point ("float"), is supported by the use of two consecutive registers. Modbus protocol is inherently "big
endian", therefore, Modbus by the Module defaults to having the high order register first for double and
float. If the low order register comes first on the device being accessed, check the "swapped" box.
If you have "swapped" turned around, you will quickly recognize it. If floating point data is reversed, a 1.0
becomes 2.2779508e-41, which simply rounds to zero. The pattern is not as predictable as the 1.0
example would suggest. A floating point 1.1 becomes negative 107609184. If 32-bit integer data is
reversed, 1 becomes 65536.
8.2 Modbus RTU Slave Register Mapping
The mappings shown below are used when the BB2-7010 is treated as a Modbus RTU Slave. All objects are
accessed as holding registers. Analog registers MUST be read as a register pair, and will return IEEE754
floating point.
Analog Input Object
Must be read/written as
Floating Point (IEEE 754)
register pair, most significant
register first.
ANALOG INPUT
Object
Modbus Registers
AI #1
1 (read 1,2 as pair)
AI #2
3 (read 3,4 as pair)
AI #3
5 (read 5,6 as pair)
AI #300
599 (read 599,600 as pair)
Analog Output Object
Must be read/written as
Floating Point (IEEE 754)
register pair, most significant
register first.
ANALOG OUTPUT
Object
Modbus Registers
AO #1
1001 (read 1001,1002 as pair)
AO #2
1003 (read 1003,1004 as pair)
AO #3
1005 (read 1005,1006 as pair)
AO #100
1199 (read 1199,1200 as pair)
Analog Value Object
Must be read/written as
Floating Point (IEEE 754)
register pair, most significant
register first.
ANALOG VALUE
Object
Modbus Registers
AV #1
2001 (read 2001,2002 as pair)
AV #2
2003 (read 2003,2004 as pair)
AV #3
2005 (read 2005,2006 as pair)
AV #100
2199 (read 2199,2200 as pair)
Binary Input Object
Read/written as a single
holding register, any non-zero
value written will result in bit
set.
BINARY INPUT
Object
Modbus Registers
BI #1
3001
BI #2
3002
BI #3
3003
BI #300
3300
Binary Output Object
Read/written as a single
holding register, any non-zero
value written will result in bit
BINARY OUTPUT
Object
Modbus Registers
BO #1
4001
BO #2
4002
8. Configuring BB2-7010 as a Modbus RTU Slave
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