Why do all irradiance registers return zero at midday?

Confirm the sensor is powered and oriented correctly. Also verify the ModbusCloud Gateway is polling the correct unit ID and IP address. Some SunSpec devices only expose model 306 data after the SunSpec magic marker and Common Model block are successfully read; ensure auto-discovery has completed before reading model-specific registers.

The gateway connects but the id register shows an unexpected value. What now?

Register id (address 0) must return 306 for this model block. If it returns a different value, the Gateway is reading a different model at that offset. SunSpec register addresses are relative to each model's starting offset, not absolute. Confirm the base address of model 306 on your device matches what is configured in the template.

Why does the tmp register show implausible values given its unit is listed as W/m²?

The SunSpec Model 306 specification defines tmp as a temperature point. The W/m² unit in this template reflects a data entry issue in the source definition. Treat the raw register value as a temperature reading and apply any scale factor your device documents. Cross-check against the device's native interface to confirm the correct engineering unit.

Values appear intermittently, then the poll times out. What is causing this?

SunSpec sensors on TCP can drop connections if multiple clients poll simultaneously. Ensure no other Modbus TCP client, such as a local datalogger or SCADA system, is holding an open connection to port 502 at the same time as the ModbusCloud Gateway. Reduce poll frequency if the sensor's connection queue is saturated.

Free Modbus template: SunSpec Model 306 — Reference Point Model

SunSpec Model 306 is the Reference Point Model, a standardized SunSpec information block used in solar PV monitoring to capture site irradiance conditions. Deployed on any SunSpec-compliant weather station or irradiance sensor with Modbus TCP, this template reads the model header registers (id, l) and three irradiance channels: global horizontal irradiance (ghi), plane-of-array irradiance (a), and a second irradiance channel (v), plus an environmental temperature point (tmp). Importing this template into your ModbusCloud portal gives installers consistent, vendor-neutral irradiance data alongside inverter performance registers for yield verification.

Name	Address	Function	Type	Unit	Access
id SunSpec model identifier, fixed value 306 for this model.	0	FC03	uint16		R
l Model block length in Modbus registers, per SunSpec framing.	1	FC03	uint16		R
ghi Global horizontal irradiance measured at the reference point, W/m².	2	FC03	uint16	W/m2	R
a Plane-of-array irradiance at the reference point, W/m².	3	FC03	uint16	W/m2	R
v Second irradiance channel at the reference point, W/m².	4	FC03	uint16	W/m2	R
tmp Environmental temperature reading at the reference point, W/m².	5	FC03	uint16	W/m2	R