Ethercat Protocol

Ethernet frames format

The protocol itself is fully compliant with the Ethernet standard. In fact, ethercat frames are sent on top of ethernet frames. So, you have the traditional ethernet frame with the following characteristics:

• MAC source: MAC address of the Network Interface (XX:XX:XX:XX:XX:XX)
• MAC destination: broadcast address (FF:FF:FF:FF:FF:FF)
• Ethernet type: 0x88A4 (value for ethercat frames)

Ethercat frames content

The remainder of the frame contains two things:

1. One header
2. One or several datagrams

Ethercat frame header

The header of an Ethercat frame contains:

• Length (11 last bits)
• A valid indicator (bit 12)
• A type (bits 13 .. 16)
  • Ethercat command= 0x001

Ethercat datagram

A datagram contains:

1. A header
2. Some data
3. Working count

Ethercat datagram header

The header contains the following fields

• Command (first 8bits)
  • Auto-increment physical Read = 1
  • Auto-increment physical Write = 2
  • Configured address Physical Read = 4
  • Configured address Physical Write = 5
  • Broadcast read = 7
  • Broadcast write = 8
• Index (8bits)
• Slave Address (16bits)
• Offset Address (16bits)
• Length (16bits), with:
  • Length (Last 11 bits)
  • Valid indicator (from bit 3 to 5)
  • Round trip (bit 2)
  • Last indicator - indicate if this is the last ethercat datagram (bit 1)
• Interrupt (16bits)
• Padding to complete the ethernet frame

Useful links

• Ethercat protocol definition for Wireshark: http://wiki.wireshark.org/Protocols/ethercat