Looking for a sanity check before I sign off a design.
Solar site, fairly remote: 24 string inverters on two RS-485 buses (Modbus RTU), a weather station that only has a serial radio link to the cabin, and some 433 MHz temperature probes on the transformer. None of it speaks IP. The subcontractor’s proposal is “MQTT-SN for the non-IP devices, CoAP for the weather station, then into Hermetry”. I’ve never seen either on a Hermetry page, and I’d rather not end up with a translation box nobody can maintain.
What do people actually use to get non-IP field devices into Hermetry?
What is MQTT-SN, is it a different MQTT?
A variant for sensor networks without TCP, it needs its own gateway to turn it into normal MQTT anyway.
Hermetry takes MQTT 3.1.1 and HTTPS from devices, plus LoRaWAN through its own network server and polled SNMP and cameras. No MQTT-SN, no CoAP. So for anything non-IP the answer is a gateway on site that speaks the field side and talks MQTT up. Two ways I’ve done it:
Licensed gateway with Hermetry Edge. It reads Modbus RTU over RS-485 and Modbus TCP (depends on the model), dials out over TLS with its own certificate, buffers during outages, and each inverter appears as its own child device. You claim it with the code on its local Cloud page, nobody types credentials on site.
Your own gateway. A Pi or industrial PC reads the buses with pymodbus, reads the weather station’s serial radio and a 433 MHz receiver, and publishes for all of them on v1/gw/connect and v1/gw/telemetry:
{"INV-07": {"ts": 1793000000000, "values": {"p_ac_kw": 41.2, "e_day_kwh": 188.4}},
"WX-1": {"values": {"irradiance": 612, "t_amb": 9.8}}}Mark the device “This device is a gateway” when you add it, or every v1/gw message is refused and the session gets closed after three. If the inverters ever need commands, you send them to the child device and the gateway receives them on v1/gw/rpc. Your own gateway has to pass them on to the bus, a licensed one does that for you.
For a site that is probably unattended for weeks, I’d pay for the licensed box.
For the inverters check if they do SunSpec Modbus maps, saves you writing a register map per model.
so the gateway counts as one device or 26?
The gateway plus every child, so 27 with the weather station, more with the probes.
Sent the subcontractor back to the drawing board. We’re going with a licensed gateway on the inverter buses, and a small Pi gateway for the weather radio and the 433 MHz probes since the gateway model we picked has no input for those. Twilson, the inverters do SunSpec, that halved the register work.
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