ECAT Industrial Ethernet Utility

EtherCAT Frame Decoder

Decode EtherCAT hexadecimal frames into the EtherCAT frame header and individual datagrams. Inspect command codes, datagram index, addressing fields, data length, circulating and more-datagrams flags, IRQ values, payload bytes and Working Counter values.

✓ EtherType 88A4 ✓ Multiple Datagrams ✓ ADP / ADO ✓ Logical Address ✓ WKC

ECAT

EtherCAT Frame Decode

● Ready

Input Format Ethernet mode expects destination MAC, source MAC and EtherType 88 A4 before the EtherCAT payload.

Length Validation Trailing bytes can occur when captures contain Ethernet padding or additional capture data.

EtherCAT Hex Bytes Hex bytes may be separated with spaces, commas, colons or hyphens. Continuous hexadecimal input and 0x-prefixed bytes are also accepted.

Important: the two-byte EtherCAT frame header is little-endian. Bits 0–10 contain the length of the EtherCAT datagram area, bit 11 is reserved and bits 12–15 contain the EtherCAT protocol type. Each EtherCAT datagram then carries its own command, addressing, data length and Working Counter.

EtherCAT Frame Decode Result Decoded

Decoded EtherCAT Frame —

EtherCAT Type —

Header Length —

Reserved Bit —

Datagrams —

EtherType —

Destination MAC —

Source MAC —

Payload Offset —

Datagram Bytes —

Trailing Bytes —

Working Counters —

Total Input Bytes —

#	Offset	Command	Index	Address	Length	C	M	IRQ	Data	WKC

EtherCAT Datagram Area

—

Frame Breakdown

—

What Is an EtherCAT Frame?

EtherCAT is an industrial Ethernet protocol designed for fast deterministic communication between controllers and distributed field devices. EtherCAT traffic can be carried directly inside an Ethernet II frame using EtherType 0x88A4.

Inside the Ethernet payload, an EtherCAT frame begins with a two-byte header. That header describes the length and protocol type, followed by one or more EtherCAT datagrams.

EtherCAT Ethernet II Frame

Ethernet II:

Destination MAC     6 bytes
Source MAC          6 bytes
EtherType           2 bytes
EtherCAT payload    variable
Optional padding
Ethernet FCS        often removed by capture hardware

EtherCAT directly over Ethernet normally uses EtherType 0x88A4. Packet-capture software frequently omits the physical Ethernet FCS, so this calculator does not require a final Ethernet CRC.

EtherCAT Frame Header

The EtherCAT payload begins with a 16-bit little-endian header.

Bits 0–10
Length

Bit 11
Reserved

Bits 12–15
Type

The length specifies the number of EtherCAT datagram bytes following the two-byte EtherCAT header.

EtherCAT Protocol Type

The upper four bits of the EtherCAT header contain the protocol Type field. For ordinary EtherCAT command datagrams, the commonly encountered type value is 1.

Header word:
0x100C

Length:
0x00C
= 12 bytes

Type:
0x1

Because the header is transmitted little-endian, that value appears on the wire as bytes 0C 10.

EtherCAT Datagram Structure

CMD           1 byte
IDX           1 byte
ADP           2 bytes
ADO           2 bytes
Length/C/M    2 bytes
IRQ           2 bytes
DATA          N bytes
WKC           2 bytes

The minimum datagram therefore consumes twelve bytes even when its data length is zero.

EtherCAT Command Codes

Code	Command	Description
0x01	APRD	Auto Increment Physical Read
0x02	APWR	Auto Increment Physical Write
0x03	APRW	Auto Increment Physical Read Write
0x04	FPRD	Configured Address Physical Read
0x05	FPWR	Configured Address Physical Write
0x06	FPRW	Configured Address Physical Read Write
0x07	BRD	Broadcast Read
0x08	BWR	Broadcast Write
0x09	BRW	Broadcast Read Write
0x0A	LRD	Logical Read
0x0B	LWR	Logical Write
0x0C	LRW	Logical Read Write
0x0D	ARMW	Auto Increment Read Multiple Write
0x0E	FRMW	Configured Address Read Multiple Write

EtherCAT Datagram Index

The IDX byte is used to associate a transmitted datagram with its corresponding response. It is an eight-bit value and is independent of the command code and device address.

