9 Working with Modbus Devices
9.1 Overview
The DXM Controller has two physical RS-485 connections using Modbus RTU protocol.
The master Modbus RS-485 port is for the DXM Controller to act as a Modbus master device to control internal and
external Modbus slave devices.
The Modbus master RS-485 port is labeled M+ and M- on the DXM Controller. The Modbus slave port is used when another
Modbus master device wants to communicate with the DXM Controller when the DXM Controller is a Modbus slave device.
The Modbus slave RS-485 port is labeled S+ and S1 on the DXM Controller.
Ethernet
Modbus RS-485
(slave port)
Processor Modbus
Control/Data
External
Modbus
Slaves (2-10)
Modbus RS-485
(master port)
Modbus
SID 199
Modbus
SID 1
Modbus
SID 201
Modbus
SID 200
Local
Registers
GW/MH
Radio
Display
I/O Base
The DXM Controller has dual Modbus roles: a Modbus slave device and a Modbus master device. These run as separate
processes.
The Modbus slave port can only access the DXM Controller local registers. To operate as a Modbus slave device, the DXM
Controller needs to be assigned a unique Modbus slave ID as it pertains to the host Modbus network. This slave ID is
separate from the internal Modbus slave IDs the DXM Controller uses for its own Modbus network. The DXM Modbus slave
ID is defined through the LCD menu. Other Modbus slave port parameters are defined by using the DXM Configuration
Tool.
The DXM Controller operates the Modbus master port. Each device on the master port must be assigned a unique slave ID.
There are slave IDs that are reserved for internal devices in the DXM Controller.
DXM Internal Modbus Slave IDs (factory default)
Modbus Slave ID
Device
1
Gateway (PE5) or MultiHop (HE5) ISM Radio—MultiHop wireless devices connected to the internal MultiHop radio should
be assigned Modbus Slave addresses starting at 11.
199
Local Registers—Internal storage registers of the DXM Controller
200
I/O Base Board—All data and parameters for each input or output of the DXM Controller.
201
LCD Display—The user has access to the LED indicators on the DXM Controller.
9.2 Assigning Modbus Slave IDs
Assign the DXM Modbus Slave ID only if a Modbus master device is reading or writing the DXM Controller Local Register
data through the Modbus RS-485 slave port (S+, S-).
Set the DXM Slave ID from the LCD menu under System > DXM Slave ID. The DXM Controller can have any unique slave
ID between 1 and 246, depending upon the host Modbus network. Other RS-485 slave port parameters are set in the DXM
Configuration Tool under the Settings > General tab.
DXM Master Configuration—When the DXM Controller operates as a Modbus master device, use the DXM Configuration
Tool to configure read or write operations of the DXM Modbus network. The DXM Controller communicates with all internal
and external peripheral devices using the external Modbus bus RS-485 (M+, M-)
DXM100-Bx Wireless Controller Instruction Manual
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