page 9
Action
BACnet
Modbus
Persistent setting?
Object for
setting
Object for
status
Register for
setting
Register for
status
Set bus control
state
BO,0 = 1
BI,0
00101.0 = 1 00201.0
No
Set start
BO,1 = 1
BI,2
00101.1 = 1 00201.3
No
Set stop
BO,1 = 0
BI,2
00101.1 = 0 00201.3
No
Set frost risk/
Frost risk detected
BO,4 = 1
BI,7
00101.7
00204.4
No
Temperature
control Enabled
BI,1
00201.4
Table 3: Data points associated with the state machines in Figure 7 and Figure 8.
In situations where the coil preheat function is not needed and has been disabled, the state diagram is as in
Figure 8. The data points associated with the state transitions are the same as above.
Figure 8: State machine for MIXIT with the preheat function disabled.
Even simpler, if the internal frost protection function has also been disabled, the only thing that can bring
MIXIT to the frost state is either a signal on the fieldbus (BACnet: BO,4/Modbus 00101.7) or a signal at a
digital input from a frost thermostat.
Figure 9 shows how to interface to MIXIT, when the coil preheat function in MIXIT is enabled. The example
in the figure is based on BACnet objects but the corresponding Modbus registers are listed in Table 2 and
Table 3. For example, as shown in Figure 9, BI,2 is read back to check that the start command BO,1 was
received by MIXIT. It is good practice always to do this, because if a command was for example not
registered, it shall be resent.
Notice!
The control loop in MIXIT runs each 1 second asynchronous with the fieldbus communication.
Therefore, up to 1 second may elapse before a command takes effect.
