SunSpec Modbus Specification Review: Key Updates and Next Steps
We are excited to share that the specification review for several SunSpec Modbus documents is now complete! These revisions bring important improvements to interoperability, performance, and compliance across the distributed energy industry. Our commitment to advancing open standards is reflected in these changes, which ensure that SunSpec Modbus remains a robust tool for seamless communication across the Distributed Energy Resource (DER) ecosystem.
Below, we provide a comprehensive breakdown of the updates made to various SunSpec specifications, followed by interoperability resolutions to address past challenges and next steps that outline our path forward.
Key Specification Updates
- SunSpec Device Information Model Specification
- New Fields Added: Introduction of Detailed Description and Standards fields to enhance clarity.
- Mandated Function Code 6: For single register write operations (Section 6.3).
- RTU Devices Support Broadcast: Reinforces the existing requirement from the base Modbus standard, ensuring consistent communication functionality (Section 6.3).
- Read/Write Timing Update: A maximum delay of 1000 ms is enforced (Section 6.5).
- SunSpec DER Information Model Specification
- New ‘Modbus Enable’ Section: Described in Section 3.3 to improve the utility and understanding of SunSpec Modbus.
- Typos Fixed: Minor typographical corrections throughout the document.
- SunSpec Modbus Conformance Test Procedures
- Expanded Test Coverage:
- Added Broadcast and Device Address write tests for RTU devices.
- Introduced new tests: Single register write, single register read, and rejection tests.
- Timing Update: Specified allowable delay of 1000 ms for read operations following a write.
- SunSpec Modbus Conformance for IEEE 1547 Test Procedures
- Initial Release with New Compliance Tests:
- Mandatory Points: Confirming essential data points for IEEE 1547 compliance.
- Scale Factor Test: Verifying the correct implementation of scale factors.
- SunSpec Information Model Reference Sheet
- New Fields Added: Detailed descriptions and standards for improved reference.
- Model Name Updates: Adjusted names to resolve potential conflicts.
Interoperability Resolutions
- Modbus Function Codes: Compliance with certain Modbus function codes was previously assumed but led to interoperability issues.
- Action: SunSpec has mandated these codes and added corresponding tests to ensure devices support them.
- Broadcast Functionality: Some RTU devices lacked broadcast support, causing communication issues.
- Action: The specifications now explicitly require broadcast support, verified through new test procedures.
- Modbus Enable Clarification: There was confusion about how to implement Enable fields.
- Action: A new section clarifies the correct use of the enable field in the 700 series models.
- Timing Consistency: Inconsistent timing requirements created confusion.
- Action: A definitive delay of 1000 ms has been added to ensure consistent read/write performance.
- Model Clarification: Questions about duplicate points and model implementation arose.
- Action: Detailed descriptions have been added, and duplicate points are marked to maintain backward compatibility.
- Energy Storage Updates: SunSpec is upgrading the 800 series models through a working group focused on improving large-scale site support.
Next Steps
- Security Considerations:
- SunSpec acknowledges the need for enhanced security measures.
- Potential Action: Develop a Secure SunSpec Modbus Certification Program to certify devices implementing TLS.
- SunSpec Docs Version for Certification/Implementation:
- Some stakeholders have asked which version of SunSpec specifications applies to certifications and IEEE 1547 compliance.
- Resolution: The latest specifications available on the SunSpec website should be followed for implementation and certification purposes. However, it’s important to note that the 700 series models are the primary reference for IEEE 1547 compliance.
