How to set the DC Master communication area?

* Question

How to set the DC Master communication area?

* Answer

To set the DC Master communication area, you need to define and configure the memory regions used for exchanging data between the master and its connected slaves. This communication area is crucial for real-time, deterministic control in distributed systems, such as motion controllers using EtherCAT or similar fieldbuses.

1. Understanding the DC Master Communication Area

The DC Master communication area typically consists of:

Send Area (Output): Master writes data to this area; slaves read from it.

Receive Area (Input): Slaves write data here; master reads it.

These areas are mapped in the master’s memory and used for cyclic process data exchange. Each area must be:

Predefined in size

Correctly aligned in memory

Consistently structured to match slave devices’ data expectations

2. Configuration Steps (Generic Process)

Step 1: Define Buffer Sizes

Determine the number of bytes needed for input and output data.

c

#define SEND_AREA_SIZE   128   // e.g., 128 bytes for outputs#define RECV_AREA_SIZE   128   // e.g., 128 bytes for inputs

Step 2: Allocate Memory

Allocate buffers for the send and receive areas.

c

uint8_t sendArea[SEND_AREA_SIZE];uint8_t recvArea[RECV_AREA_SIZE];

Step 3: Set Communication Area in Master Configuration

If you’re using a DC Master API (e.g., via C/C++), use functions provided by the SDK:

c

DCMaster_SetSendArea(sendArea, SEND_AREA_SIZE);

DCMaster_SetReceiveArea(recvArea, RECV_AREA_SIZE);

For platforms like EtherCAT:

Use the configuration tool (e.g., TwinCAT, SOEM, Codesys) to map PDOs (Process Data Objects) into the communication memory.

Step 4: Map Slaves’ Process Data

Assign each slave’s input/output to specific offsets within the send/receive areas.

c

// Example: Set 16 bytes to Slave 1 output starting at offset 0

DCMaster_MapOutput(slaveID = 1, offset = 0, size = 16);

Step 5: Activate Cyclic Communication

Start the DC Master’s real-time loop:

c

DCMaster_Start();

3. Example in EtherCAT Context

If you are using EtherCAT with DC synchronization:

The communication area is configured by:

Mapping RxPDOs (outputs) and TxPDOs (inputs)

Setting memory offsets and data size

In TwinCAT:

Open I/O configuration

Select slave device → Map PDO entries → Assign to variables

Link to memory (e.g., Global Variable List)

4. Best Practices

Summary

To set the DC Master communication area, you must:

Define input/output buffer sizes

Allocate and assign memory

Map each slave’s data to a specific location

Use master SDK or configuration tools to initialize and start communication

This setup ensures fast, synchronized exchange of control and feedback data in real-time applications such as robotics, CNC, or industrial automation.