Modbus Gateways - User Guide
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Version EN-092019-1.21
5.1.3
Process data
The Modbus registers representing the process data of the PROFIBUS devices start with register number 0
for both input data and output data. While the input data is mapped to input registers the output data is
mapped to holding registers. Input data and output data of a device are mapped contiguously to the
Modbus registers. In other words the process data of a module can start or end in the middle of a register.
The registers to which the process data is mapped are found in the configuration report, listing the table
Slots
for each configured device with the columns
Input Registers
and
Output Registers
. See also chapter
and the related
for details on how to create a report.
In the example above the 5 bytes process data of
Analog Input 1
are mapped to registers 0 and 1 plus the
high byte (first byte) of register 2. The letter
H
behind the start register number 0 indicates that the process
data starts in the high byte of register 0. Similarly, the letter
H
behind the end register number 2 indicates
that the process data ends in the high byte of register 2. The low byte (second byte) of register 2 is
therefore not used by process data of this module.
The 5 bytes process data of
Analog Input 2
are mapped to the low byte (second byte) of register 2 and the
registers 3 and 4. The letter L behind the register number indicates that the process data starts or ends in
the low bytes of the registers.
As a result of the contiguous mapping, register 2 includes both the last byte of the process data of the first
module and the first byte of the process data of the second module.
The following diagram shows the mapping of the process data to the registers.
The process data of further devices start at the next register.
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