Add initial modbus documentation (#24619)

Co-authored-by: Franck Nijhof <frenck@frenck.nl>

source/_integrations/nibe_heatpump.markdown

@@ -22,18 +22,16 @@ The Nibe Heat Pump integration allows you to control and monitor [Nibe Heat Pump
 
 Supported devices:
 
-- F1145
-- F1155
-- F1245
-- F1255
-- F1345
-- F1355
+- F1145/F1155
+- S1145/S1155
+- F1245/F1255
+- F1355/F1355
 - F370
 - F470
 - F730
 - F750
-- SMO20
 - SMO40
+- SMOS40
 - VVM225
 - VVM310
 - VVM320
@@ -46,7 +44,14 @@ Supported devices:
 
 To communicate with the pump there are a few different connection methods available depending on hardware. At the moment the following methods are supported.
 
-- [UDP Gateway using NibeGw](#udp-gateway-using-nibegw)
+- [Connection Methods](#connection-methods)
+  - [UDP Gateway using NibeGw](#udp-gateway-using-nibegw)
+    - [ESPHome](#esphome)
+    - [Arduino](#arduino)
+    - [Raspberry Pi / Linux](#raspberry-pi--linux)
+  - [Modbus Connection](#modbus-connection)
+    - [TCP/IP](#tcpip)
+    - [RCU](#rcu)
 
 ### UDP Gateway using NibeGw
 
@@ -75,3 +80,25 @@ A standard Linux application for running the Nibe Gateway software is also avail
 
 - [Documentation from OpenHAB](https://www.openhab.org/addons/bindings/nibeheatpump/#raspberry-pi-or-other-linux-unix-based-boards)
 - [Source code in OpenHAB Contrib](https://github.com/openhab/openhab-addons/tree/main/bundles/org.openhab.binding.nibeheatpump/contrib/NibeGW/RasPi)
+
+### Modbus Connection
+
+Home Assistant support connecting to Nibe heat pumps over Modbus over either TCP or RCU like connections. When connecting to the pump inside a Home Assistant, the Modbus connection string must be specified.
+
+#### TCP/IP
+
+The newer S-series pump exposes a built-in TCP Modbus server that can be used to communicate with the pump. To enable support for this integration, the pump must be connected to your local network and network (5.2 - Network settings) and Modbus (7.5.9 - Modbus TCP/IP) must be enabled. More details on the Modbus support in the pump can be found in Nibe document [M12676EN](https://partner.nibe.eu/nibedocuments/29795/M12676EN.pdf).
+
+- `tcp://[IP OR HOSTNAME]`
+- `tcp://[IP OR HOSTNAME]:502`
+
+#### RCU
+
+If your system is equipped with a MODBUS40 accessory, this can be used to control your pump. To connect Home Assistant to the MODBUS40 accessory, a RS485 USB dongle is needed, which can be locally connected to your Home Assistant system, or on a separate computer exposed via [ser2net](https://linux.die.net/man/8/ser2net)
+
+- `serial://[DEVICE PATH]` for direct local connection
+- `rfc2217://[IP OR HOSTNAME]:[PORT]` for a [ser2net](https://linux.die.net/man/8/ser2net) type proxy
+
+<div class='note info'>
+Support for RCU-based communication is currently untested.
+</div>