EtherCAT Composite Types#

This document describes how the TwinCAT EtherCAT Master generates composite data types for terminal process data, and which types are created for different terminal categories.

Overview#

When TwinCAT compiles an EtherCAT configuration, it creates composite types (structured data types) for each terminal’s process data. These types group related fields into single addressable units.

The EtherCAT Master generates these types automatically based on the PDO (Process Data Object) definitions in each terminal’s ESI XML file.

Type Generation Process#

From XML PDO to Composite Type#

The ESI XML defines PDOs with individual entries:

<TxPdo Fixed="1">
    <Index>#x1a02</Index>
    <Name>AI Standard Channel 1</Name>
    <Entry>
        <Name>Status</Name>
        <DataType>UINT</DataType>
        <BitLen>16</BitLen>
    </Entry>
    <Entry>
        <Name>Value</Name>
        <DataType>INT</DataType>
        <BitLen>16</BitLen>
    </Entry>
</TxPdo>

TwinCAT generates a composite type:

Type Name: "AI Standard Channel 1_TYPE"
ADS Type: 65 (BIGTYPE)
Size: 4 bytes
Members:
  - Status (UINT, offset 0, 2 bytes)
  - Value (INT, offset 2, 2 bytes)

Naming Convention#

TwinCAT constructs type names by appending _TYPE to the PDO name:

PDO Name	Generated Type Name
`AI Standard Channel 1`	`AI Standard Channel 1_TYPE`
`CNT Inputs`	`CNT Inputs_TYPE`
`AO Output Channel 1`	`AO Output Channel 1_TYPE`
`Inputs`	`Inputs_TYPE`

Note: The type name always uses the first channel’s name (e.g., “Channel 1”), even when the same type is used for all channels.

Channel Replication#

For multi-channel terminals, TwinCAT creates:

One type definition shared across channels
Multiple symbol instances using that type

Example for EL3104 (4-channel analog input):

Type: "AI Standard Channel 1_TYPE" (defined once)

Symbols:
  - AI Standard Channel 1 (uses AI Standard Channel 1_TYPE)
  - AI Standard Channel 2 (uses AI Standard Channel 1_TYPE)
  - AI Standard Channel 3 (uses AI Standard Channel 1_TYPE)
  - AI Standard Channel 4 (uses AI Standard Channel 1_TYPE)

Composite Types by Terminal Category#

Analog Input Terminals (EL31xx, EL32xx, EL33xx)#

Standard Analog Input (EL3004, EL3104, etc.)

PDO: AI Standard Channel N

Member	Type	Offset	Description
Status	UINT	0	Status word (underrange, overrange, error flags)
Value	INT	2	Signed 16-bit analog value

Total size: 4 bytes

High-Resolution Analog Input (EL3602, EL3612)

PDO: AI Inputs Channel N

Member	Type	Offset	Description
Status	UINT	0	Status word
Value	DINT	2	Signed 32-bit analog value (24-bit resolution)

Total size: 6 bytes

Thermocouple/RTD Input (EL3202, EL3204, EL3314)

PDO: RTD Inputs Channel N or TC Inputs Channel N

Member	Type	Offset	Description
Status	UINT	0	Status word
Value	INT	2	Temperature value (scaled)

Total size: 4 bytes

Analog Output Terminals (EL4xxx)#

Standard Analog Output (EL4002, EL4004, EL4102)

PDO: AO Output Channel N

Member	Type	Offset	Description
AnalogOutput	INT	0	Signed 16-bit output value

Total size: 2 bytes

Some terminals include status feedback:

Member	Type	Offset	Description
AnalogOutput	INT	0	Output value
Status	UINT	2	Output status

Total size: 4 bytes

Digital Input Terminals (EL1xxx)#

Simple Digital Input (EL1004, EL1008)

PDO: Channel N^Input or Inputs

For single-bit access:

Member	Type	Offset	Description
Input	BOOL	0 (bit N)	Single digital input state

For grouped access (all channels in one word):

Member	Type	Offset	Description
Inputs	BYTE/UINT	0	Packed digital inputs

Digital Output Terminals (EL2xxx)#

Simple Digital Output (EL2004, EL2008)

PDO: Channel N^Output or Outputs

Member	Type	Offset	Description
Output	BOOL	0 (bit N)	Single digital output state

For grouped access:

Member	Type	Offset	Description
Outputs	BYTE/UINT	0	Packed digital outputs

Counter/Encoder Terminals (EL5xxx)#

Incremental Encoder (EL5101)

