SunSpec Model 403 — String Combiner (Current)
SunSpec Model 403 is the standard information model for a DC string combiner in a photovoltaic system, covering current-based monitoring across all connected PV strings. The template reads total DC output current, cumulative amp-hour yield, DC bus voltage, combiner temperature, maximum rated current, and the number of active strings, alongside combiner-level and vendor-specific event bitfields. Per-string scale factor registers enable precise decoding of individual string data in the repeating block. Using Modbus TCP, this template gives installers and asset managers a vendor-neutral view of combiner health and string-level production directly inside the ModbusCloud portal.
- Protocol
- Modbus TCP
- Connection
- Ethernet, Modbus TCP
- Registers
- 15, read only
- Brand
- SunSpec
- Last updated
Registers (15)
15 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 | ||||
| dcamax | A | Read | |||
| n | Read | ||||
| evt | Read | ||||
| evtvnd | Read | ||||
| dca | A | Read | |||
| dcahr | Ah | Read | |||
| dcv | V | Read | |||
| tmp | °C | Read | |||
| indca_sf | Read | ||||
| indcahr_sf | Read |
Loading registers…
Send your device list
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 from all strings to verify expected combiner yield against irradiance data.
- Track cumulative amp-hour production over time to compare seasonal performance across multiple PV arrays.
- Alert on combiner event flag changes to detect faults such as overcurrent or string disconnection events quickly.
- Verify combiner temperature stays within operating limits during peak summer generation periods.
- Compare the number of active strings against the installed count to spot silent string failures early.
Installation
Enable Modbus TCP on the string combiner or its gateway device under the communication or Modbus section, and note the assigned IP address and TCP port (default 502).
Connect the combiner to the same LAN segment as the ModbusCloud Gateway, ensuring no firewall rules block port 502 between the two devices.
In the ModbusCloud portal, add the ModbusCloud Gateway, then import the SunSpec Model 403 template and set the correct Modbus unit ID matching the combiner's slave address.
Spot-check the live value of register dca (address 11) against a clamp-meter reading at the combiner output to confirm correct scaling and connectivity.
Things to watch for
Current readings look wrong or off by a factor of ten. Why?
SunSpec uses signed scale factor registers. The raw integer in dca must be multiplied by 10 raised to the power of dca_sf. If your Modbus client does not apply this automatically, read dca_sf first and apply it manually. A dca_sf value of -1 means divide the raw value by 10.
The gateway connects but all registers return zero or 0x8000. What now?
A raw value of 0x8000 on an int16 register is the SunSpec 'not implemented' sentinel, meaning the device does not populate that point. Confirm the combiner firmware supports Model 403 fully. Also verify the Modbus unit ID matches the device slave address, as a wrong ID returns valid frames but with meaningless data.
The portal cannot reach the combiner even though it is on the network. What should I check?
Confirm the ModbusCloud Gateway and the combiner are on the same IP subnet. Check that no managed switch applies VLAN isolation between them. Verify the combiner's Modbus TCP server is enabled and listening on port 502, and that the IP is static or DHCP-reserved to prevent address changes after reboots.
How do I interpret the evt and evtvnd bitfields?
Both are 32-bit bitfields where each bit represents a distinct fault or status condition. The evt register follows the SunSpec standard combiner event definitions. The evtvnd register contains vendor-specific flags whose meaning depends on the specific combiner manufacturer. Consult the device datasheet to map individual bits to fault names.
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.
Want to see how it works?
