Important News for Manufacturers Seeking UL 1741 SB Compliance

Many smart inverter and energy storage system manufacturers are incorporating the SunSpec Modbus interface into their products to achieve UL 1741 SB compliance. If your company matches this description, we have some important information to share.

The SunSpec Modbus IEEE 1547-2018 Profile Specification and Implementation Guide defines the specific requirements for achieving IEEE 1547.1 interoperability as mandate in clause 6 of the IEEE specification. This document points to and incorporates two other SunSpec specifications: the SunSpec Device Information Model Specification and the SunSpec Modbus Conformance Test Procedures.

Changes to SunSpec Specifications

On May 11, SunSpec made updates to the SunSpec Device Information Model Specification and the SunSpec Modbus Conformance Test Procedures to make an implicit Modbus requirement explicit and testable in the SunSpec Modbus context. We recommend you review these changes.

In a nutshell, section 6.5 of the SunSpec Device Information Model Specification states:

“It is common practice with Modbus devices to perform a read operation after a write operation to verify that the values were successfully written.

When a read operation is performed, the values returned MUST reflect the values contained in the last write request that returned a successful response. The successful write and subsequent read of the values only confirms successful information exchange and does not imply any updates to operational behavior of the device associated with the information.”

A corresponding test procedure was added to the SunSpec Modbus Conformance Test Procedures. Section 2.4.4 (MOD-3 Point Write Verification) of this document states:

“This test validates that all implemented adjustable points in the model can be written and then read without delay. The value read must match the value that was written.”

Section defines the procedure for this test:

“Verify all implemented adjustable points can be written and then read without delay. An automated methodology must be used to perform the write and read sequence to reduce the delay as much as possible and to model typical device to device communication.”


Since the point of this test is to mimic device-to-device interaction, the best way to execute the test is by programmatic, client-to-server communication. Ad hoc testing that is reliant on human interaction is generally insufficient. If you have questions about this, please join the SunSpec Modbus working group or send us an email.


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