CMD:
04

IDX:
21

The datagram index is:
0x21 = 33

EtherCAT ADP and ADO

Physical EtherCAT commands carry two 16-bit address fields: ADP and ADO. Their interpretation depends on the command type.

ADP
Address Position / Configured Station Address

ADO
Address Offset

For auto-increment commands, ADP participates in position-based addressing. For configured physical commands, ADP can represent the configured station address and ADO identifies the register or memory offset.

EtherCAT Logical Address

Logical commands such as LRD, LWR and LRW use the four bytes occupying ADP and ADO as one 32-bit logical address.

Address bytes:

00 10 00 00

Little-endian logical address:

0x00001000

The calculator therefore reports both ADP/ADO and the combined logical address when a logical command is detected.

EtherCAT Datagram Length

Each datagram contains a 16-bit little-endian field whose lower eleven bits represent its data length.

Datagram length word:

Bits 0–10
Data Length

Bit 14
Circulating / C flag

Bit 15
More Datagrams / M flag

Reserved bits are retained separately by the parser so that unexpected values do not silently alter the calculated payload length.

More Datagrams Flag

The M bit indicates that another EtherCAT datagram follows the current datagram in the same EtherCAT frame.

M = 0
Current datagram is final

M = 1
Another datagram follows

The decoder follows the actual encoded datagram lengths and can list multiple commands from a single frame.

EtherCAT Working Counter

Every EtherCAT datagram ends with a two-byte Working Counter, commonly called WKC. EtherCAT slave processing modifies the counter according to whether the addressed operation was successfully processed.

WKC bytes:

01 00

Little-endian:

0x0001
= 1

A WKC of zero can indicate that no applicable slave processed the datagram, while larger values can represent successful participation by one or more devices. The exact expected counter depends on the topology and command.

EtherCAT IRQ Field

The two-byte IRQ field is present in each EtherCAT datagram before the data area. It is decoded as a 16-bit little-endian value and shown without assigning application semantics that cannot be proven from the packet alone.

EtherCAT vs Standard Ethernet

EtherCAT still uses standard Ethernet framing, MAC addresses and physical-layer technology, but EtherCAT processing differs from normal store-and-forward application communication. EtherCAT slave controllers can process relevant data while the frame passes through them.

Feature	EtherCAT	Typical Ethernet Application
EtherType	0x88A4 for native EtherCAT	Depends on upper protocol
Datagrams per Ethernet frame	One or more	Protocol-specific
Working Counter	Yes	No equivalent Ethernet field
Device processing	On-the-fly architecture	Usually full-frame reception first

EtherCAT Frame Decoder FAQs

What EtherType does native EtherCAT use?

Native EtherCAT over Ethernet uses EtherType 0x88A4.

How long is the EtherCAT frame header?

The EtherCAT header at the beginning of the EtherCAT payload is two bytes.

Is the EtherCAT header little-endian?

Yes. The 16-bit EtherCAT header is interpreted in little-endian byte order.

What does APRD mean?

APRD means Auto Increment Physical Read.

What does FPRD mean?

FPRD means Configured Address Physical Read.

What does LRW mean?

LRW means Logical Read Write.

What is the EtherCAT WKC?

WKC means Working Counter. It is a two-byte value modified as addressed devices successfully process the datagram.

Can one EtherCAT frame contain multiple datagrams?

Yes. Multiple EtherCAT commands can be packed into the same EtherCAT frame.

What does the M flag mean?

The M flag indicates that another EtherCAT datagram follows the current one.

Does this decoder require an Ethernet FCS?

No. Ethernet packet captures commonly omit the physical Frame Check Sequence, so the tool decodes the Ethernet header and EtherCAT payload without requiring an Ethernet CRC.

Can this tool identify PDO signal names?

No. Raw EtherCAT datagrams do not contain the engineering names and scaling of PDO entries. Those require the matching EtherCAT configuration or device description.

Decode EtherCAT Commands and Working Counters

Inspect EtherCAT frame headers, physical and logical addresses, datagram commands, payload lengths, multi-datagram flags, IRQ fields and Working Counter values directly from hexadecimal industrial Ethernet traffic.