EtherCAT Basics

State Machine

The EtherCAT state machine is a fundamental concept on setting up an EtherCAT network.

Devices in an EtherCAT network can be in one of the following states:

State	Configuration	Process Data	Electrical Output	Description
INIT	No	No	No	Device can be configured.
PREOP	Yes	No	No	Device can be configured.
SAFEOP	No	Yes	No	Like production but no electrical output.
OP	Yes	Yes	Yes	Production Mode.
BOOT	Yes	No	No

When Ethercrab initializes the network it transfers the devices from INIT to PREOP state.
We can then identify machines and send the configurations to the devices accordingly.
Once the devices are configured we can move them indirectly to OP state.

Now we can read and write process data from the devices.

Network Topology

EtherCAT supports a variety of network topologies like:

Line
Ring
Star
Tree
...

In our case we don't need to worry about the topology as the EtherCAT master (Ethercrab) will take care of it. Most likely we will use a line topology as just daisy chaining the devices together.

Master / Slave

EtherCAT is a master/slave architecture. The EtherCAT is a device as well as a software. Slaves are also devices and need a slave software. The slave software is the Beckhoff firmware which we can't change. The master can be Beckhoff's TwinCAT, SOEM (C++), PySEOM (Python) or in our case Ethercrab (Rust). The master is responsible for the network topology, device discovery, distributed clock synchronization, the state machine and the communication with the slaves. Ethercrab gives us the tools to send SDO requests and access to the PDO data for each individual device.

Only the master can initiate communication. The slaves can only modify the data in the frame.

Buscouplers like EK1100 are not EtherCAT masters but also slaves.

EtherCAT Frame

EtherCAT builds on top of Ethernet. Packets are always broadcasted and traverse the whole network. The frame will eventually return to the master. The devices can read and write to the same frame so often the sent data will return similarly. A "working count" keeps track of the number of devices that have processed the frame.

Each frame can contain multiple EtherCAT datagrams. Each datagram can contain multiple EtherCAT commands. The commands specify the method of addressing and the kind of reading and writing. All this logic is handled by the EtherCAT master (Ethercrab).

Methods of Communication

Process Data / PDU: The process data is a frame that is frequently sent to the devices. It contains an array of binary data. The schema of the data is configured beforehand. The EtherCAT Master (Ethercrab) negotiates with the devices how many bytes they need at what position. The amount of bytes depends on the PDO Assignment which can sometimes be configured but has always a default
Service Data / SDO
- Mailbox
  - CoE: The CoE (CANopen over EtherCAT) is a protocol for a key-value store inside the device. The CoE is reset to default when the d

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