Register address 0 returns an unexpected value. What is wrong?

Model 128 is part of a chained SunSpec block. Its absolute Modbus address depends on where it is placed in the device's model chain, not on address 0 of the device. Use SunSpec auto-discovery or consult the device's allocation table to find the correct base address before reading relative offsets.

The modena flag is set but the inverter injects no reactive current. Why?

The function will not activate if the voltage deviation stays inside the deadband defined by DbVMin and DbVMax. Confirm those boundaries are set to non-zero values appropriate for your grid voltage, and check that the blocking-zone voltage BlkZnV is not masking the response.

Gradient values read back as unexpectedly large or small numbers.

ArGraSag and ArGraSwell are scaled integers. Divide the raw register value by 10 raised to the power of the argra_sf register (a signed int16 that may be negative). Always read argra_sf in the same poll cycle and apply it before interpreting the gradient registers.

Writing a new gradient setpoint had no visible effect on the inverter.

Several SunSpec-compliant inverters treat Model 128 registers as read-only or require a separate activation sequence. Verify that modena (address 5) is set to the ENABLED bit value, that the inverter firmware supports write access to this model, and that no higher-priority control mode is overriding the setting. Writing incorrect gradient values can also push the inverter outside permissible grid-code limits, so validate setpoints before applying them to live plant.

Free Modbus template: SunSpec Model 128 — Dynamic Reactive Current

Name: SunSpec Model 128 — Dynamic Reactive Current
Brand: SunSpec
SKU: aaad3be7-b294-4818-845f-40854cec0787

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Free Modbus template: SunSpec Model 128 — Dynamic Reactive Current | ModbusCloud

Registers (15)

15 read / 11 write

Name	Address	Function	Type	Unit	Access
id SunSpec model identifier, fixed value 128 for this model.	0	FC03	uint16		R
l Length of the model block in registers, read-only.	1	FC03	uint16		R
argramod Reactive current gradient mode: edge (0) or center (1).	2	FC03	uint16		R/W
argrasag Reactive current gradient for voltage sag, percent rated current per percent voltage deviation.	3	FC03	uint16	%ARtg/%dV	R/W
argraswell Reactive current gradient for voltage swell, percent rated current per percent voltage deviation.	4	FC03	uint16	%ARtg/%dV	R/W
modena Enable flag for the dynamic reactive current function.	5	FC03	bool		R/W
filtms Low-pass filter time constant applied to voltage measurement, seconds.	6	FC03	uint16	Secs	R/W
dbvmin Lower deadband boundary; voltage deviations below this produce no reactive current.	7	FC03	uint16	% VRef	R/W
dbvmax Upper deadband boundary; voltage deviations above this produce no reactive current.	8	FC03	uint16	% VRef	R/W
blkznv Block zone voltage threshold; lower boundary below which reactive current is suppressed.	9	FC03	uint16	% VRef	R/W
hysblkznv Hysteresis voltage for the block zone, as percent of VRef.	10	FC03	uint16	% VRef	R/W
blkzntmms Block zone hold time before reactive current support is suppressed, milliseconds.	11	FC03	uint16	mSecs	R/W
holdtmms Hold time reactive current support remains active after voltage recovers, milliseconds.	12	FC03	uint16	mSecs	R/W
argra_sf Scale factor for ArGraSag and ArGraSwell register values.	13	FC03	int16		R
vrefpct_sf Scale factor for all VRef percentage registers in this model.	14	FC03	int16		R