“The transmission rate now plays only a secondary role where the data is shared through the conductor control system on the conductor level of the mill or company group, i.e. in the control room of the PC. The safe transmission of very large amounts of data (up to the megabytes) is of more importance. Again, if an industrial Ethernet is needed, worldwide internet update of the technology is connected.”
Certificated Engineer and Associate Professor
German Müllerschule, Bad Harzburg
Communication solutions applied in the industrial field started to be commissioned in 1983 after the microprocessing technology in the automation technology was mainly being used in the memory solutions in 70s. These interconnected components consist of workstations and hosts via common interfaces and transmission tools (e.g. copper wire) and programmable logic controllers (SPS) and measurement systems. Participants establish a communication system together with transmission tools. Such systems, such as a personal computer, are of course included in the devices, but they will not be considered in the context of this article.
SERIAL AND PARALLEL DATA TRANSMISSION
In principle, serial and parallel processes are applied during transmission. In Table I, the standard interfaces of the personal computer (PC) developed by the IBM company “master version” are shown.
The length of the transmission line is limited due to its electrical properties. At larger distances, the signal must be prepared via “repeaters”. In parallel processes, their number is equivalent to the data bit and the controller lines, and therefore the parallel process is hardly ever used in industrial environments as it is burdensome.
The RS-232 standard defines the point-to-point connection of two participants. In the industrial sector, in general, many devices are required to be connected to the network, in other words, to connect with more than one interface point. These are very common in laboratory and special devices (e.g. scanners).
STANDARDS OR NORMS IN COMMUNICATION
Each communication task can be roughly divided into two parts as transmission -oriented and user-oriented parts.
Let’s take a random number measuring laboratory device as an example: at the end of the measurement, a data telegram containing the results is transmission red by the device to a serial standard interface. In order to be able to receive and evaluate this data, it is imperative that the same interface and the same configurations (transmission speed, number of data bits, etc.) and the same processes are operated in order to be able to detect transmission errors in the receiving device (personal computer) and ensure successful communication.
A software program has to interpret the data bits to create the actual information in the user-oriented part and to check whether the entire data transmission is complete and error-free. The measurement data received in this way can then be processed by another program.
Their use-oriented parts (protocol), which are essentially standardized in industrial communication, have manifested themselves as different processes that are standardized in the same way or composed of producer specific transmission processes (see Table 2).
The length of the transmission line is often dependent on the reported transmission rate per second (Baudrate) in bits; in other words, the higher the data rate, the lower the possible net strain.
An RS485 standard defines a network with multi-point capability in which multiple devices with the same authority (master) are connected (see figure 4, Bus). RS485 also creates the physical basis for the data bus system “PROFIBUS” and Siemens specific MPI. Many machines and devices only have an RS485 interface that allows physical connection. The relevant protocol must be applied in order to communicate with this terminal.
The IEEE 802-Standard (better known as Ethernet) was a project of the Institute of Electrical and Electronics Engineers (IEEE) in the 80’s and sets the standard for local area networks (LANs) in office environments. Through continuous improvements in hardware and software programs, a large number of transmission tools and access processes have been standardized (e.g. Wireless LAN, WLAN as per IEEE802.11).
OPEN DATA TRANSMISSION SYSTEMS
The impartial or standardized data bus systems are also defined as “open” (Table 3). Such data bus systems (e.g. PROFIBUS DP) are data transmission systems developed for all industrial interventions, with all components coming from different manufacturers. The encompassing data bus systems are generally represented by a group of users, for example the PROFIBUS user organization, PNO .
Participants in communications send telegrams containing different types and quantities of data. The telegrams are directed directly to a group (Multicast) or to all participants (Broadcast). The length of telegrams is limited, so large amounts of data are distributed to multiple telegrams with blockages. Essentially the communication process consists of three parts:
• Connection configuration
• Data transmission
• Connection process
The participant to connect is referred to as the “Client” (service requestor), and the other participant is called the “Server” (service provider).
Expectations from data bus systems
Depending on the nature of intervention areas, there are different expectations regarding transmission technology, transmission rate and amount of data. Expectations regarding the failure safety in industrial areas and unfavorable ambient conditions (shaking, heat, etc.) are different from expectations for the office environments where the amount of data to be transferred is of great significance.
Example of intervention in open data bus systems
There are different expectations from the data bus systems used in the milling operations. Let’s explain this with three different examples.
Sensor - Reactor - Level
In a given packaging system for small packagings managed by SPS, high cycle rates are required. The secondary information of sensors such as full detectors and accumulation detectors, etc. must be recorded quickly and for example, the commands transmitted to the valves must be observed rapidly. The amount of data is limited to a few dozen bits. One such bus system for such an application is the Reactor - Sensor - Interface (AS - 1). Thus, the data is cyclically read by the individual sensors at a high rate (max. 5 ms) and written into individual reactors. Compared to most data bus systems, the data in AS-I is improved by being modulated to the supply voltage of the data bus system and transmitted to the connection line. Data lines and power supply lines are generally laid out separately.
