5 Implementation of Modbus TCP
5.7 Bit areas: mapping of input-discrete- and coil-areas
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XI/ON: XNE-GWBR-2ETH-MB
10/2011 MN05002008Z-EN
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5.7
Bit areas: mapping of input-discrete- and coil-areas
The digital in- and outputs can be read and written (for outputs) as registers in the data area of
the packed in- and output process data.
In order to set for example a single output (single coil), the following functions are available for
reading and writing single bits:
• FC1 ("Read Coils"),
• FC2 ("Read Discrete Inputs"),
• FC 5 ("Write Single Coil")
• FC15 ("Write Multiple Coils")
Data mapping in the input-discrete- and coil-areas:
• Mapping: input-discrete-area
All digital inputs are stored in this area (offset "0").
• Mapping: Coil-area
All digital outputs are stored in this area (offset "0").
5.8
Error behavior of outputs
In case of a failure of the Modbus communication, the outputs’ behavior is as follows,
depending on the defined time for the Watchdog (register 0x1120, page 54):
• Watchdog = 0 ms (default setting)
→
outputs hold the momentary value
• Watchdog > 0 ms
→
outputs switch to "0" after the watchdog time has expired
Note
In the packed process data, the digital I/O data are stored following the variable in-
and output data area of the intelligent modules, which means they are stored with a
variable offset, depending on the station’s I/O-configuration.
Note
Setting the outputs to predefined substitute values is not possible in Modbus TCP.
Eventually parameterized substitute values will not be used.
