SunSpec Model 133 — Basic Scheduling
SunSpec Model 133 is the Basic Scheduling model defined by the SunSpec Alliance for inverter controls, applied across any SunSpec-compliant solar inverter or DER device. Over Modbus TCP, this template reads the model identifier and block length for discovery, the number of supported schedules and points per schedule, and exposes the active-schedule bitmask and the model-enable flag as read/write registers. Using ModbusCloud Gateway with this template, installers can verify that timed control programs are loaded and active, and track scheduling state without manual device access.
- Protocol
- Modbus TCP
- Connection
- Ethernet, Modbus TCP
- Registers
- 6, read and write
- Brand
- SunSpec
- Last updated
Registers (6)
6 read, 2 write
| Name | Unit | Access | Address (Hidden until you enter your email) | Function (hidden) | Type (hidden) |
|---|---|---|---|---|---|
| id | Read | ||||
| l | Read | ||||
| actschd | Read and write | ||||
| modena | Read and write | ||||
| nschd | Read | ||||
| npts | Read |
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Use cases
- Verify that the Basic Scheduling model is enabled on a SunSpec-compliant inverter after commissioning.
- Monitor the active-schedule bitmask to confirm which timed control programs are running at any moment.
- Alert on unexpected changes to the actschd register that may indicate a scheduling conflict or firmware reset.
- Track the number of supported schedules and points per schedule across a multi-site inverter fleet.
- Report scheduling model enable status over time to document compliance with site energy management programs.
Installation
Enable Modbus TCP on the target SunSpec-compliant inverter under its communication or Modbus settings, and note the assigned IP address and TCP port (default 502).
Connect the inverter to the same LAN as the ModbusCloud Gateway, ensuring no firewall blocks port 502 between the Gateway and the inverter.
In the ModbusCloud portal, add the Gateway, import the SunSpec Model 133 template, and set the correct unit ID and base register offset matching the inverter's SunSpec model chain.
Spot-check the id register to confirm it returns 133 and the nschd register to verify the device reports a non-zero schedule count.
Things to watch for
Why does the id register return an unexpected value?
SunSpec models are chained in sequence, so the base address of Model 133 is not fixed. If id does not return 133, the configured start address in the portal is pointing to a different model block. Use SunSpec model discovery by scanning from the device base address to locate the correct offset for Model 133.
All registers read zero or not-implemented. What is wrong?
The device may not implement Model 133. Not all SunSpec-compliant inverters include the Basic Scheduling model. Confirm the device's PICS (Protocol Implementation Conformance Statement) or firmware release notes list model 133 as supported before polling.
The gateway connects but modena reads false. How do I enable scheduling?
Writing true (1) to the modena register activates the Basic Scheduling model. On some devices this register is protected by a session or password mechanism. Ensure no other Modbus client holds an exclusive session on the device, then write modena via the portal.
Is it safe to write the actschd or modena registers remotely?
Use caution. Writing actschd activates or deactivates time-based control programs on a live inverter, which can alter power output or grid export behaviour immediately. Always confirm the intended schedule index before writing, and consult the device documentation and site operator before modifying write registers remotely.
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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