Abstract
Nowadays, most control systems are based on programmable logic controllers (PLCs) and on commercial programs, but the use of conventional PCs is becoming an increasing by popularity and competitive alternative. After an introduction that underlines the positive and negative aspects of both solutions, EtherCAT, a hard real-time industrial Ethernet protocol that allows the use of both conventional PCs and open-source software, is analyzed. Two different EtherCAT master implementations are compared, which are based on a commercial and on a open-source software, respectively. In order to highlight differences and provide a short reference, a simple example was implemented in the two environments.
TopBackground
Before dealing with the EtherCAT protocol, a rapid overview of the more popular RTEs (Cena, 2011) will be provided. Basically, today, there is a wide range of RTEs, that differs in many aspects, e.g., the performance, the network topology, the use of special designed hardware, the synchronisation protocol, and so on.
EtherNet/IP and PROFINET IO are two examples of RTEs that rely on a star topology based on switches. The first relies on normal switches and uses the Common Industrial Protocol (CIP) to manage communications. Real-time data are exchanged through UDP and watchdogs are used to guarantee real-time requirements. Real-time messages are usually sent according to a producer/consumer paradigm and in multicast. The use of conventional switches degrades the determinism of this protocol due to the queuing of packets in the output ports of switches. The CIP Sync protocol, based on the IEEE 1588 protocol, is used for the synchronisation of the network nodes. On the other side, PROFINET IO follows a very different approach, where modified switches are used to obtain determinism. PROFINET IO divides the network bandwidth into four phases depending on traffic priorities, namely RED, ORANGE, GREEN and YELLOW. The RED traffic is the one with highest priority. This phase is characterised by a reserved bandwidth in the domain of the time, enforced by means of the modified switches. In this phase only RED traffic can be sent. The size and number of RED packets is defined and scheduled “a priori” during network configuration, so this phase is characterized by high determinism. In the ORANGE phase the infrastructure is reserved for ORANGE traffic which is routed using the conventional Ethernet MAC protocol, while the GREEN phase is used for best effort transmissions. The YELLOW phase allows the sending of packets with the same priority as those sent in the GREEN phase. Those packets must be completely transferred within the end of the YELLOW phase. The PTCP protocol, that is a modification of the last version of IEEE 1588 protocol, is used to synchronise switches.
Key Terms in this Chapter
Process Data: Real-time control data, exchanged between the control application and remote devices by means of the communication network.
TwinCAT: A popular commercial EtherCAT master.
Industrial Control Applications: Applications programmed to control an industrial production process.
IEC 61131-3: A standard that illustrates the programming languages used to program industrial control applications.
EtherLab EM: A popular open-source EtherCAT master.
Industrial Network Protocols: Protocols used in industrial networks, developed in order to guaranties a certain degree of performance.
Datagrams: A part of the EtherCAT frame where EtherCAT commands are encoded.
Master/Slave: An approach where only the master node can begin the communication with one or more slaves.
EtherCAT master: The application that coordinates the communication in an EtherCAT network.
EtherCAT: A hard real-time, relatively new, industrial network protocol.