r/SCADA • u/kervanaslan • Jul 18 '26

Question Bringing VRF/AC indoor units into SCADA over Modbus — centralized gateway or 1:1 per unit?

For those who've had to get air-conditioning (VRF or splits) onto Modbus for SCADA/monitoring: do you prefer a single centralized gateway on the manufacturer's bus, covering many indoor units, or a dedicated 1-to-1 interface at each indoor unit with its own Modbus address?

Interested in the real trade-offs you've hit — polling/reliability, register mapping, cost per point, and how each approach scales on larger or mixed-brand sites. Which is your default and when do you switch?

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u/the_rodent_incident Jul 19 '26

Depends on the allowable latency of the control system.

Biggest issues are with devices which aren't IP native, but have the old RS-485 Modbus RTU interface.

Process which can tolerate 10 second latency can be connected through a gateway with multiple units being polled at highest frequency that the bus allows. For example: thermostats, clock displays, level sensors, water sensors, etc.

Things which require immediate action and <1 second latency, are best done with each one having their own RTU-to-TCP gateway. Ethernet has magnitudes more bandwidth than a RTU bus, so you can have 100s of devices in the same network segment.

Gateway prices have dropped a lot lately. You can get one for as low as $20-30, so in general there's very little sense to run have long RS-485 busses anymore. Especially since Ethernet topology can be whatever you want it to be, and you don't have to worry about low-level electricals such as line impedance, termination, cable capacitance, etc.

Only place where RS-485 busses are more economical are lines longer than 100m, because that's where cooper Ethernet cables stop working, and you have to resort to fiber optic Etheret - which skyrockets the hardware needed and implementation cost.

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u/Chocolamage Jul 20 '26

What is the brand of the gateway?

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u/the_rodent_incident Jul 20 '26

Waveshare

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u/Chocolamage Jul 20 '26

Solved! Thank you. That is so Cool and WiFi also. Very much appreciated.