PDO: ENC Status (input) and ENC Control (output)

ENC Status (Input):

Member	Type	Offset	Description
Status	UINT	0	Encoder status flags
CounterValue	UDINT	2	32-bit counter value
LatchValue	UDINT	6	Latched counter value

Total size: 10 bytes

ENC Control (Output):

Member	Type	Offset	Description
Control	UINT	0	Control word
SetCounterValue	UDINT	2	Value to set counter

Total size: 6 bytes

Up/Down Counter (EL1502, EL1512)

PDO: CNT Inputs (input) and CNT Outputs (output)

CNT Inputs:

Member	Type	Offset	Description
Status	UINT	0	Counter status
CounterValue	UDINT	2	Current counter value

Total size: 6 bytes

CNT Outputs:

Member	Type	Offset	Description
Control	UINT	0	Counter control
SetCounterValue	UDINT	2	Preset value

Total size: 6 bytes

Oversampling Terminals (EL3702, EL4732)#

Oversampling Analog Input (EL3702)

These terminals produce arrays of samples per cycle:

PDO: Ch1 CycleCount and Ch1 Samples

Member	Type	Offset	Description
CycleCount	UINT	0	Number of samples this cycle
Samples	ARRAY[0..99] OF INT	2	Array of oversampled values

Total size: 202 bytes (2 + 100*2)

Communication Terminals (EL6xxx)#

Serial Interface (EL6001, EL6021)

PDO structure varies by protocol mode. Common pattern:

Input:

Member	Type	Offset	Description
Status	UINT	0	Communication status
DataLength	UINT	2	Received data length
Data	ARRAY OF BYTE	4	Received data buffer

Output:

Member	Type	Offset	Description
Control	UINT	0	Communication control
DataLength	UINT	2	Send data length
Data	ARRAY OF BYTE	4	Send data buffer

Special Types#

Working Counter State#

All terminals include a diagnostic symbol:

PDO: WcState^WcState

Member	Type	Offset	Description
WcState	BOOL	0	Working counter valid (communication OK)

This is typically a single bit, not a composite type.

InfoData (Optional)#

Some terminals provide extended diagnostic information:

PDO: InfoData^State and InfoData^TxPdoState

Member	Type	Offset	Description
State	UINT	0	Terminal state
AdsAddr	UINT	2	ADS address

Type Inheritance and Variants#

PDO Selection#

Many terminals offer multiple PDO mappings (Standard, Compact, Complete). The composite type depends on which PDO mapping is selected:

EL3104 PDO Options:

AI Standard - Status + Value (4 bytes)
AI Compact - Value only (2 bytes)
AI Complete - Full status word + Value (4 bytes)

The generated type name changes accordingly:

AI Standard Channel 1_TYPE
AI Compact Channel 1_TYPE

Revision-Specific Types#

Different terminal revisions may have different PDO structures. TwinCAT generates appropriate types based on the specific revision detected.

Summary Table#

Terminal Category	Common Type Names	Typical Size
Analog Input (16-bit)	`AI Standard Channel 1_TYPE`	4 bytes
Analog Input (24-bit)	`AI Inputs Channel 1_TYPE`	6 bytes
Analog Output	`AO Output Channel 1_TYPE`	2-4 bytes
Digital Input	`Inputs_TYPE` or individual bits	1-2 bytes
Digital Output	`Outputs_TYPE` or individual bits	1-2 bytes
Counter/Encoder Input	`CNT Inputs_TYPE`, `ENC Status_TYPE`	6-10 bytes
Counter/Encoder Output	`CNT Outputs_TYPE`, `ENC Control_TYPE`	6 bytes

EtherCAT Composite Types#

Overview#

Type Generation Process#

From XML PDO to Composite Type#

Naming Convention#

Channel Replication#

Composite Types by Terminal Category#

Analog Input Terminals (EL31xx, EL32xx, EL33xx)#

Analog Output Terminals (EL4xxx)#

Digital Input Terminals (EL1xxx)#

Digital Output Terminals (EL2xxx)#

Counter/Encoder Terminals (EL5xxx)#

Oversampling Terminals (EL3702, EL4732)#

Communication Terminals (EL6xxx)#

Special Types#

Working Counter State#

InfoData (Optional)#

Type Inheritance and Variants#

PDO Selection#

Revision-Specific Types#

Summary Table#

Related Documentation#

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