Overview

Passive prometheus exporter

The primary function of this code is to sniff the messages between the data logger and the inverter, and turn these passively collected values into prometheus exporter values which can be scraped. This gives a local source of data with higher resolution than Solis Cloud (it's updated at 1 minute intervals) and continues to work even if Solis Cloud is down.

Modbus message injection

The secondary (optional) feature is rather more dubious: you can enable a modbus TCP gateway which allows you to inject modbus messages onto the RS485 bus to the inverter.

Modbus does not support multi-master operation, and the data logger is unaware this is going on, so it's quite possible you will stomp on the data logger's own messages. The code here takes some care to check that the line is idle before transmitting, but the data logger almost certainly doesn't.

Modbus messages are protected by a CRC, so the worst that should happen is that occasionally a message fails, in which case the master should retry; but it's possible that Bad Things™ will happen.

I added this functionality because I needed a remote control to set timed charge/discharge on the inverter (which sits in my attic). Since I'm only going to do this once or twice a day, I'm prepared to accept the risk of an occasional corrupted message on the modbus link. It has worked fine for me so far.

A safer way to do this would be to build a modbus proxy with two RS485 interfaces, and sit it in between the data logger and the inverter (acting as a slave to the logger, and a master to the inverter).

Alternatively you could disconnect the data logger, so that your computer is acting as the only modbus master, but then you'd lose the Solis Cloud logging. It would also be up to you to send the periodic messages to poll the inverter.

I chose not to do this, because I wanted to keep the Solis Cloud monitoring running (it has a nice app, and it can be used for support purposes by my PV supplier).

Security warning

If you enable the modbus TCP gateway, make sure it's bound to 127.0.0.1, and if you expose it on the network, use firewall rules. You definitely do not want random unauthorized sources updating the parameters of your inverter.