7 Working with Modbus Devices
The DXM700 has two physical RS-485 connections using Modbus RTU protocol.
The master Modbus RS-485 port is for the DXM700 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 DXM700. The Modbus slave port is used when another
Modbus master device wants to communicate with the DXM700 when the DXM700 is a Modbus slave device.
The Modbus slave RS-485 port is labeled S+ and S1 on the DXM700.
Figure 31. DXM700 system overview
Ethernet
RS-232
RS-485 (master)
RS-485 (slave)
USB
Local Registers
(Modbus ID 199)
Processor/Outputs (Base Board)
Outputs (Modbus ID 203)
Processor Controlled
Modbus Data Traffic
Control
Cellular Modem
Outputs
Local Registers
Non-Volatile
Local Registers
Float
Local Registers
Integer
ISM Radio (Modbus ID 1)
User Display (Modbus ID 201)
Gateway or MultiHop
LED / LCD Display
The DXM700 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 DXM700 local registers. To operate as a Modbus slave device, the DXM700
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 DXM700 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 Software.
The DXM700 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 DXM700.
DXM Internal Modbus Slave IDs (factory default)
Modbus Slave ID
Device
1
DX80 Performance Gateway or MultiHop 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 DXM700
203
Base Board Outputs—Outputs of the DXM700.
201
LCD Board—The user has access to the LED indicators on the DXM700.
7.1 Assigning Modbus Slave IDs
Assign the DXM Modbus Slave ID only if a Modbus master device is reading or writing the DXM 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 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 configuration
software under the
Settings
>
General
tab.
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®
DXM700-Bx Wireless Controller
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