SunSpec Model 401 — String Combiner (Current)
SunSpec Model 401 is a standardised Modbus TCP information model for DC string combiners in photovoltaic installations. It defines a fixed header block followed by a variable repeating section, one record per input string, allowing any SunSpec-compliant combiner to expose per-string and aggregate data over a single interface. The template reads total DC current, cumulative amp-hours, DC bus voltage, enclosure temperature, maximum current rating, string count, and both standard and vendor event bitmasks. Using Modbus TCP, the ModbusCloud Gateway ingests these registers continuously, giving installers and asset managers a single pane of glass for combiner health across multiple sites.
- Protocol
- Modbus TCP
- Connection
- Ethernet, Modbus TCP
- Registers
- 13, read only
- Brand
- SunSpec
- Last updated
Registers (13)
13 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 |
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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 current and DC voltage continuously to verify combiner output matches design expectations.
- Track cumulative amp-hours over time to detect gradual string capacity fade across seasons.
- Alert on non-zero event bitmask values to catch combiner faults before they escalate to inverter trips.
- Compare per-string current readings against the maximum rating to identify underperforming or open strings.
- Report enclosure temperature trends to verify adequate ventilation in combiner box installations.
Installation
Enable Modbus TCP on the string combiner or its gateway device under the communication or network settings menu, 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 rule blocks port 502 between the two devices.
In the ModbusCloud portal, add the Gateway, import the SunSpec Model 401 template, and assign the correct unit ID matching the combiner's Modbus slave address.
Trigger a live poll and verify that registers dca, dcv, and tmp return plausible values; confirm dca_sf, dcahr_sf, and dcv_sf are applied before comparing to nameplate data.
Things to watch for
Current and voltage readings look wrong by a factor of ten. Why?
SunSpec uses signed scale factor registers (dca_sf, dcv_sf, dcahr_sf) as power-of-ten exponents. A raw dca value of 1234 with dca_sf of -1 means 123.4 A. Confirm your Modbus client applies the scale factor before displaying or alerting on any measurement register.
The gateway connects but the repeating string block returns 0x8000. What now?
0x8000 is the SunSpec not-implemented sentinel for int16 registers. The combiner may support fewer strings than the block length suggests, or a string input has no CT attached. Read the n register first to determine how many repeating records are actually populated, and ignore slots beyond that count.
Model length register l returns an unexpected value. How do I parse the block correctly?
SunSpec Model 401 is variable-length. The number of repeating string records equals (l minus the fixed header length) divided by the per-record register width. Your Modbus client must read l first and calculate the repeating block size dynamically rather than assuming a fixed register count.
The evt or evtvnd register shows a non-zero value. What should I check?
Both evt and evtvnd are 32-bit bitmasks where multiple flags can be active simultaneously. Cross-reference the active bits against the SunSpec String Combiner event flag table. Common causes include current out of range on one or more strings, a communication fault to a sensing module, or a vendor-specific hardware condition reported in evtvnd.
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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