PDO vs. SDO

In EtherCAT, values are sent and received from most drives using CANopen PDO’s and SDO’s. In most applications, RapidCode API users need not be aware of these low-level features.

  • PDO (Process Data Object) → Real-time data sent to/from the MotionController to each drive/node for every sample period.

  • SDO (Service Data Object) → Service Channel messages which are not exchanged every cycle.

What are PDOs?

PDOs or Process Data Objects are the values exchanged cyclically with every real-time sample of the MotionController (1kHz by default). For servo drives, these are values such as control/status information and position demand/feedback.

Multi-Threading Note

Writing to the same PDO values is not Multi-Threading safe.

Writing to different PDO values is Multi-Threading safe.

PDO - Reading & Writing

Using RapidSetup

Open the RapidSetup tool then go to Tools > Network Data

Here you can see which PDO’s are being exchanged between our controller and your device/node(s).

Using RapidCode API

Reading Inputs

public void NetworkInputs()
{
    int inputCount = controller.NetworkInputCountGet();
    
    for (int i = 0; i < inputCount; i++)
    {
        int size     = controller.NetworkInputBitSizeGet(i);
        int offset   = controller.NetworkInputBitOffsetGet(i);
        string name  = controller.NetworkInputNameGet(i);
        UInt64 value = controller.NetworkInputValueGet(i);
    }
}

Reading and Writing to Outputs

public void NetworkOutputs()
{
    int outputCount = controller.NetworkOutputCountGet();
             
    for (int i = 0; i < outputCount; i++)
    {
        int size      = controller.NetworkOutputBitSizeGet(i);
        int offset    = controller.NetworkOutputBitOffsetGet(i);
        string name   = controller.NetworkOutputNameGet(i);
        UInt64 value  = controller.NetworkOutputValueGet(i);

        //If you intend to write outputs, you only need to do this once.
        controller.NetworkOutputOverrideSet(i, true);

        //Anytime you want to write a specific output value.
        controller.NetworkOutputOverrideValueSet(i, value); //←Write to a PDO output.
    }
}

Be sure you know what you are doing if you choose to Override an Output. Any automated process which was writing to it, will no longer have control and will not be aware that someone else is in charge.

PDO - Injection

You can inject exchanged information by editing the EtherCATNodeInfo.xml file.

Let's say we want to read/write from the latch status, control, and positions SDO’s because communicating directly with the SDO takes too much time.

Follow the Example below for a solution.

PDO Injection Using EtherCATNodeInfo.xml File

In the EtherCATNodeInfo.xml file I can see that Yaskawa (for example) by default has the following PDO’s configuration:

<PDOs>
    <PDOAssignment Index="0x1601" IsOutput="True"  Include="False" />
    <PDOAssignment Index="0x1a01" IsOutput="False" Include="False" />
    <PDOAssignment Index="0x1600" IsOutput="True"  Include="True" RemoveContent="0x6060 0x6072" />
    <PDOAssignment Index="0x1a00" IsOutput="False" Include="True" />
</PDOs>

Take a look at the Yaskawa-SGD7S.xml ESI file, located in the ESI folder inside your RMP folder:

<Object>
    <Index>#x60B8</Index>
    <Name>Touch probe function</Name>
    <Type>UINT</Type>
    <BitSize>16</BitSize>
    <Flags>
        <Access>rw</Access>
        <PdoMapping>RT</PdoMapping>
    </Flags>
</Object>

<Object>
    <Index>#x60B9</Index>
    <Name>Touch probe status</Name>
    <Type>UINT</Type>
    <BitSize>16</BitSize>
    <Flags>
        <Access>ro</Access>
        <PdoMapping>T</PdoMapping>
    </Flags>
</Object>

<Object>
    <Index>#x60BA</Index>
    <Name>Touch probe 1 position value</Name>
    <Type>DINT</Type>
    <BitSize>32</BitSize>
    <Flags>
        <Access>ro</Access>
        <PdoMapping>T</PdoMapping>
    </Flags>
</Object>

<Object>
    <Index>#x60BC</Index>
    <Name>Touch probe 2 position value</Name>
    <Type>DINT</Type>
    <BitSize>32</BitSize>
    <Flags>
        <Access>ro</Access>
        <PdoMapping>T</PdoMapping>
    </Flags>
</Object>

You can add Entries by editing the EtherCATNodeInfo.xml file by making the following changes:

<PDOs>
    <PDOAssignment Index="0x1601" IsOutput="True"  Include="False" />
    <PDOAssignment Index="0x1a01" IsOutput="False" Include="False" />
    <PDOAssignment Index="0x1600" IsOutput="True"  Include="True" RemoveContent="0x6060" />  
    <PDOAssignment Index="0x1a00" IsOutput="False" Include="True"  />
        <AddEntry Name="Touch probe 1 position value" Index="0x60BA" SubIndex="0" BitLen="32" DataType="DINT" />
        <AddEntry Name="Touch probe status"           Index="0x60B9" SubIndex="0" BitLen="16" DataType="UINT" />
        <AddEntry Name="Touch probe 2 position value" Index="0x60BC" SubIndex="0" BitLen="32" DataType="DINT" />
    </PDOAssignment>
</PDOs>

This works well for Inputs <Access>ro</Access> like Touch probe values and status. But it does not work currently for Outputs <Access>rw</Access> such as the Touch probe function (0x60B8). Right now if you add an output in, we will write 0 into it every cycle. Unless you Override the sent value (see next section).

PDO - Output Value Override

Using RapidSetup

All you need to do is set the value under the OVERRIDE VALUE column and then check the checkbox under the OVERRIDE column. (See image below)

Using RapidCode API

Enabling Override Feature

motionController.NetworkOutputOverrideSet(index, outputOverride);

Where:

index → the PDO Network Output index to read from. outputOverride → boolean that will enable or disable the override feature for specific PDO index.

Setting Override Value

motionController.NetworkOutputOverrideValueSet(index, value);

Where:

index → the index of the PDO Network Output to Override. outputOverride → Your desired value (must be an unsigned 64-bit value, you may need to use a union)

What are SDOs?

SDOs or Service Data Object are messages in a confirmed service with a kind of handshake. They are used for the access to entries of the object dictionary. Especially the configuration for the requested behavior of the drive adapted to the various possible applications is done by these objects.

SDO’s are not exchanged during every cycle, they are done periodically.

(Not as fast as PDO’s since they must wait for network response)

SDO - Reading & Writing

Using RapidSetup

The easiest way to read a SDO or write to an SDO is by using our RapidSetup utility then click on your drive node. (see images below)

Using RapidCode API

SDO Writing

axis.NetworkNode.ServiceChannelWrite(index, subIndex, byteCount, value);

Where:

index → the memory address to write to. subIndex → the sub index to write to. byteCount → the number of bytes to write. value → the numeric value to write.

SDO Reading

axis.NetworkNode.ServiceChannelRead(index, subIndex, byteCount);

Where:

index → the memory address to read from. subIndex → the sub index to read from. byteCount → the number of bytes to read.

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