Why does the gateway return an 'illegal function' error for the power register?

The Plexlog data-logger exposes the PV power register via function code 4 (Read Input Registers). Using function code 3 (Read Holding Registers) against the same address causes exception code 1. Confirm the template is using FC4 and that your Modbus TCP port matches the one enabled in the logger's communication settings.

The gateway connects but the power value reads zero or null. What now?

First check that the Modbus TCP server is enabled on the Plexlog device, not just the API. Then confirm the unit ID is set to 1. If it still reads zero, verify that at least one inverter is actively reporting to the logger; the logger will not forward data from inverters it has lost contact with.

Which TCP port does the Plexlog data-logger use for Modbus?

Plexlog documents both port 502 (standard Modbus TCP) and port 503 depending on logger model and firmware configuration. Check the communication settings in the Plexlog web interface and ensure the same port is entered in the ModbusCloud portal. Mismatched ports cause silent connection failures.

The power register returns an unexpectedly large or negative value. What could cause this?

The register is typed as int32 and spans two consecutive 16-bit Modbus words. If the client reads only one word or reverses word order, the decoded value will be incorrect. Confirm the template is decoding both words as a big-endian int32, and check that no scaling factor has been applied accidentally in the portal configuration.

Free Modbus template: Plexlog Plexlog (pv)

Name: Plexlog Plexlog (pv)
Brand: Plexlog
SKU: fafbbb8b-bc20-47bd-ada4-31130943f0c1

Plexlog is a PV data-logger and monitoring platform used to aggregate output from inverters of many manufacturers into a single portal. The Plexlog (pv) template connects to the device's Modbus TCP interface, reading the total PV generation power register at address 0 via function code 4. With this single register in your ModbusCloud portal, you can trend solar production in real time, build historical yield charts, and trigger alerts when output drops unexpectedly. It is a compact, practical integration for any site where a Plexlog data-logger is already the on-site aggregator.