Why do the id and l registers return unexpected values?

SunSpec devices chain multiple models in memory starting at address 0, 40000, or 50000. If you target offset 0 but the device places Model 305 further along the chain, the id register will reflect a different model. Scan the device map from its base address to locate the block where id equals 305.

The lat and long registers read zero even though the device has GPS.

A value of zero for a signed int32 coordinate is valid mathematically but usually indicates the GPS receiver has not yet acquired a fix. Allow several minutes after power-up for satellite acquisition, and ensure the antenna has an unobstructed sky view. Persistent zeros suggest antenna or firmware issues.

The tm register returns garbled characters instead of a time string.

Model 305 stores time as a multi-register string in hhmmss.sssZ format across six consecutive 16-bit registers. Some Modbus masters read only one register. Ensure your read request covers the full six-register span starting at address 2, and that your client interprets the result as a contiguous ASCII string, not individual integers.

Latitude starts at address 32 but my tool expects offset 22. Which is correct?

There is a known discrepancy between the SunSpec Environmental Model PDF (which places Latitude at offset 22) and the authoritative SMDX and XLS specifications (which place it at offset 32, matching this template). Trust the SMDX-derived register addresses used here; they align with the OpenEMS implementation.

Free Modbus template: SunSpec Model 305 — GPS

SunSpec Model 305 is the GPS environmental model defined by the SunSpec Alliance for distributed energy resource devices. It exposes location and time data from an on-device GPS receiver over Modbus TCP, delivering UTC time, UTC date, a free-text location label, latitude, longitude, and altitude. The template reads all eight registers in the model's fixed block, giving your ModbusCloud Gateway a precise, timestamped geographic reference for any SunSpec-compliant inverter, meter, or controller that implements Model 305. This makes site identification and time-synchronisation auditing straightforward without manual configuration.

Name	Address	Function	Type	Unit	Access
id SunSpec model identifier, fixed value 305 for GPS.	0	FC03	uint16		R
l Model length in Modbus registers following this field.	1	FC03	uint16		R
tm UTC time from GPS receiver, formatted hhmmss.sssZ.	2	FC03	string	hhmmss.sssZ	R
date UTC date from GPS receiver, formatted YYYYMMDD.	8	FC03	string	YYYYMMDD	R
loc Free-text location label or site description string.	12	FC03	string	text	R
lat Geographic latitude in degrees, signed integer.	32	FC03	int32	Degrees	R
long Geographic longitude in degrees, signed integer.	34	FC03	int32	Degrees	R
alt Altitude above sea level in meters, signed integer.	36	FC03	int32	meters	R