ICC XLTR-1000 Instruction Manual page 138

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• Specific bits within the database can be accessed as either coils (0X
references) or discrete inputs (1X references).
• 32-bit register accesses are supported in a variety of options and formats.
• Because the transaction is handled locally within the gateway, write data
checking is not available. For example, if a write is performed to a register
with a data value that is out-of-range of the corresponding data element, no
Modbus exception will be immediately returned.
• Configuration tip: Improved network utilization may be obtained by
appropriately grouping contiguous register assignments in the database. In
this way, the "read multiple registers", "read input registers" and "write
multiple registers" functions can be used to perform transfers of larger blocks
of registers using fewer Modbus transactions compared to a situation where
the read/write registers were arranged in an alternating or scattered fashion.
9.9.2.2 Holding & Input Register Mappings
The Modbus RTU slave driver provides read/write support for holding registers
(4X references) and read-only support for input registers (3X references). Both
holding registers and input registers access the same data. For example, reading
Holding Register 4 returns the same data as reading Input Register 4. By default,
registers are mapped into the database using the following scheme:
Register 1 is mapped to address 0,
Register 2 is mapped to address 2,
Register 3 is mapped to address 4,
:
Arithmetically, the register-to-address relationship can be described via Equation
8:
Additionally, a register remap object can be created to map a register to a
different address in the database, or to map a register that is outside of the
default mapping into the database. Refer to section 8.6.12.2 for more information
on configuring register remap objects.
For clarity, let's use Equation 8 in a calculation example with a remap object.
Let's assume we have defined a register remap object to remap register 25 to
database address 62. This means that instead of register 25 mapping to address
48 (as it would with the default mapping), it will now map to address 62. Now say
we wish to read registers 24 and 25. We already know that register 25 maps to
database address 62, so we must use Equation 8 to calculate what address
register 24 is mapped to. Using the equation, we can determine that register 24 is
mapped to database address 46. So reading registers 24 and 25 will return data
from addresses 46 and 62 in the database, respectively.
= × (
address 2 register
− )
1
137
Equation 8