Concept
MP-C Modbus
The MP controller Modbus RTU (RS-485) subnetwork allows standard Modbus devices to be connected to the controller. An adapter is required for converting the controller RS-485 Com port with RJ45 interface to screw terminals.
MP-C controller models with “-M” in the product name do not support Modbus.
A separate power supply unit (PSU) is needed to power the Modbus devices on the network.
The Modbus RTU protocol is used for the communication. The Modbus communications is characterized by communication between client device(s), which initiate communication and make requests of server device(s), which process requests and return an appropriate response. The MP controller acts as the Modbus client device and the connected (hosted) devices act as server devices.
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 .
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 .
The cable shield should be connected to earth ground at one location (preferred at the hosting controller) and should be connected through to the next segment at each device without additional connections to ground.
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 reconfigure the RS-485 Com A port (RJ45) on the MP-C controller for the Modbus network. The RS-485 Com A port is by default configured for the sensor bus. The Modbus communication parameters can be freely configured regardless of that. For more information, see Communication Ports on MP-C-15A, -18A, -18B, -24A, and -36A .
The maximum number of Modbus devices that can be connected to an MP 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 MP controller depends on the communication capacity of the controller's Modbus network.
The MP controller Modbus network supports up to 20 connected Modbus devices with the following restrictions:
Maximum of 1,000 Modbus registers per network
64-bit Modbus registers are supported, which can be used in energy metering.
Hardware Overview
MP-C Controllers
MP-C Models
Non-isolated RS-485 Adapter
Wiring
RS-485 Communications
Cable Routing
Cable Selection
Communication Ports on MP-C-15A, -18A, -18B, -24A, and -36A
SpaceLogic MP-C Pro