The id register returns a value other than 126. Why?

The SunSpec model block for Model 126 may be located at a different offset on this inverter. SunSpec devices use a self-describing chain of models starting at a base address (commonly 40001). Scan forward through the chain using the l (length) register of each model until you reach the block whose id field equals 126, then use that block's start address.

Volt-VAR mode is enabled but the inverter does not respond. What now?

Confirm modena is written as 1 and that actcrv points to a valid curve index between 1 and ncrv. Some implementations require the curve data registers (V and VAr points in the repeating block) to be populated before the model header enable flag is accepted. Also check that no firmware-level grid-code lock overrides remote Modbus control.

Scale-factor registers v_sf or deptref_sf read as 0x8000. Is that correct?

A value of 0x8000 is the SunSpec not-a-number (NaN) sentinel, meaning the scale factor is not supported by this device. If the scale factor is NaN, treat the associated curve point values as unscaled integers. Verify firmware is up to date, as some older versions report NaN for optional scale factors even when the feature is active.

Is it safe to write actcrv or modena while the inverter is producing power?

Writing actcrv switches the reactive-power curve immediately (subject to wintms delay and rmptms ramp). An abrupt curve switch during high-generation periods can cause a step change in reactive power that trips protection relays or violates grid-code ramp-rate limits. Always use wintms and rmptms to stage transitions, and test changes during low-load conditions before applying them to live production sites.

Free Modbus template: SunSpec Model 126 — Static Volt-VAR

Name: SunSpec Model 126 — Static Volt-VAR
Brand: SunSpec
SKU: cff20da3-056b-4423-a57d-3ea0c3c2b948

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Free Modbus template: SunSpec Model 126 — Static Volt-VAR | ModbusCloud

Registers (12)

12 read / 5 write

Name	Address	Function	Type	Unit	Access
id SunSpec model ID, fixed value 126 identifying this model.	0	FC03	uint16		R
l Length of the model block in 16-bit registers.	1	FC03	uint16		R
actcrv Index of the active Volt-VAR curve; write to select a stored curve.	2	FC03	uint16		R/W
modena Enable flag; set to 1 to activate Volt-VAR control mode.	3	FC03	bool		R/W
wintms Time window in seconds before the new curve takes effect.	4	FC03	uint16	Secs	R/W
rvrttms Reversion timeout in seconds; 0 disables automatic reversion.	5	FC03	uint16	Secs	R/W
rmptms Ramp time in seconds for transitioning to the new curve output.	6	FC03	uint16	Secs	R/W
ncrv Number of Volt-VAR curves supported by this device, read-only.	7	FC03	uint16		R
npt Number of curve points supported per curve, read-only.	8	FC03	uint16		R
v_sf Scale factor exponent applied to voltage curve points.	9	FC03	int16		R
deptref_sf Scale factor exponent applied to the reactive-power dependent reference.	10	FC03	int16		R
rmpincdec_sf Scale factor exponent applied to ramp increment and decrement rates.	11	FC03	int16		R

SunSpec Model 126 — Static Volt-VAR

Overview

Use cases

Installation

Things to watch for

Registers (12)

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