Bit of a non-technical one, apologies in advance.
We run two press lines, about 30 Modbus RTU devices (energy meters, a couple of Eurotherm controllers, press counters). The integrator is quoting us licensed gateways with Hermetry Edge. My finance director keeps asking why we are paying for boxes on site at all when “the cloud does everything”. And our maintenance lead says the opposite, he wants everything processed locally because “the internet goes down”.
I need to decide this month. What actually runs where? If the line’s broadband drops for an hour on a Friday night, do I lose data? Do alarms still fire? I care about downtime first, then cost.
Not an answer to the whole thing, but Modbus RTU devices can’t talk to any cloud by themselves, something on site has to poll them. So you need a box on site either way. You’re really choosing which box.
Fair. The integrator said we could also use a Raspberry Pi. Is that a real option or a hobby thing?
It’s real, the gateway topics are documented and anyone can implement them. But then you own the patching, the SD card dying, the buffering logic, credentials on the box. From the security side I’d only sign off on a Pi if someone is named as owner of it.
We went through the same thing at our hotels last year, so here is what I learned (I’m not an engineer, so this is the manager version).
Edge is software on the licensed gateway. It reads your Modbus devices on site, and each meter or controller shows up in Hermetry as its own child device. The gateway dials out over TLS, nothing on your site has to accept incoming connections.
If the broadband drops, licensed gateways can keep the readings locally and send them when the link is back. Each reading carries the time it was taken, so the history fills in at the right place. How much it can hold depends on the model, so ask for the figure for your model before you rely on it. Our integrator had to check that for us.
What surprised me was that alarm rules and flows run in Hermetry, on the cloud side, and the box only reads and buffers. So during an outage nobody gets an alarm e-mail, and when the backlog arrives the alarm rules look at the late readings by their own rules. Anything that must stop a press in a fault stays in the PLC or the controller, never in the cloud. Our maintenance lead was happy once he heard that.
The page that settled it for us: Licensed gateways and Hermetry Edge
That’s the answer I needed. Safety interlocks stay in the PLCs where they already are, Edge buffers the meters, and the cloud does the reporting and alarms. I asked the integrator for the buffer size of their model, they came back with roughly two days at our polling rate, which is plenty. Finance is satisfied because the box is replacing a Pi we would have had to look after ourselves.
One more thing for the Friday night case: add an offline alarm on the gateway. When the gateway goes offline its children go offline too, so one alarm tells you the whole line went quiet.
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