Same field plot project. The node now reports fine, and we want to add a proper gateway in the shed: a Pi 3B (not B+, that’s what we have) reading 6 Modbus soil probes over an RS-485 HAT and sending them to Hermetry as children. There’s an Ethernet cable from the farm building with a PoE switch at the other end, but no mains socket in the shed.
The official PoE HAT says it fits the 3B+ and 4 only, the 3B has no 4-pin PoE header. Is there a sensible way to power a 3B from PoE, or should I just beg for a 3B+?
An 802.3af splitter with a micro-USB 5 V output works on a 3B, I run one in my greenhouse. Get one rated 2.4 A, the 1 A ones brown out when the RS-485 HAT and Wi-Fi are both busy.
Aisha’s right, and I’d add a few things from running Pi gateways on a production floor.
Buy a splitter that says 802.3af/at on it and lets the switch negotiate, never a passive injector. Check the cable run, long thin patch leads drop enough voltage that the Pi throttles. You can see that from the Pi itself:
$ vcgencmd get_throttled
throttled=0x50005 # bit 0 = under-voltage now, bit 16 = has happened since boot
I’d send that value to Hermetry with the probe readings. The gateway can report its own telemetry on v1/telemetry as a normal device, while the probes go on v1/gw/telemetry as children. Then an alarm rule on the gateway’s profile for pi.undervolt == 1 tells you the supply is marginal before the SD card gets corrupted.
Remember that when the gateway goes offline, every child goes offline with it. So the gateway’s offline alarm is the one that matters, and the children’s offline alarms will all fire too unless their delay is longer. We put the gateway on 300 s and the children on 900 s. The child behaviour is described here: Gateways and devices behind a gateway.
And keep the SQLite buffer you’d use anyway, a PoE switch reboot during a firmware update is a 2-minute outage.
Ordered a 2.4 A af splitter and it’s been up 9 days. Throttled was 0x50005 on the first day with a 15 m flat patch cable, swapped it for proper Cat6 and it’s 0x0 since. Added the undervolt key and the gateway/children offline delays like you said.
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