BB2-7030 User Guide – Rev. 1.0
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You may access any BACnet object as a Modbus register using the above register number
calculations. You also have the option of creating a “virtual Modbus device” using the server
map. Furthermore, you have the option of using Modicon notation (40001 for holding register 1,
etc) to create your virtual device. You set this map up on the Server Map page under Modbus
TCP Setup.
IMPORTANT:
The definition of Input versus Output object is from the perspective of
the BACnet network. Therefore your Modbus client should Write to Input objects to provide
input to BACnet, and Read from Output objects to receive output from BACnet. Attempting to
write a BACnet Output object from Modbus will not work properly. You must think of your
Modbus device as the physical I/O being accessed from BACnet. If you want to make your
Modbus device write to an Output object on another BACnet device, use the BACnet client
mapping to translate a local Input to remote Output on the BACnet side.
For each register to be mapped into the custom map, enter the server address where this register
should appear, the format it should be presented in, and the source of the data. Scale factor is
optional. The source data will be multiplied by this to produce the data in the mapped server
register. Offset is optional. This value will be added to the source data after multiplying by the
scale factor.
Bit field and fill allow compiling register contents derived from multiple sources if the bit field is
defined (nonzero). The source data will be limited to the number of bits represented in the bit
field (which is a hexadecimal value), and shifted into the position represented by '1' bits in the
field. Fill bits will be logically OR'ed into the result before being presented by the server.
Consecutive server map entries that reference the same server address will all be OR'ed together
and presented at that address. Duplicate map entries that reference the same server address but
are not listed in consecutive order following the first instance will be skipped. No special bit
