My understanding of modbus is that its a master slave system.  One unit must be designated the master, others are slaves.  i dont know how you do this.  You can definitely connect multiple slaves, doubtless up to some max number which I dont know.

Thats probably not definitive enough for which I apologise.  Maybe someone will come along with a definitive answer.

Technically, no, Modbus cannot have multiple connections, but you can decode the Modbus data using an RS-485 interface and suitable software (a Modbus sniffer). The Homely becomes the Modbus master (requesting the data from the Samsung); only one master is allowed.

That being said, I have tried multiple connections (Homely and Home Assistant), and it does work most of the time, but I would not recommend it. It can be flaky and very slow.

It's far easier to use a simple off-the-shelf interface (M5 Stack Atom + RS485 base ATOMIC RS485 Base | The Pi Hut) and wire directly to the F1/F2 terminals. You will get much more data and controls available using this method (using the Samsung NASA protocol). There are multiple software options for this method on GitHub. I use ESP-Home and Home Assistant which works great.

@arundalep Thank you that's useful info. I have modbus with Planet Devices which gives me plenty of data but no control. I have just fitted the long awaited wifi kit and linked to SmartThings and am about to try Havenwise. Should that not be successful I would try Homely via the modbus

Thanks for the various replies.

I've tried to summarize the external monitoring methods:

Method 1 - Modbus via interface (MIM-B19N)
protocol: Modbus over RS485
access: Samsung Modbus interface MIM-B19N + RS485 to USB adaptor to computer / RS485 to TTL adaptor for microcontroller
notes:
a) Modbus will only support a single client.
b) possible to sniff data using a Modbus sniffer, but can be slow and flaky ( https://renewableheatinghub.co.uk/forums/postid/44593)

Method 2 - Connection via F1 / F2 terminals on control kit (MIM-E03*N)
protocol: NASA (see: https://wiki.myehs.eu/wiki/NASA_Protocol) over RS485
access:
- RS485 to USB / Ethernet / WiFi converter for connection to computer
- RS485 to TTL converter for connection to microcontroller (such as ESP32)
notes:
a) use of official Samsung MIM-C02N RS485 to USB interface expensive but provides some additional functionality, such as the downloading of Samsung firmware updates ( https://community.openenergymonitor.org/t/monitoring-your-samsung-ashp-controller/27638)
b) use of free Samsung SNET-Pro2 software provides easy access to some input and output data, allows you to change FSVs (field setting values), but not variables associated with the outdoor unit. And,allows logging to file in Excel spreadsheet format.
c) an ESPHome module has been developed to allow integration with Home Assistant ( https://github.com/omerfaruk-aran/esphome_samsung_hvac_bus)

Method 3 - Connection via F3 / F4 terminals on control kit (MIM-E03*N)
protocol: NASA over On-Off-Keyed (OOK) data line over a DC transmission line
access:
- requires a THVD8000 RS-485 transceiver to decouple bus signal from DC transmission line.
- RS485 to USB / Ethernet / WiFi converter for connection to computer
- RS485 to TTL converter for connection to microcontroller (such as ESP32)
- Samsung remote controller can remain connected
notes:
a) required circuitry appears to be involved, with - for example - the carrier frequency of the transceiver needing to be set by the adjusting resistor dip-switches. See:
https://github.com/70p4z/samsung-nasa-mqtt/issues/1
https://www.ti.com/lit/ug/sllu140a/sllu140a.pdf?ts=1743411742358&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FTHVD8000EVM
b) being actively developed by Topaz ( https://community.openenergymonitor.org/u/Topaz/summary), who plans to market a cheap pbc to facilitate access to these terminals). For more details of his project see:
https://community.openenergymonitor.org/t/contribution-samsung-ashp-nasa-link-mqtt-bridge-home-assistant/26744/9
https://github.com/70p4z/samsung-nasa-mqtt

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Apparently the amount and frequency of data that can be viewed and controlled varies by method, with Modbus offering the least and connection to F3 / F4 terminals the most. A screenshot of showing data and settings for the F3 /F4 terminal method can be see here: https://github.com/70p4z/samsung-nasa-mqtt. It would be useful for those familiar with the Modbus and F1 / F2 methods what data or settings are missing for these interfaces.

Thanks