Why do dca, dcv, and dcw all read zero or 0x8000?

A raw value of 0x8000 (32768 for int16) is the SunSpec 'not implemented' sentinel. The combiner has not populated those points, often because no strings are connected or the device firmware requires a minimum number of configured inputs. Check that string inputs are active and the combiner is producing DC current.

Scaled current values look wildly wrong. What is happening?

SunSpec uses signed int16 scale factor registers (dca_sf, indca_sf, etc.) as powers of ten. A value of -2 means divide the raw integer by 100. If your Modbus master treats these as unsigned, the negative exponent becomes a large positive number and the scaled result is nonsense. Ensure your client reads scale factor registers as signed int16.

The gateway connects but register n always reads 0.

Register n reports the number of configured input strings. On many combiners this is set during commissioning via the device web interface or a DIP switch bank. If strings have not been configured in the combiner itself, n will remain 0 and per-string blocks will not be populated. Verify the combiner's input count setting matches your physical installation.

The gateway cannot reach the combiner on TCP port 502.

SunSpec Modbus TCP devices typically listen on port 502, but some combiners use a non-standard port or require Modbus TCP to be explicitly enabled in firmware. Check the combiner's communication settings, confirm the IP address is reachable with a ping, and verify the unit ID matches the configured Modbus slave address.

Free Modbus template: SunSpec Model 404 — String Combiner (Advanced)

Name: SunSpec Model 404 — String Combiner (Advanced)
Brand: SunSpec
SKU: 245926ff-cc04-42f3-aa27-2b5302ea3448

Try the demo Shop

Free Modbus template: SunSpec Model 404 — String Combiner (Advanced) | ModbusCloud

Registers (23)

23 read / 0 write

Name	Address	Function	Type	Unit	Access
id SunSpec model identifier, value 404 for this model.	0	FC03	uint16		R
l Model block length in 16-bit Modbus registers.	1	FC03	uint16		R
dca_sf Scale factor exponent applied to combiner DC current readings.	2	FC03	int16		R
dcahr_sf Scale factor exponent applied to combiner amp-hour accumulator.	3	FC03	int16		R
dcv_sf Scale factor exponent applied to combiner DC voltage readings.	4	FC03	int16		R
dcw_sf Scale factor exponent applied to combiner DC power readings.	5	FC03	int16		R
dcwh_sf Scale factor exponent applied to combiner DC energy accumulator.	6	FC03	int16		R
dcamax Rated maximum DC current capacity of the combiner, amperes.	7	FC03	uint16	A	R
n Number of active input strings configured on the combiner.	8	FC03	uint16		R
evt Combiner-level SunSpec event flags, 32-bit bitmask.	9	FC03	uint32		R
evtvnd Vendor-specific combiner event flags, 32-bit bitmask.	11	FC03	uint32		R
dca Total aggregated DC current output from all strings, amperes.	13	FC03	int16	A	R
dcahr Cumulative amp-hours delivered by the combiner since last reset.	14	FC03	uint32	Ah	R
dcv Combined DC bus voltage at the combiner output, volts.	16	FC03	int16	V	R
tmp Combiner enclosure or electronics temperature, degrees Celsius.	17	FC03	int16	°C	R
dcw Total DC power output from the combiner, watts.	18	FC03	int16	W	R
dcpr DC performance ratio of the combiner array, percent.	19	FC03	int16	Pct	R
dcwh Cumulative DC energy delivered by the combiner, watt-hours.	20	FC03	uint32	Wh	R
indca_sf Scale factor exponent for per-string current readings.	22	FC03	int16		R
indcahr_sf Scale factor exponent for per-string amp-hour accumulators.	23	FC03	int16		R
indcv_sf Scale factor exponent for per-string voltage readings.	24	FC03	int16		R
indcw_sf Scale factor exponent for per-string power readings.	25	FC03	int16		R
indcwh_sf Scale factor exponent for per-string energy accumulators.	26	FC03	int16		R