The RP controller Modbus RTU (RS-485) subnetwork allows standard Modbus devices and the KNX Modbus gateway (RP-C-EXT-KNX) to be connected to the controller. An adapter is required for converting the controller RS-485 Com port with RJ45 interface to screw terminals.
The Modbus RTU protocol is used for the communication. The RP controller acts the Modbus client and the connected devices act as servers.
For standard Modbus devices, a separate power supply unit (PSU) is needed to power the Modbus devices on the network.
For the KNX Modbus gateway, the RP controller provides both 24 VDC power supply and communications through the Modbus network connection.
It is recommended to use the Non-isolated RS-485 Adapter (SXWNISORS48510001) to provide screw termination of the Modbus device wiring and help avoid miswiring. The adapter converts an RS-485 RJ45 interface to screw terminals. For more information, see Non-isolated RS-485 Adapter .
For the KNX Modbus gateway, the RS-485 Power Adapter (SXWNISORS485P10001) can preferably be used to convert the RP controller RS-485 interface from RJ45 port to screw terminals because this adapter supports 24 VDC power supply from the controller to the KNX Modbus gateway. For more information, see RS-485 Power Adapter .
The maximum total length of the Modbus (RS-485) network is 72 m (236 ft).
The Modbus (RS-485) network cable should be routed in a continuous daisy-chain bus configuration without cable stubs. For more information, see Cable Routing .
Cable selection is important to achieve good performance and reliability of the Modbus (RS-485) network. The cable must provide twisted pairs and be specified for RS-485 data communications. It is recommended to use a shielded, low-capacitance twisted pair cable with an impedance of 120 ohm and a wire size (cross-sectional area) of 22 to 24 AWG (0.33 to 0.20 mm²). For more information, see Cable Selection .
When the controller is installed in a space that handles conditioned air or return air, the Modbus network cables and IP network cables frequently must be plenum-rated to meet applicable building codes.
You need to configure which RS-485 (RJ45) port on the RP-C controller to use for the Modbus network. You can configure to use either the RS-485 Com A or Com B port, which by default are configured for the sensor bus and room bus respectively. For more information, see RP-C Communication Ports . The Modbus communication parameters can be freely configured regardless of which RS-485 port on the RP-C controller that is used for the Modbus network.
The maximum number of Modbus devices that can be connected to an RP-C controller depends on the type of Modbus device and the number of Modbus registers.
The maximum number of Modbus registers that can be connected to and managed by an RP-C controller depends on the communication capacity of the controller's Modbus network.
The RP-C Modbus network supports up to 10 connected Modbus devices with the following restrictions:
Maximum of one KNX Modbus gateway (RP-C-EXT-KNX)
Maximum of 250 Modbus registers per network
The RP controller Modbus network does not support 64-bit Modbus registers.
The KNX Modbus gateway has the default Modbus address 1. The address can be configured through the ETS. For more information, see Configuring the Modbus Device Settings in the KNX Modbus Gateway .