Decentralized Development of SPS (area periphery)
For an SPS, the data bus system (e.g. PROFIBUS DP) that periodically transmits data from the individual parts of the mill (see 1/2008) reaches a data amount of about 100 Bytes (1 Byte = 8 Bit). The expectations regarding the transmission rate are determined by the program execution time of the central unit of the SPS. Its magnitude is about 10 ms.
Administrations that can program memory within total operation take partial tasks into account. Production data is networked with each other so that mutual exchange of information is possible. This sharing of data occurs only according to needs, i.e. periodically under normal conditions. Thus, the expected rate of transmission is less. However, there is a fairly high amount of data (several hundred kilobytes). For this purpose, industrial Ethernet is generally used today.
The transmission rate now plays only a secondary role where the data is shared through the conductor control system on the conductor level of the mill or company group, i.e. in the control room of the PC. The safe transmission of very large amounts of data (up to the megabytes) is of more importance. Again, if an industrial Ethernet is needed, worldwide internet update of the technology is connected.
So basically the expected rate of transmission at the Sensor / Controller field or in the decentralized environment is not high and the amount of data is low. Conversely, in the conductor level communication, the transmission time is low but the data amount is high.
Data Transmission Tools
Twisted copper wires (double or multi-wire) are often used as transmission tools and rarely coaxial lines are used due to their burdensome installation. Light wave links (LWL) are not susceptible to electro-magnetic disturbances and are therefore placed in fault-sensitive media. The wireless connections do not require mechanical connection to the devices, so they are used in specific applications such as communication of transport vehicles or filling level measurement systems installed at a distance.
The possible physical cabling (topology) of the networks is shown in Figure 4. Data transmission topology (PROFIBUS), whi ch is generally a transmission tool for all users, is known, i.e. a tree (connection of multiple data transmission paths via Repeater, for example PROFIBUS) and star-like node connection (industrial Ethernet) which can be connected to a network in the same way.
The connection between the data transmission components is generally via socket connections (see Figure %)
Authorization Process for the Data Transmission Path
Authorizations must be made in the data transfer path as the transmission tools (data lines) must be open to all users. If all users have the same privilege, such as networking of the PC workstations, then we can talk about the Multi - Master - Bus. If there are hierarchical differences between the devices (e.g. SPS and input model groups), we can talk about a system called Master - Slave - Bus. The known authorization processes are described below.
Token communication authorizes an entry which continues to multiply according to a specified order among the authorized users (Master) and after the end of a certain period of time. The user with the token can communicate with all other users.
The advantage of the token process is that the duration of the communication is known because each user can only have Token for a certain period of time. The disadvantage is that the connection of new users and Token’s management is quite burdensome. The data transmission path in PROFIBUS is based on Token process on the one hand.
CSMA / CD – Process
In the “classic” Ethernet, all participants in the data transmission path are in contact with each other in the CSMA / CD process, which is a standard under IEEE 802.3, and communication can begin as long as the data transmission path is open. By limiting the length of the telegraph, it is necessary to terminate the transmission of the participant, and the transmission tool becomes free again. In the CSMA-CD process, a participant can participate directly in the communication after being physically connected via data transmission.
Depending on the burst time of the signal (about 70% light speed), in rare cases, two participants may be trying to transmit at the same time. This causes a collision that will be recognized by both participants due to the telegraph structure. Their communication is interrupted and they start randomly afterwards. That is, in this case, the possibility of collision increases in very high data growth and the transfer rate reaches to an unforeseeable level. Until the 90s, the process regarding the collision lines was standard between the office and industrial networks (e.g. SINEC H1). Then the star-shaped connection started to be used with individual stud cables known as the normal structure and transmitted to the switch. Collisions are prevented here by the use of buffered active switches. The process is now used on industrial Ethernet in the industrial field or on PROFINET in media fields.
A controlled transmission is available in the Master - Slave - System. Slave which is an auxiliary and decentralized structure and records data of the machines on the milling operations or reads the filling level measurement device sends data to Master – Structure group (e.g. SPS) only when requested by them. For example, Master - Slave - process is available in PROFIBUS DP.
PROFIBUS DP (PROcess FIeldBUS, decentralized peripheral), now based on EN 50170, has been developed for industrial automation in Germany as an open data transmission system at the end of the 80’s and has been standardized under DIN 19245 in 1991. The then-three standardized formulations (FMS, DP and PA) are now widely used today in the worldwide standard of continuous auxiliary peripheral SPS networks (input and output structure groups, frequency converters, control systems, etc.) and in PROFIBUS PA explosion hazard environments. Figure 9 shows a typical PROFIBUS configuration consisting of Busmaster (SPS) and Slaves. Almost all automation technology devices can be used as PROFIBUS-DP interfaces.
Figure 10 shows the fields of use of the data transmission paths system described in the hierarchy of automation in a mill.