SunSpec Model 64020 — Mersen GreenString
The Mersen GreenString is a string-level monitoring card system for mid- and large-scale photovoltaic combiner boxes. Using Modbus RTU over isolated RS485, this SunSpec Model 64020 template reads per-string DC voltage and current across up to seven channels, a frequency sensor, up to five auxiliary and one probe temperature inputs, and a main board temperature. It also exposes three relay status registers. Connecting the GreenString to ModbusCloud lets O&M teams detect string underperformance, thermal anomalies, and relay events without visiting the site.
- Protocol
- Modbus RTU
- Connection
- RS485
- Registers
- 32, read only
- Brand
- SunSpec
- Last updated
Registers (32)
32 read, 0 write
| Name | Unit | Access | Address (Hidden until you enter your email) | Function (hidden) | Type (hidden) |
|---|---|---|---|---|---|
| id | Read | ||||
| l | Read | ||||
| aux0tmp | °C | Read | |||
| aux1tmp | °C | Read | |||
| aux2tmp | °C | Read | |||
| aux3tmp | °C | Read | |||
| aux4tmp | °C | Read | |||
| probetmp | °C | Read | |||
| maintmp | °C | Read | |||
| sensorv_sf | Read | ||||
| sensora_sf | Read | ||||
| sensorhz_sf | Read | ||||
| sensor1voltage | V | Read | |||
| sensor2voltage | V | Read | |||
| sensor3voltage | V | Read | |||
| sensor4voltage | V | Read | |||
| sensor5voltage | V | Read | |||
| sensor6voltage | V | Read | |||
| sensor7voltage | V | Read | |||
| sensor1current | A | Read | |||
| sensor2current | A | Read | |||
| sensor3current | A | Read | |||
| sensor4current | A | Read | |||
| sensor5current | A | Read | |||
| sensor6current | A | Read | |||
| sensor7current | A | Read | |||
| sensor8 | Hz | Read | |||
| relay1 | Read | ||||
| relay2 | Read | ||||
| relay3 | Read | ||||
| resetaccumulators | Read | ||||
| reset | Read |
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Use cases
- Monitor per-string DC current on all seven channels to detect underperforming or failed strings early.
- Compare sensor1voltage through sensor7voltage across strings to identify soiling or shading imbalances.
- Track aux0tmp through aux4tmp and maintmp to alert on combiner box overheating before equipment is damaged.
- Verify relay1, relay2, and relay3 status changes to confirm protection devices operated correctly after a fault.
- Report historical string current trends over multiple seasons to support yield-guarantee and O&M reporting.
Installation
Confirm the GreenString card is wired to the RS485 bus with correct polarity (A/B), a 120-ohm termination resistor at each bus end, and a unique Modbus unit ID set on the card's DIP switches or configuration interface.
Connect the ModbusCloud Gateway to the same RS485 bus using the Gateway's RS485 terminals, matching baud rate and parity to the GreenString factory defaults (typically 9600 baud, 8N1).
In the ModbusCloud portal, add a new RTU device, import the SunSpec Model 64020 Mersen GreenString template, and assign the unit ID that matches the card's hardware setting.
Spot-check sensor1current and maintmp in the portal's live register view to confirm polling is active and scale factors sensorv_sf and sensora_sf are applied correctly before saving the configuration.
Things to watch for
All sensor readings show zero or 0x7FFF. What is wrong?
The GreenString reports 0x7FFF (32767) as the SunSpec not-implemented sentinel for int16 points when a sensor is absent or not connected. Check that each physical sensor is wired to its designated input on the probe or auxiliary card. Unconnected channels will always return this sentinel.
Voltage and current values look wrong by a factor of ten or one hundred. Why?
The raw register values for sensor1voltage through sensor7voltage and sensor1current through sensor7current must be multiplied by 10 raised to the power of sensorv_sf or sensora_sf respectively. If your Modbus client does not apply these SunSpec scale factors automatically, the displayed values will be off by an order of magnitude. Read the scale factor registers first and apply them in your client.
The gateway connects but no data arrives from the GreenString card.
Verify the RS485 bus has exactly one 120-ohm termination resistor at each physical end of the cable run. A missing or doubled termination causes signal reflections that corrupt frames. Also confirm the unit ID on the card matches the ID configured in the portal, and that baud rate and parity settings agree on both ends.
The resetaccumulators or reset registers are in the template. Is it safe to write to them?
Both registers act as control commands: writing to resetaccumulators clears stored energy or diagnostic counters, and writing to reset triggers a soft device restart, which will interrupt data collection momentarily. Do not issue these commands during active monitoring sessions or from automated rules without a deliberate operator action, as unintended resets can corrupt accumulated data and may affect connected protection relay state.
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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