Industrial protocols emerged at around the same time as control elements. The first programmable control device appeared in 1968 and along with it communications with sensors and actuators.
At the beginning of the 1970s fieldbuses already existed, although not standardised, and in 1979 ModBus emerged to enable communications between different devices and to create the first industrial communication network, thus allowing users to interact with each other using computers with controllers.
In 1999 the Modbus/TCP specification was created: the first protocol to use TCP for transport layer, IP network level and Ethernet for lower layers.
The 1980s: the golden age of protocols
The 1980s represented the greatest period of expansion for control systems due to the fact that numerous protocols were defined during these years.
Some were proprietary, thus implying that they were closed-specification protocols which only their vendors or creators knew about and could therefore use. In some cases, specific chipsets that included the protocol specification were created so that partners and other vendors could use them. An example of one of these instances is Modbus Plus.
- Industrial Protocols -
The emergence of various protocols created the need to create information converters -or gateways- so that information could be moved between them. These gateways allowed for information to be read by the different network devices and also enabled the exchange of information. These gateways were in charge of translating lower-level protocol information used in the field into higher-level protocol information used in the application or ’user’ layer.
Gateways allow networks that use different protocols as well as different means of communication to interconnect, such as the RS-485/RS-232 Gateways.
The eruption of several industrial communication protocols was not followed by an adjustment to the degree of security to the same extent. As a result, the level of security demanded from protocols concerned the physical environment with the primary aim of withstanding adverse environmental conditions without losing information.
The switch from serial to Ethernet communications
OOriginally, industrial communication protocols worked by using the point-to-point method using standard serial communication, pair copper wire or other proprietary methods. For situations that employed serial communication, the specification used was RS-232, such as the Modbus protocol. Later on in the 1980s with the emergence of sensors based on microprocessors, communication requirements grew and point-to-point communication evolved into a multi-point specification by means of RS-422 and RS-485 communications. This evolution gave rise to field buses such as Profibus.
The Tower of Babel of specifications
Apart from communications based on serial specifications, many vendors developed their own specifications both for hardware as well as for communications. For example, the FieldBus H1 protocol is based on the IEC 1158-2 specification just like many other protocols such as Profibus, etc.
- Specific cable for the Profibus PA protocol according to the IEC 1158-2 standard -
The variety of protocols and cables made it quite difficult to integrate different devices and different protocols into a single system, while at the same time making it necessary to use several specific protocols for each level of the ISA-95 pyramid.
A step towards unification: Ethernet
Ethernet had already appeared a few years earlier. Its appearance made it appear as though the problems surrounding a lack of feasible unification of the physical and data link layers used in field buses from different manufacturers could be solved.
The use of Ethernet entailed some advantages early on both for vendors and customers. These benefits included unification of the physical environment with subsequent cost savings; improved management since a different technician is no longer needed for each proprietary protocol; and the possibility of reaching a wider market given the ability to develop applications for more standardised, superior protocols.
However, the initial phase of Ethernet introduction in industrial communications also sparked criticism, among which stand out:
- A lack of resistance to aggressive environments.
- A lack of adaptations made to standards involving certain industrial sectors.
- Specification changes that were made too quickly compared with the life cycle of industrial devices./li>
- An inability to guarantee packet arrival times.
- An inability to feed end devices.
- A lack of security both in the network electronics as well as in the end device due to the “plug&play” function that enables communication with any device as soon as it is connected.
Ethernet in the industrial system
Ethernet has undergone several improvements in order to be compatible with the peculiarities of these systems and to be completely accepted by this industry. These adjustments have led to what is known as Industrial Ethernet which features the following characteristics:
- Physical evolution which lets elements function under extreme conditions (humidity, temperature, etc.).
- Increase in the Mean Time Between Failures (MTBF), thus quadrupling the expected lifetime of office equipment.
- Use of redundant power feeds with the possibility of using continuous and alternate power as well as Power on Ethernet (PoE).
- Evolution towards high-availability topologies using redundancy protocols, according to the IEC 62439-3 standard.
- Improved time constraints thanks to new network electronics, thus reaching levels of determinism that are even better than the old industrial networks.
- Network security using integrated measures such as MAC port security, protection from denial-of-service attacks, protection from flood of packets, etc.
- Addition of network conditions to the Human Machine Interface, indicating network devices as elements of the SCADA network; data collection using common protocols (SNMP) or specific protocols (OPC).
- Integration in control and supervisory systems (ModBus/TCP, Ethernet/IP, ProfiNET) by means of user profiles that allow end devices to announce themselves on the network and to automatically be recognised.
- M12 M12 connector for industrial Ethernet -
The protocols used in industry have stemmed from both the evolution of old protocols that were based on serial communication and from the development of new standards based on new technologies.
Some examples of the evolution of old protocols are Modbus/TCP, DNP3, Profinet, etc. These protocols employ the functional and security advantages offered by both Ethernet and TCP/IP in order to provide better data transmission capabilities to control systems. Thus, the majority of protocols are based on merging the original data protocol with the data protocol of an Ethernet frame.
Other means of transmission
Apart from Ethernet protocols, other next-generation industrial protocols are entering the scene and are employing wireless or physical transmission methods, such as Power Line Communications (PLC).
Some examples of wireless protocols that we can mention are WirelessHART, an evolution of HART which is used over wireless transmission methods, and ZigBee, a next-generation protocol. Some examples of PLC protocols are Lonworks and G3PLC.
The energy sector, thanks to the utilisation of smart grids, has also introduced several next-generation protocols such as PRIME, DLMS/COSEM, etc; this sector has also presented its own evolution from serial to Ethernet protocols, for example from IEC-101 to IEC-104.
- Current industrial protocols -
These new protocols, which are almost completely based on TCP/IP, incorporate security measures that their predecessors lacked. Some security measures that we can highlight, besides those already offered by Ethernet, are information authentication and encryption.
Despite the fact that the introduction of Ethernet into the industry seemed as though it would eventually lead to the use of a single protocol, several protocols continue to exist today. For this reason, solutions such as OPC remain necessary to the standardisation of communications in control systems.