He is currently increasing development of instrumentation and control equipment that communicate via digital signals to replace analogue.
It is therefore important to know some characteristics about the transmission thereof, of the physical means through which they are sent and the advantages of digital instrumentation over the conventional field. Let's consider this part as transmitters and valves for digital technology.
Different Digital Instruments today
The famous analog and digital signals
Although the 4-20 mA signal as an example of signal transmission standard is still used in most cases in recent years have seen a gradual transformation from analog to digital instrumentation. Today, change is occurring analog signals to digital.
The analog transmission of information is characterized by continuous change of signal amplitude. In process engineering, 4-20 mA signal mentioned above is transmitted in analog form pure. A current proportional to the measured value in the case of transmitters, flow between them and the instruments that are as a driver, a meter or a recorder. In such an analog signal, however, the information content is very restricted, only the current value and the presence or absence of it can be determined.
Unlike a digital signal does not change continuously, but is transmitted in discrete packets. It is not immediately interpreted, it must first be decoded by the receiver. The method of transmission is another: as electrical pulses that vary between two different voltage levels. In terms of process engineering, there is no limitation on the signal content and any additional information from a transmitter for example, can be transmitted with the measured variable.
One of the advantages of digital transmission is the elimination of unnecessary conversions from analog to digital. In this case, the analog signal is sampled. A higher sampling rate with a finer resolution, better conversion. But costs also increase, so there must be a compromise between cost and precision.
A quick review of concepts of Digital Communication
In analog communication, information is transmitted through the signal amplitude. In digital communication, the signal is composed of a series of voltage pulses is sent from the transmitter to the receiver via a transmission medium. This may be a cable, fiber optics or radio. The information is usually contained in the changes between two voltage levels that can take the logical values "1" and "0". Actual voltage levels and tolerances depend on the standard interface used. The voltages that fall outside the bands are not considered as data.
Individual data units represented by the values 0 and 1 are known as bits. These values are of course the basis of the binary number system. The binary system is used for all calculations within the microprocessor which appears as the heart of a digital instrument processes. The set of 8 bits is called a byte which represents the building block of all alphanumeric values used for example by an operator to communicate with a microprocessor.
Since digital communication involves at least two teams, both should be able to interpret the signal. That is why we use control codes in which a specific sequence of bits, said one of the teams being transmitted and how. Data codes are used for the information itself. The same rules apply whether we are talking to a microprocessor or multiple devices on a network are communicating with each other. ASCII code (American Standard Code for Information Interchange) is probably the best known example of a control code and data.
Transmission media that we have?
The more information provided, and higher transmission speed, the greater the demand for better features for the transmission medium. This is particularly true for industrial network communication, where conditions are far from ideal, for example due to possible interference of electrical machines. For this reason, the best means of transmission depends largely on the application.
- Cables Twisted (Twisted Cable)
It is the most economical solution for data transmission, allows transmission speeds up to 375 KBit / s on lines up to 300 m long. In many cases, use shielded twisted pairs and providing greater immunity to interference. In this case the distance can reach 1200 m. You can also use stranded cables, provided they do not cause interference between them.
According to the standard FIP, two pairs of double-shielded cables allow transmission speeds of 1 Mbit / s over distances up to 2000 m.
In all cases however, the communication cable must be kept separate from power cables when handling heavy loads.
- Coaxial Cables
They allow high-speed transmission with the added benefit that can take many messages simultaneously. The bandwidth is up to 10 MHz cables are more expensive than twisted and are rarely encountered in the field.
- Fiber Optic Cables
Its transmission capacity is 5 times higher than the coaxial cable. The fiber optic cable contains a single fiber glass which for reasons of stability is surrounded by several protective covers so that is almost as thick as a coaxial cable. These cables are easy to build. During the transmission, electrical signals are converted into light signals. This means that the usual factors such as interference electromagnetic fields have no influence.
Most fiber optic cables allow transmission speeds in the range of Gigabits / s. Due to the more complicated method of connection, this media is the most expensive. On the other hand, since it is relatively new and still in constant development, will in the future to replace copper cables for data transmission.
Braided Cable
Coaxial Cable
Fiber Optic Cables
Figure 1 - physical transmission media
