Process data

The process data overview lists the detailed PDO selection. These data are not usually necessary for operation under TwinCAT, since they can be simply configured from the configuration interface via the process data preselection.

Preselection of process data

An EtherCAT device usually offers several different process data objects (PDO) for input and output data, which can be configured in the System Manager, i.e. they can be activated or deactivated for cyclic transmission. See further below for the corresponding overview.

From TwinCAT 2.11 on, with the EtherCAT devices intended for the purpose according to the ESI/XML description, the process data for input and output can be activated simultaneously by appropriate predefined sentences, the so-called "predefined PDO".

In the "Process Data" tab, the EL3255 has

Process data 1:
“Process Data” tab

the following “predefined PDO” sentences (input data only):

Process data 2:
TwinCAT System Manager with the PDO selection

In detail the sentences are composed as follows:

Operating mode

Name

SM2, PDO assignment

SM3, PDO assignment

SM-synchron “frame-triggered”

5 Ch. Standard (default setting)

-

0x1A00
0x1A02
0x1A04
0x1A06
0x1A08

 

4 Ch. Standard

-

0x1A00
0x1A02
0x1A04
0x1A06

 

3 Ch. Standard

-

0x1A00
0x1A02
0x1A04

 

2 Ch. Standard

-

0x1A00
0x1A02

 

1 Ch. Standard

-

0x1A00

 

5 Ch. Compact

-

0x1A01
0x1A03
0x1A05
0x1A07
0x1A09

DC-synchron

5 Ch. Standard

-

0x1A00
0x1A02
0x1A04
0x1A06
0x1A08

Explanation of the process data

The default process image (5 Ch. Standard) encompasses the following data:

Process data 3:
Standard EL3255 process image

The EL3255 (A) has 2-byte variables at its disposal with different bit meanings. These can be seen by expanding the tree (A). They are also displayed in the detail view (B) if the appropriate display function (C) is activated. Alternative process images can be set via the Predefined PDO (D).

The bit meaning i.e. offset position can then be taken from the memory assignment display (F) on the basis of the point notation, also taking into account the variable size (E). "71.2" means here that the 2nd bit (counting method 0,1,2, etc.) or 3rd bit (counting method 1,2,3, etc.) in the status word indicates the overrange. The user requires this information in the PLC if the status word is to be divided into its bit meanings.

Both the collective name e.g. Status and the individual bit variable e.g. Overrange can be linked, but not both at the same time.

Input data

Collective name

Name

Description / function

Bit position [0 - 15]

Status

Underrange

Indicates that the electrical measuring range is undershot

0

 

Overrange

Indicates that the electrical measuring range is exceeded

1

 

Limit 1

Evaluation limit 1

Setting according to CoE object 0x80n0:07, see also Notes

2

 

Limit 2

Evaluation limit 2, setting based on CoE object 0x80n0:08, see also Notes

4

 

Error

An error has occurred,
Error Codes

66

 

Sync error

Signals a synchronization error if “distributed clocks” are activated.

13

 

TxPDO State

Validity of the process data, TRUE means invalid process data on this channel

14

 

TxPDO Toggle

Changes its state 0/1 each time process data are exchanged.

15

Value

 

16-bit measured value 0 - x7FFF

 

WcState

 

Setpoint during operation: 0

Each datagram of the EL3255 indicates its processing state here. This allows the EL3255 to be monitored for correct process data communication.

 

InfoData (State)

 

Setpoint during operation: 8

Status display of the “EtherCAT state machine”

 

AdsAddr

 

AMS address of the responsible EtherCAT Master in the format "0.0.0.0.0.0". In addition, the port number valid for this Slave.
Required for acyclic accesses to the CoE at runtime.

 

Evaluation

For the regular operation of the EL3255 (as for every analog input terminal), proceed as follows via the diagnostic depth:

If the "EL3255" device diagnostics signals no communication, then the subordinate "Channel" level is also no longer to be considered ready for operation.

Calculation of process data

The concept “calibration”, which has historical roots at Beckhoff, is used here even if it has nothing to do with the deviation statements of a calibration certificate. Actually, this is a description of the vendor or customer calibration data/adjustment data used by the device during operation in order to maintain the assured measuring accuracy.

The terminal constantly records measured values and saves the raw values from its A/D converter in the ADC raw value object 0x80nE:01. After each recording of the analog signal, the correction calculation takes place with the vendor and user calibration data as well as the user scaling, if these are activated (see following picture).

Process data 4:
Calculation of process data

Calculation

Designation

XADC

Output of the A/D converter

XF

Output value after the filter

YH = (XADC – BH) x AH x 2-14

Measured value after vendor calibration,

YA = (YH – BA) x AA x 2 -14

Measured value after vendor and user calibration

YS= YA x AS x 2-16 + BS

Measured value following user scaling

Legend

Name

Designation

Index

XADC

Output value of the A/D converter

0x80nE:01

XF

Output value after the filter

-

BH

Vendor calibration offset (not changeable)

0x80nF:01

AH

Vendor calibration gain (not changeable)

0x80nF:02

BA

User calibration offset (can be activated via index 0x80n0:0A)

0x80n0:17

AA

User calibration gain (can be activated via index 0x80n0:0A)

0x80n0:18

BS

User scaling offset (can be activated via index 0x80n0:01)

0x80n0:11

AS

User scaling gain (can be activated via index 0x80n0:01)

0x80n0:12

YS

Process data for controller

-

Process data 5:

Measurement result

The accuracy of the result may be reduced if the measured value is smaller than 32767 / 4 due to one or more multiplications.

Note about the 1-byte status of earlier EtherCAT terminals

Previous analog input terminals from Beckhoff (e.g. EL31x2) had a status byte instead of the status word that is now implemented and therefore a 3-byte interface. 8 additional bits now offer extended diagnostic options, wherein the default process image of the EL31xx now encompasses 4 bytes, status word and value word. The bit meanings of the LowByte are retained; Limit1 and Limit2 as 2-bit types are shown in the case of the EL31xx.

Process data 6:
3-byte interface of the EL31x2

If the 3-byte interface for linking to the analogue input channel is implemented in existing PLC projects, the TwinCAT System Manager nevertheless offers the possibility to link the EL31xx with a 4-byte interface.

To do this, open the link dialog as usual by double-clicking on the variable and activate the AllTypes checkbox. As a result, variables with differing sizes are also offered for linking. Select the corresponding 1-byte input variable for your application. The 8 bit coverage is confirmed in the following SizeMismatch dialog, Fig. Element-orientated process image under TwinCAT 2.10.

Process data 7:
Connecting the 4-byte interface of the EL31xx to a 3-byte interface existing in the project