Introduction to EtherCATOctober 27, 2020 by Carlos Aguilar
Learn about EtherCAT and some important considerations to make when choosing options for communication and networking.
EtherCAT is an Ethernet-based Fieldbus protocol that is designed to connect Industrial devices at high speed giving real-time communication between the controller and devices like servo drives or drives for motor synchronization. One of its advantages is that it can use Ethernet networks to communicate with industrial devices in real-time.
Real-time means that all the information is delivered without excessive delay and with very low variations in jitter time, so this protocol can ensure constant and high-speed communication.
Due to its characteristics, EtherCAT protocol is often seen in robotic applications, packaging machines, automated assembly machines, safety applications, and others. All of these application examples have a common factor that they require fast communications.
EtherCAT Vs. Ethernet
EtherCAT is based on the same standard as Ethernet. EtherCAT uses IEEE 802.3, which contains all physical and data link standards to communicate all devices in a Local Area Network (LAN), such as an industrial shop floor.
Analyzing the 7 layers of the OSI model and comparing EtherCAT with a TCP/IP package, EtherCAT only uses the layer 1 and layer 2 for transmitting the data. Looking at the table in bold, we can see the data link and physical layers that are common for both.
7 Layers of the OSI Model
In layers 3-7, the EtherCAT deviates from a normal Ethernet model because EtherCAT reduces the payload information sent in a typical Ethernet frame. It doesn’t use the network and transport layers because the network is much more simple and does not need to be burdened with the extra layers. For example, the frame doesn’t have an IP packet in its payload. The application data is on the frame directly and only puts some header information.
Standard 802.3 Frame
|Preamble||Start of frame delimiter||
|Payload||PAD||Frame Check Sum|
An EtherCAT frame looks similar to an Ethernet frame. The difference is that the Ethernet Type indicates that it is an EtherCAT frame with the identifier 0x88A4, causing the sending and receiving devices to recognize the simpler frame. The payload in an Ethernet frame is normally the network package that contains the information of the network, this changes for EtherCAT.
In Ethernet, the frame has the information of one device sending data directly to another, or more typically, the same data sent to all the connected devices (multicast or broadcast). In EtherCAT, one frame can send information originating from any device and going to any other device on the network. It can do this because the payload of the frame has a specific space for each device to write data to send. This is called a ‘datagram’. Each datagram has a header with information on what devices can access the data, and if it can be written or read-only.
The EtherCAT graphic showing what the cables look like. Image courtesy of EtherCAT.
By putting multiple devices’ information into one frame, the network will have fewer frames passing through to all the devices, reducing delay and jitter. In Ethernet, one frame is sent to each device and must contain more information to reach its destination. The simplicity of EtherCAT also makes the connection reliable. Having more frames in the network can make the jitter higher.
Ethernet communicates with a lot of devices and moves a high volume of information like images, videos, etc. However, EtherCAT connects devices at higher speed with a lower volume of information like 32-bit Registers, analog I/Os, digital I/Os, etc. It gives time accuracy with timestamps added. EtherCAT can connect up to 65,535 in one segment, so large network sizes are not a problem.
For EtherCAT, it’s not necessary to have a network element like a switch to send data. Every device has 2 ports to connect each other and all devices can forward frames, however, a switch or hub can still be connected to the network, if necessary. The network topologies supported by EtherCat are:
With the ring topology, EtherCAT has redundancy and has a fast recovery from a broken link. This topology means that from the main controller device, each following device has an incoming port as well as outgoing to the next device. The last device finally has a connection back to the main controller. It's a very simple topology, and this equipment has 2 ports to create a ring network without needing to add networking equipment that can reduce some costs. If any network cable is broken around the ring, there is still a connection to every device.
EtherCAT has the flexibility to move Ethernet information through the network. This function is called EoE (Ethernet over EtherCAT). This EtherCAT is able to send all data that Ethernet can move like web pages, emails, files, etc.
Cables for EtherCAT and Important Considerations
As mentioned before, the layer 1 of EtherCAT is the same as Ethernet. Due to this, cables and connectors of Ethernet can be used in an EtherCAT network.
It's not usually recommended to use standard cables that are used to connect computers. The physical scenario is different for EtherCAT because this protocol is for industrial applications. One important factor is the electromagnetic noise. Industrial Ethernet cables have conductive shields to prevent an electromagnetic induction to the cable.
Another important factor is that all mechanical and chemical conditions are different. The cables may be installed in places with high or very low temperature, so the material of the cable jacket is very important.
Connectors need to be an industrial degree and can be an RJ45 with ground or M8 connectors, these connectors are used for IP67 protection.
Because EtherCAT has the same physical layer as Ethernet, depending on the equipment specs, we can use twisted-pair or optical fiber to connect the devices, and different category cables of Ethernet depending on the communication speed. This means it can send information at speeds of 10Mbps, 100 Mbps, or 1Gbps depending on the characteristics of the equipment's Ethernet port and cable category.
EtherCAT can be better in applications that need high-speed communication and better time accuracy. EtherCAT is also more suitable for creating a simpler network because the end devices reduce the need for switches and hubs which means less hardware.