BECKHOFF EtherCAT Technology Section I - Manual (Page 24)

EtherCAT Protocol
I-4 Slave Controller – Technology
2 EtherCAT Protocol
EtherCAT uses standard IEEE 802.3 Ethernet frames, thus a standard network controller can be used
and no special hardware is required on master side.
EtherCAT has a reserved EtherType of 0x88A4 that distinguishes it from other Ethernet frames. Thus,
EtherCAT can run in parallel to other Ethernet protocols
1 .
EtherCAT does not require the IP protocol, however it can be encapsulated in IP/UDP. The EtherCAT
Slave Controller processes the frame in hardware. Thus, communication performance is independent
from processor power.
An EtherCAT frame is subdivided into the EtherCAT frame header followed by one or more EtherCAT
datagrams. At least one EtherCAT datagram has to be in the frame. Only EtherCAT frames with Type
1 in the EtherCAT Header are currently processed by the ESCs. The ESCs also support IEEE802.1Q
VLAN Tags, although the VLAN Tag contents are not evaluated by the ESC.
If the minimum Ethernet frame size requirement is not fulfilled, padding bytes have to be added.
Otherwise the EtherCAT frame is exactly as large as the sum of all EtherCAT datagrams plus
EtherCAT frame header.
2.1 EtherCAT Header
Figure 2 shows how an Ethernet frame containing EtherCAT data is assembled.
Ethernet Header
Destination Source
EtherType
0x88A4
Destination Source
Destination Source
EtherType
0x0800
Destination Source
EtherType
0x0800
IP Header
UDP Header
dest. port 0x88A4
EtherType
0x88A4
Ethernet Data
EtherCAT Data
EtherCAT Header
FCS
FCS
FCS
FCS
FCS IP Header VLAN Tag
Datagrams
EtherCAT Header
EtherCAT Header
Length Res. Type
48 bit 48 bit 16 bit 32 bit
11 bit 1 bit 4 bit
160 bit 64 bit
32 bit
46-1500 Byte
16-1478 Byte
Datagrams
Datagrams
44-1498 Byte
16-1478 Byte
64-1522 Byte
UDP Header
dest. port 0x88A4
Destination Source
EtherType
0x88A4
FCS EtherCAT Header Datagrams
16 bit 44-1498 Byte
Basic EtherCAT frame
EtherCAT in
UDP/IP frame
Basic EtherCAT frame with
VLAN tag
EtherCAT in UDP/IP frame
with VLAN tag
Basic EtherCAT frame
Ethernet frame
EtherCAT frame header
VLAN Tag
Figure 2: Ethernet Frame with EtherCAT Data
Table 2: EtherCAT Frame Header
Field Data Type Value/Description
Length 11 bit Length of the EtherCAT datagrams (excl. FCS)
Reserved 1 bit Reserved, 0
Type 4 bit Protocol type. Only EtherCAT commands (Type = 0x1) are supported
by ESCs.
NOTE: The EtherCAT header length field is ignored by ESCs, they rely on the datagram length fields.
1 ESCs have to be configured to forward non-EtherCAT frames via DL Control register 0x0100[0].