SunSpec Model 402 — String Combiner (Advanced)
SunSpec Model 402 is the advanced string combiner information model defined by the SunSpec Alliance for multi-string DC collection points in photovoltaic systems. This template reads the combiner-level registers over Modbus TCP, covering total DC current, DC bus voltage, instantaneous power, power as a percentage of rated capacity, cumulative energy in watt-hours, amp-hour accumulation, enclosure temperature, and both standard and vendor event bitmasks. Five scale-factor registers ensure correct decimal placement across all measured quantities. Installers use this template with the ModbusCloud Gateway to monitor string combiner health, catch early fault events, and track long-term DC yield without proprietary software.
- Protocol
- Modbus TCP
- Connection
- Ethernet, Modbus TCP
- Registers
- 18, read only
- Brand
- SunSpec
- Last updated
Registers (18)
18 read, 0 write
| Name | Unit | Access | Address (Hidden until you enter your email) | Function (hidden) | Type (hidden) |
|---|---|---|---|---|---|
| id | Read | ||||
| l | Read | ||||
| dca_sf | Read | ||||
| dcahr_sf | Read | ||||
| dcv_sf | Read | ||||
| dcw_sf | Read | ||||
| dcwh_sf | Read | ||||
| dcamax | A | Read | |||
| n | Read | ||||
| evt | Read | ||||
| evtvnd | Read | ||||
| dca | A | Read | |||
| dcahr | Ah | Read | |||
| dcv | V | Read | |||
| tmp | °C | Read | |||
| dcw | W | Read | |||
| dcpr | Pct | Read | |||
| dcwh | Wh | Read |
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If a device is not in the library yet, we build the template for free as soon as we have the register data.
Use cases
- Monitor total DC output current and voltage to verify the combiner is operating within its rated envelope.
- Track cumulative watt-hours over time to compare actual DC yield against expected generation for the array.
- Alert on non-zero event bitmask values in evt to detect overcurrent, ground faults, or wiring anomalies early.
- Compare dcpr against historical baselines to identify performance degradation across seasons or after maintenance.
- Log enclosure temperature alongside power output to correlate thermal stress with combiner derating events.
Installation
Verify that the SunSpec Model 402 block is active on the target device by confirming the DID register returns 402; consult the combiner manufacturer's documentation under the communication or Modbus section to enable Modbus TCP if not already active.
Connect the combiner to the same Ethernet network as the ModbusCloud Gateway, ensuring TCP port 502 is reachable and no firewall blocks the connection.
In the ModbusCloud portal, add the Gateway, import the SunSpec Model 402 template, and assign the correct Modbus unit ID matching the combiner's configured slave address.
Spot-check the live dca and dcv readings in the portal against the combiner's local display or data logger to confirm scaling factors (dca_sf, dcv_sf) are being applied correctly.
Things to watch for
Current and power readings look ten times too large or too small. Why?
SunSpec uses signed 16-bit scale factors (dca_sf, dcw_sf, dcwh_sf) as exponents of ten. If your Modbus master applies the raw integer without multiplying by 10 raised to the scale-factor value, readings will be off by an order of magnitude. Confirm the portal is reading the scale-factor registers at addresses 2 through 6 and applying them to each corresponding measurement.
The gateway connects but all registers return 0x8000 or 0xFFFF. What now?
These are SunSpec not-implemented sentinel values. The target device may not support every register in Model 402, or the model block was not discovered correctly. Verify the DID at address 0 returns exactly 402 and that the block length register (l) matches the expected size. Some combiners require a Modbus TCP discovery scan starting at register 40001 or 50001 before the model block becomes accessible.
The evt register shows a non-zero value. What does each bit mean?
The evt field is a 32-bit SunSpec standard bitmask where each bit position maps to a defined combiner fault condition such as low-string current, overcurrent, or ground fault. Consult the SunSpec String Combiner Models specification for the full bit definition table. The evtvnd register holds additional manufacturer-specific flags; check the combiner device manual for those bit definitions.
I cannot reach the combiner over TCP even though it is on the same LAN.
Confirm that the combiner's Modbus TCP server is enabled (some devices ship with it disabled by default) and that it is listening on port 502. Check that the unit ID configured in the ModbusCloud portal matches the slave address set on the device. Also verify no VLAN separation or firewall rule is blocking port 502 between the Gateway and the combiner.
Knowledge base articles for this device
SunSpec Modbus: one register map for many inverters
SunSpec Modbus is the open register map SMA, Fronius, SolarEdge and Kostal share. The layout, int+SF vs float, the TCP port per brand and reading a mixed fleet.
Fronius Modbus: read a Symo GEN24 over Modbus TCP
Fronius Modbus explained: enable Modbus TCP on the Symo GEN24 on port 502, choose between int+SF and float, read the right SunSpec registers and scale them correctly. Practical guide for solar installers.
SMA Modbus: reading a Sunny Boy over Modbus TCP (installers)
SMA Modbus explained: enable the Modbus TCP server on port 502, choose between the SMA profile (unit ID 3) and SunSpec (unit ID 126), read the right registers, and handle the NaN values at night. Practical guide for installers.
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