# SDO Overview [LEGACY]

```{warning}
**Legacy documentation.** This page documents the CANopen implementation shipped with MD firmware
**2.5.x**. It is kept for users who still run that firmware and is no longer updated.

The CANopen stack has been rewritten for the current firmware and is documented in
[CANopen](md_canopen). The two object dictionaries are not compatible, so do not read values across
from that section.
```


## 1. Introduction

**SDOs (Service Data Objects)** in CANopen provide a **request/response mechanism** to access and
configure any object in the **Object Dictionary**.\
Unlike PDOs, SDOs are **slower** but allow reading/writing large or complex data structures.

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## 2. SDO Communication Concept

- SDO communication is always between a **client** and a **server**.
- The **client** initiates the transfer (read or write), and the **server** responds.
- SDOs can transfer data **up to 4 GB** using segmented communication if needed.

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## 3. SDO Message Structure

SDO messages are carried using **CAN-ID 0x600 + Node-ID** for **client → server**, and **0x580 +
Node-ID** for **server → client**.

| **Field**              | **Description**                                                 |
| ---------------------- | --------------------------------------------------------------- |
| CAN-ID                 | 0x600 + Node-ID for requests, 0x580 + Node-ID for responses     |
| Command Specifier (CS) | Defines the type of operation: upload, download, segment, abort |
| Index / Subindex       | Identifies the object in the Object Dictionary                  |
| Data                   | Up to 4 bytes (or segmented for longer data)                    |

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## 4. SDO Commands (Command Specifiers)

| **Command**                   | **Hex Value** | **Description**                        |
| ----------------------------- | ------------- | -------------------------------------- |
| Initiate Download (Expedited) | 0x2x          | Write up to 4 bytes in one message     |
| Initiate Upload(Expedited)    | 0x4x          | Read up to 4 bytes in one message      |
| Initiate Download (Segmented) | 0x2x          | Start writing more than 4 bytes        |
| Initiate Upload (Segmented)   | 0x60          | Start reading more than 4 bytes        |
| Segment Download / Upload     | 0x00 / 0x01   | Transfer next 7 bytes segment          |
| Abort Transfer                | 0x80          | Stop current transfer in case of error |

> Note: x => second bytes depend on the length of the data

> Note: Expedited SDO is used for **small data** (≤ 4 bytes), segmented SDO is used for **larger
> data** (> 4 bytes).

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## 5. Example of SDO Transfer

**Write 16-bit value 0x1234 to object 0x6040:00 (Controlword) on node 0x05:**

| **CAN-ID** | **CS** | **Index** | **Subindex** | **Data**  |
| ---------- | ------ | --------- | ------------ | --------- |
| 0x605      | 0x23   | 0x40 0x60 | 0x00         | 0x34 0x12 |

**Read the same object:**

| **CAN-ID** | **CS** | **Index** | **Subindex** | **Data**  |
| ---------- | ------ | --------- | ------------ | --------- |
| 0x605      | 0x40   | 0x40 0x60 | 0x00         | –         |
| 0x585      | 0x43   | 0x40 0x60 | 0x00         | 0x34 0x12 |

> Warning: The index and the data must be send in little-endian

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## 6. Advantages of SDO

- Access **any object** in the Object Dictionary
- Supports **large data transfers** with segmentation
- Ensures **reliable request/response communication**
- Essential for **configuration, diagnostics, and parameterization**

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## 7. Conclusion

SDOs complement PDOs in CANopen by allowing **flexible and reliable data access**, especially for
configuration and larger or less time-critical data.

✅ Use SDOs for reading/writing configuration parameters\
✅ Use PDOs for real-time process data\
✅ Combine both for a fully functional CANopen system