Instructions

UM-0085-B09 DT80 Range User Manual Page 180
RG
This job sets up the following CVs for access by the Modbus client:
•
1CV, 2CV and 3CV contain the three measured temperatures. A scaling factor of 10.0 is applied so that it can
be returned with one decimal place (range -3276.8 to +3276.7)
• 4CV is designed to be accessed as an output register, i.e. the Modbus client writes to it. This channel variable is
used as the temperature setpoint, so it is scaled in the same way as the other temperature CVs.
• 5CV and 7CV return the number of logged data and alarm records for the main A schedule. In this example the
store file size is relatively large (30 days data @ 15s scans), so the number of logged records may exceed the
capacity of a single 16-bit register. They are therefore defined as 32-bit long integers (
MBL). (Channel variables
6CV and 8CV have been skipped because their associated Modbus registers are used for returning the 32-bit
5CV and 7CV values.)
• 9CV returns the fan speed in RPM. A standard 16-bit integer is OK here. (The SETMODBUS command for this
CV could have been omitted because the values it is setting are the default values).
The digital channels of interest are 1DSO (fan control) and 2DSO (watering system). Since the Modbus client can access
these channels directly, there is no need to include them in the DT80 job.
One final point is that because all relevant data are returned by Modbus, there is no point having the DT80 return real
time data via its standard command interface – it would just clutter the screen if the operator ever needed to connect to
the DT80 using DeTransfer. All channels are therefore set to "working'" (
W) or "no return" (NR) channels.
Notice: all Modbus-accessible CVs have been grouped into one contiguous block. This is not essential, but it will improve the efficiency
of the Modbus link because the client can request all relevant CVs in one command.
 Modbus Client Configuration
It is now necessary to configure the SCADA software package to suit the DT80 channel usage described above. This is
highly application dependent but in very general terms the steps involved will typically include:
• configuring communications details
• designing a mimic screen incorporating the required measurement and control fields
• associating each element of the mimic with the correct Modbus register address
So in this case the first step might be to select the software package's "generic PLC" device driver and then create a
"DT80" device instance. As a minimum, the DT80's IP address would need to be entered here. If the driver or application
provides a "test" facility, you may at this point be able to try manually reading and writing specific Modbus registers.
The mimic screen for this application might consist of:
• three thermometer displays showing the measured temperatures
• an entry field where the operator can set the desired setpoint
• two numeric indicators showing number of logged data and alarm records
• a fan icon with on/off and RPM indicators
• a button to turn the watering system on or off
The final step would typically then be to edit the properties of each control and indicator to specify their behaviour. This
would generally mean specifying:
• which slave device to use. This may involve selecting the DT80's "device instance" from a list of connected
Modbus slave devices
• the Modbus register number
• the data type (signed or unsigned or floating point, 16 or 32 bits, byte/word ordering)
• the scaling factor. This is often presented as a span, similar to a DT80 Spans (Sn) (P65). That is, you specify
the "device range" (min/ max returned value) and the corresponding "display range" (min/ max value to display)
– which then creates a linear scaling curve.
• the scan rate (how often to read/ write the value from/to the DT80)
• other details such as units