Networks are a great modern convenience, and understanding them can be very important for engineers and managers alike.
This question came up from a vendor during a meeting hosted in our office. Since college, my understanding of how Ethernet works led me to believe that the answer is “No, Ethernet is not deterministic.” But how about the industrial Ethernet networks that are prevalent on the factory floor and used for your Manufacturing Intelligence system?
Does it matter?
Let’s examine why a deterministic network is important. For most control applications, information transmission or receipt is not time-sensitive. However, for faster applications such as motion control, the very nature of the control loop requires a predictable amount of time between the receipt of new data for it work properly. This control scheme requires a deterministic network: one for which the exact time to expect the message is known and predictable. For time-sensitive variables in any control loop, such as motion control, a message arriving late can be very bad news. This can also be the case for messages arriving early.
EtherNet/IP is an Application Layer managed by ODVA that sits on top of the lower network layers. This allows the use of standard networking hardware. The application layer it uses is called CIP (Control and Information Protocol). Utilizing the correct physical layout and the correct switches as well as ensuring that non-EtherNet/IP devices remain off of the subnet, will yield a fast, real-time network. However, this configuration will not provide the level of determinism necessary for motion control. For those applications that need more determinism, EtherNet/IP adds CIPSync, which is an implementation of IEEE 1588, to synchronize clocks throughout the network.
EtherCAT uses a Master/Slave model and some specialized hardware to manage network traffic and is managed by ETG (EtherCAT Technology Group). This model uses specialized hardware. Therefore, in order to maintain the nature of the network, the types of devices that can be used is limited. Also, it is difficult to connect two segments of the network together due to the Master/Slave configuration. However, by locating routers and switches between the Master and Slave, the configuration will yield a fast, real–time, Deterministic network. http://www.ethercat.org/
Ethernet Powerlink (EPL) uses a Master/Slave model without any specialized hardware to manage network traffic and is managed by EPSG (Ethernet POWERLINK Standardization Group). Like EtherNet/IP, EPL does not use specialized hardware, therefore EPL is easier to implement, but must be well engineered. By ensuring that non-EPL devices are never connected to the network segment, this model will provide a real-time, Deterministic network. EPL segments can be connected but must comply with the IEEE 1588 standard to synchronize data between segments. http://www.ethernet-powerlink.org/
PROFInet uses a hardware based solution to manage network traffic and is managed by PROFIBUS & PROFINET International (PI). Each node of a PROFInet network is highly synchronized at the hardware level to allow determinism. Because the devices on the PROFInet network use specialized hardware once connected together they form a real-time, Deterministic network. http://www.profibus.com/
These implementations make deterministic Ethernet on the factory floor possible. With the speeds that Ethernet can now achieve and the addition of the IEEE 1588 synchronization, Ethernet is by far a better choice for control networks than any of the slower proprietary deterministic networks of the past. The evidence of this is that these Ethernet networks are now being certified for safety applications. The IEC 61508 standard applies to these safety systems. Look for manufacturers that comply with this standard and provide a safety integrity level (SIL) rating of their network model.