EtherCAT Process Data Object (PDO)

A Process Data Object (PDO) is a block of user-defined data that is sent between the MainDevice and a SubDevice. A PDO is sent continuously, each loop time of the RMC. This data is called Process Data. PDO’s are typically used for data that must be continuously communicated, such as Target Position, Actual Position, Control Output, Status, etc.

Configuring the PDOs for each SubDevice is an important part of setting up an EtherCAT network. Many SubDevices provide multiple pre-configured PDO’s to choose from, usually pre-configured for the different scenarios in which the SubDevice may be used. Typically, these PDO’s are editable, but sometimes are not editable. See Configuring PDOs below.

Input and Output PDOs

For each SubDevice, there may be input PDOs and output PDOs. Many SubDevices provide both, and some provide only one. For example, a drive provides input and output PDOs, a typical sensor may provide only input PDOs, and a typical discrete output device may provide only output PDOs.

An RMC Input PDO is called a SubDevice Transmit PDO (TxPDO).

An RMC Output PDO is called a SubDevice Receive PDO (RxPDO).

Modules and PDOs

If a SubDevice supports Modules, the PDOs will be pre-defined based on the module or modules that have been selected. Modules typically define both PDO data and Init commands. See EtherCAT Modules for details.

Viewing the PDO Configuration

For each SubDevice, the PDO configuration and the resulting Process Data items can be viewed on the EtherCAT PDO Mapping and Process Data tabs:

In the RMCTools Project View, double-click EtherCAT to open the EtherCAT Editor.
In the EtherCAT Explorer pane, select a SubDevice.
Select the PDO Mapping tab.

The PDO Mapping tab lists all the PDOs that are made available by the SubDevice. Only the data in the checked PDOs will be included in the actual Process Data.
Select the Process Data tab.

The Process Data tab lists only the PDO items that will be communicated. The Direction column specifies whether the data item is an RMC input or RMC output item.

To view the Process Data items for all SubDevices on the network, in the EtherCAT Explorer pane, select the MainDevice, then select the Process Data tab.

Viewing and Forcing Process Data in Real Time

Process Data values can be viewed in real time in the EtherCAT Diagnostics:

In the EtherCAT Editor, on the toolbar, click the Start EtherCAT Diagnostics button.
In the EtherCAT Explorer pane, select a SubDevice.
Select the Process Data tab.

The Value column displays the real-time values.

Items in the Process Data tab can be forced to a value:

In the Variables section, select the desired row.
At the bottom of the Process Data tab, in the Edit Variables section, enter a value and click Force.

The Forced check box will be checked.

This will override the Process Data value until you select that row and click Release.

Configuring PDOs

After adding a SubDevice to the network, the SubDevice PDOs can be configured.

Note: Consult SubDevice documentation before editing PDOs. Each device may have specific requirements for editing PDOs, and the ESI manager does not enforce the rules. See Examples of PDO Requirements below.

To configure SubDevice PDOs:

In the EtherCAT Editor, in the EtherCAT Explorer pane, select a SubDevice.
If the SubDevice has a Modules tab, first choose a module as described in EtherCAT Modules.
On the PDO Mapping tab, do the following:
1. Review the existing PDO Mappings. Click the arrow to expand a PDO mapping.
2. If there are multiple PDOs available, check the checkbox for each desired PDO. Some SubDevices allow multiple PDOs to be selected and others allow only one.
3. To add, remove or change individual items within a PDO, click the desired PDO, then click the Edit button (located below the PDO Mapping box) to open the Edit PDO dialog.
4. In the Edit PDO dialog, select a row in the Entries table and use the Add, Delete, and Edit buttons to change the PDO. See the Edit PDO Dialog topic for more details.
Select the Process Data tab and review the Variables as a double-check that you added the desired items.
In the EtherCAT Editor toolbar, click the Download to Controller button. On the Controller menu, click Update Flash.
In the Project View, right-click EtherCAT and choose Restart Network.

Examples of PDO Requirements

Consult SubDevice documentation before editing PDOs. Each device may have specific requirement for editing PDOs, and the ESI manager does not enforce the rules. For example, some devices do not allow editing of PDO data, and some do not allow multiple PDOs simultaneously while others require multiple PDOs to send more data items.

Some specific examples of PDO limitations are:

Some Bosch Rexroth valves limit the length of the number of bytes of PDO data that can be transferred by loop time. It also has 4 PDOs, each of which is limited to 16 bytes. PDOs must be filled consecutively.
Some Omron drives provide multiple PDO entries in each direction, but only the first PDO entry can be edited. Editing the other PDO entries will appear to work but those PDOs will only provide the data they were originally configured for.

Adding and Removing PDOs

It is usually not necessary, to add or remove PDOs. Extra PDOs that are not checked normally do not cause any problems, and typically need not be removed. Existing PDOs can normally be edited and new data added to them, so entirely new PDOs typically need not be added.

However, in some situations, you may wish to remove or add PDOs. To do so:

In the EtherCAT Editor, for the selected SubDevice, choose the PDO Mapping tab.
Use the Add and Delete buttons.

For any PDO that you add, edit it as described above in Configuring PDOs.

Accessing Process Data in User Programs and Plots

To access any Process Data item via a user program or in plots, that Process Data item must first be added to the RMC I/O Data Map, which assigns that item to an addressable register in the RMC. The register can then be accessed via user programs and plots, just like any RMC register.

See the EtherCAT Input/Output Data Maps for details.

PDO vs SDO

Process Data Objects (PDOs) are for data that needs to be continuously communicated. Other data that does not need to be communicated as frequently can be transferred via Service Data Objects (SDOs). The terms PDO and SDO can be compared to a machine where the process is occurring continuously, but service happens only occasionally.

The RMC200 does not currently support SDO communication but will in a future release.

For more details, see the Service Data Object (SDO) help topic.