Operating Instructions Smart Weather Sensor
G. Lufft Mess- und Regeltechnik GmbH, Fellbach, Germany 119
19.7 Communication in Modbus Mode
For a simpler integration of WS family Smart Weather Sensors into a PLC environment the
Modbus communication protocol has been made available.
Measurement values are mapped to Modbus Input Registers. The range of values available
is basically the same as for the UMB protocol, including different unit systems.
In the interest of simple and safe integration the use of register pairs for floating point values
or 32 bit integers, which is not part of the Modbus standard, has not been applied. All
measurement values are mapped to 16bit integers using suitable scaling factors.
19.7.1 Modbus Communication Parameters
The Smart Weather Sensor can be configured for MODBUS-RTU or for MODBUS-ASCII.
The base configuration has to be done using the UMB Config Tool.
When selecting MODBUS RTU or MODBUS-ASCII with the UMB Config Tool,
communication parameters 19200 Bd, even parity, will be preselected.
Modbus operating modes: MODBUS-RTU, MODBUS-ASCII
Baud rate: 19200 (9600, 4800 or lower)
Interface Setting 8E1, 8N1, 8N2
NOTE: The Modbus communication has been tested for a poll rate of 1 sec. The proper
function of the Smart Weather Sensor with higher Modbus poll rates has not been tested.
We suggest to set the poll rate to 10 sec or slower, as, with the exception of the channels
„wind speed / wind directions fast“, which are provided for special purposes, the update rate
of the data is >= 10sec. Anyway for most of the weather data significant changes have tob e
expected more in the range of minutes.
19.7.2 Addressing
The Modbus address is deducted from the the UMB device ID (see Chap. 19.3.2).
A device with UMB device ID 1 also has the UMB address 1, etc..
The valid address range of Modbus from 1 to 247 is smaller than that of the UMB device IDs.
If a UMB device ID > 247 has been selected, the Modbus address will be set to 247.
19.7.3 Modbus Functions
The functions of conformance class 0 and 1 have been implemented as far as they are
applicable for the Smart Weather Sensor, i.e. all functions operating on register level.
Selected configuration settings
Selected configuration settings
Measurement values and status information
Selected configuration settings
(responds also to broadcast address)
19.7.3.1 Function 0x03 Read Holding Registers
The Holding Registers are used to make a selected set of adjustable parameters available
for Modbus access. As for the measurement values the parameters are mapped to 16bit
integers.
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