FEC920: USER GUIDE
Page 182
September 18
8.3.3 Read/Write Access to Internal Modbus Registers
Read/write access to any internal Modbus register is provided using pairs of BACnet objects named ‘User
Parameters’ as shown in Table 4.
30 User Parameter pairs (numbered 1 to 30) are supported.
Table 4: Modbus Register Access
This functionality allows the BACnet user to access any internal parameter that is available in the standard
Modbus address space. Pairs of values as shown in Table 4 are implemented as two BACnet Analog Value
objects. The BACnet client (typically a BMS) will write the first value with the Modbus address for the data
parameter required as shown in Figure 109. The user may obtain the Modbus address from Section 7.3 of this
manual.
Figure 109 BMS Client writes address of required parameter
The BMS client may then read or write to the data referenced by this address, as shown in Figure 110.
Figure 110 BMS Client reads/writes to data parameter
Object Name
Type
Data Parameter
Internal Path
R/W
UserParameter.
n
.Address
AV
R/W User Parameter
n
VirtualChannel.n.Alarm1.Threshold
R/W
UserParameter.n.Value
AV
VirtualChannel.n.Alarm1.Status
R/W
Note:
The data value will always be represented over BACnet as a floating point, even if the
internal source data is of a different type (e.g. Bool). Strings will not be accessible using
this mechanism.
Note:
Values written to device parameters via BACnet may be overwritten (with different
values) internally by the device firmware under certain device configurations. Due to
this, when the values of the BACnet objects are read back, they may be different to the
value requested via the preceding BACnet write command.
A5E45696052A Rev-AA
