clock, better system performance.
Figure 5 shows how an analog signal is developed to represent the position of the valve.
Figure 5
When the valve is fully open, the position indicator will send a signal of 5 volts. When the valve is fully closed position indicator sends a signal of 0 volt. The figure shows a valve position signal equivalent to 3 volts which is equivalent to 60% of the valve opening.
Figure 6
The analog - digital processes information and provides the bit number equivalent to this information. For example you are working with a four-bit converter. Therefore the resolution of the conversion is 5 / 24 = 0.3125 volts. That is why each is subtracted from this value.
The table shows the calculation in volts obtained from the analog-digital conversion. I noticed that there is an error of 0.1825 volts.
Figure 7
LONG DISTANCE - SERIAL COMMUNICATION
When communications are at large distances is necessary to work with a serial communication. This means that in a string, the bits are sent one by one from the origin to destination.
The parallel communication is well known for using printers connected to the PC's. However, the major limitation is the maximum that can be obtained.
When you need to send data in serial form, it is necessary to work with additional equipment that can send the bits one by one analog-digital converters. The most commonly used technique is to start with the most significant bit and end with the least significant bit. This convention is known as protocol and will be explained later.
