Modbus Operation; Wireless And Wired Devices; Modbus Communication Timeouts; Multihop Networks Vs Dx80 Star Networks - Banner Sure Cross DXM1200-Bx Instruction Manual

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Sure Cross DXM1200-Bx Wireless Controller

7.2 Modbus Operation

All Modbus transactions are managed by a central Modbus engine. If there are Modbus messages intended for a Modbus
slave that doesn't exist, the Modbus engine waits for a response until the timeout period is expired. This slows down the
Modbus polling loop for read and write operations.
For this reason, verify all Modbus read and write operations are intended for Modbus slave devices that are in the network.
If a Modbus slave is not in the network, either a wired or wireless device, the operation of the LCD menu system can be
compromised. Operations like Binding, Site Survey, or accessing the ISM menu may be slower. This is because all internal
devices (such as the ISM radio, LCD, etc) of the DXM1200 are also Modbus slaves.

7.3 Wireless and Wired Devices

Wireless DX80 Performance Gateway—The DX80 Performance Gateway architecture is a star architecture in which all
Nodes in the system send their data back to the Gateway. The host can access the entire network data from the Gateway,
which is Modbus slave ID 1. Because the DXM1200 will not be sending any Modbus messages across the wireless link, the
timeout parameter can be set low (less than 1 second) and the device is treated like a directly connected device.
MultiHop Master—The MultiHop master radio forms a wireless tree network using repeaters and slave devices. Each device
in a MultiHop network must be assigned a unique Modbus Slave ID and is accessed as a separate device.
For the DXM1200 to talk with a MultiHop device in the wireless network, the master MultiHop device interrogates every
message on the RS-485 bus. If they are within the wireless devices range (slave IDs 11 though 60), the message is sent
across the wireless network. To change this range, the user must adjust the offset and range setting in the MultiHop master
radio (Modbus Slave ID 1). Modbus register 6502 holds the Modbus offset, default 11. Modbus register 6503 holds the
number of Modbus slaves allowed (maximum of 100).
Modbus Slave ID Description
1 Allocated for the internal ISM radio device, either a DX80 Performance Gateway or MultiHop Master
2–10 Slave addresses available for direct connected Modbus slave devices to the master RS485 port (M+ , M-)
11–60 Allocated for wireless MultiHop radio network devices. If there is not an internal MultiHop in the DXM1200, these slave
addresses are available to use for directly connected devices.
61–198 Available to user for direct connected Modbus slave devices or the expansion of the wireless network slave IDs to go past 50
wireless devices.
199 Allocated for internal Local Register
201 Allocated for the LCD display board, the user can read/write LEDs.

7.4 Modbus Communication Timeouts

A Modbus timeout is the amount of time a Modbus slave is given to return an acknowledgment of a message sent by the
Modbus master. If the Modbus master waits for the timeout period and no response is seen, the Modbus master considers
it a lost message and continues on to the next operation.
The timeout parameter is simple to set for Modbus devices directly connected to the DXM1200, if there are no MultiHop
wireless devices. Special considerations need to be made to set the timeout parameter when a MultiHop network uses the
DXM1200 as the master radio.
Configure controllers operating wireless networks to allow for enough time for hardware transmission retries. Set the
Communications Timeout parameter to cover the expected time for messages to be sent throughout the wireless network.
For the DXM1200, the Communications Timeout parameter is the maximum amount of time the DXM1200 should wait after
a request is sent until the response message is received from the Modbus slave device. Use the DXM Configuration
Software to set the timeout parameter on the Settings > System screen (select Show advanced settings).
The default setting for the timeout parameter is 5 seconds.

7.4.1 MultiHop Networks vs DX80 Star Networks

The DX80 star Gateway collects all the data from the Nodes, which allows the host system to directly read the data from the
Gateway without sending messages across the wireless network. This allows for DX80 Gateway to be treated like any other
wired Modbus device.
In a MultiHop network, the data resides at each device, forcing the controller to send messages across the wireless network
to access the data. For this reason, carefully consider the value of the wireless timeout parameter.
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