The gateway connects but both registers return zero or no data.

Confirm the Modbus TCP port matches exactly what is configured in the Plexlog portal. The OpenGateway may run on port 503 rather than the standard 502. Check firewall or VLAN rules between the ModbusCloud Gateway and the Plexlog unit, and verify the unit ID is set to 1 unless you have changed it.

The power register shows a large unexpected negative value.

The power register at address 37 is a signed int32. A negative value means the battery is currently charging, which is correct behaviour. If the magnitude looks wrong, confirm that no scaling is applied in the template, as the Plexlog OpenGateway delivers watts directly without a divisor.

State of charge reads 65535 or another unrealistic value.

A value of 65535 (0xFFFF) on a uint16 register typically means the Plexlog has not yet associated a battery with this gateway slot, or the battery is offline. Check the Plexlog portal to confirm the storage device is connected and reporting live data internally before polling over Modbus TCP.

Why does function code 3 return an illegal-function exception on these registers?

The Plexlog OpenGateway exposes battery data exclusively on input registers, requiring function code 4. Function code 3 (holding registers) is not supported for these addresses and will return an exception code 1. Ensure the template uses FC4, which is already set correctly in this ModbusCloud template.

Free Modbus template: Plexlog Plexlog (battery)

The Plexlog energy manager (PL40, PL100, PL500+) is a PV and smart-energy monitoring and control device deployed in residential and commercial installations. This ModbusCloud template connects to the Plexlog OpenGateway over Modbus TCP and reads two core battery registers: state of charge and instantaneous power. The signed power value distinguishes charging from discharging in a single field. With these data points in ModbusCloud, operators can track battery behaviour over time, set charge-threshold alerts, and verify that connected storage is performing as expected across all managed sites.

Name	Address	Function	Type	Unit	Access
power Battery instantaneous power, signed; positive discharging, negative charging.	37	FC04	int32	W	R
soc Battery state of charge, integer percentage, 0 to 100.	36	FC04	uint16	%	R