Register id returns a value other than 502. Why?

The model block base address may be wrong. SunSpec devices place models at dynamic offsets discovered by walking the device map from the SunSpec start address. Confirm the base address of Model 502 by scanning from the SunSpec identifier register rather than assuming a fixed offset.

Current and power readings look wrong by a factor of 10 or 100. What is the cause?

SunSpec uses signed integer scale factors. The raw outa, outpw, ina, and inw values must be multiplied by 10 raised to the power of a_sf or w_sf respectively. If your integration ignores these scale factor registers, all derived readings will be off by a power of ten.

The gateway connects but outwh and inwh show zero or no change. What should I check?

Energy accumulator registers (outwh, inwh) are uint32 and increment only while the module is producing power. Verify the module's stat register is in an active operating state and that the module is not in a fault condition indicated by the evt bitfield.

Is it safe to write to the ctl, ctlvend, and ctlval registers in normal monitoring?

No. These are read-write control registers that send commands to the module. Writing incorrect values to ctl or ctlval can alter module operating mode, interrupt production, or trigger protective shutdowns. Only write to these registers if you have verified the command sequence in the device's documentation and understand the consequences.

Free Modbus template: SunSpec Model 502 — Solar Module

Name: SunSpec Model 502 — Solar Module
Brand: SunSpec
SKU: ec7c3bfd-c31a-4bf8-bcee-4073665c3bf1

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Free Modbus template: SunSpec Model 502 — Solar Module | ModbusCloud

Registers (23)

23 read / 3 write

Name	Address	Function	Type	Unit	Access
id SunSpec model identifier, fixed value 502 for Solar Module.	0	FC03	uint16		R
l Length of this model block in 16-bit Modbus registers.	1	FC03	uint16		R
a_sf Scale factor exponent applied to current measurements (A).	2	FC03	int16		R
v_sf Scale factor exponent applied to voltage measurements (V).	3	FC03	int16		R
w_sf Scale factor exponent applied to power measurements (W).	4	FC03	int16		R
wh_sf Scale factor exponent applied to energy measurements (Wh).	5	FC03	int16		R
stat Operating status of the solar module, enumerated state code.	6	FC03	uint16		R
statvend Vendor-defined module status code for manufacturer-specific states.	7	FC03	uint16		R
evt Standard event bitfield reporting module fault and alarm conditions.	8	FC03	uint32		R
evtvend Vendor-defined event bitfield for manufacturer-specific fault flags.	10	FC03	uint32		R
ctl Module control command register, selects the active control action.	12	FC03	uint16		R/W
ctlvend Vendor-defined control qualifier accompanying the ctl command.	13	FC03	uint32		R/W
ctlval Control setpoint value associated with the active ctl command.	15	FC03	int32		R/W
tms Timestamp of the current data snapshot, seconds since epoch.	17	FC03	uint32	Secs	R
outa Module output current in amperes, scaled by a_sf.	19	FC03	int16	A	R
outv Module output voltage in volts, scaled by v_sf.	20	FC03	int16	V	R
outwh Cumulative energy delivered at module output, watt-hours.	21	FC03	uint32	Wh	R
outpw Instantaneous output power in watts, scaled by w_sf.	23	FC03	int16	W	R
tmp Module temperature in degrees Celsius.	24	FC03	int16	C	R
ina Module input current in amperes, scaled by a_sf.	25	FC03	int16	A	R
inv Module input voltage in volts, scaled by v_sf.	26	FC03	int16	V	R
inwh Cumulative energy received at module input, watt-hours.	27	FC03	uint32	Wh	R
inw Instantaneous input power in watts, scaled by w_sf.	29	FC03	int16	W	R