User guide

46 MDX-ETHERNET User Guide
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13 Advanced features
13.1 Data bytes order
When data is sent over the Modbus TCP network it is transmitted as 8-bit bytes.
Therefore when a 32-bit word or 16-bit word is transmitted it is split into four or two 8-bit
bytes. It is important that the receiving node reconstructs the received 8-bit bytes in the
correct order to arrive at the 32-bit or 16-bit data value that was originally transmitted,
this order is known as the Data Endian Format and is shown in Table 13.1.
Table 13.1 Data endian format
Most Modbus TCP master controllers use little endian format by default, many also
support big endian.
13.2 Compression of cyclical data
By default, the MDX-ETHERNET uses 32-bits for each data channel, even if the target
parameter in the drive is a 16-bit, 8 bit or 1 bit parameter. This strategy (known as
casting) ensures that the cyclic data transmitted over the Modbus TCP network is kept
aligned with memory locations in 32-bit PLCs. When cyclic data compression is enabled
(Pr 15.34 = ENABLED) a data channel will only use 32-bits if the target drive parameter
is a 32-bit parameter. If the target drive parameter is 16-bits wide 16-bits will be used. If
the target drive parameter is only 1or 8 -bits wide 16-bits will be used for that particular
data channel. This is shown in Table 13.2.
Data bytes order
Pr 15.08
Default 0 (LSB first)
Range 0 to 1
Access RW
Data
endian
format
Pr 15.08
16-bit value 32-bit value
Byte order Word order Byte order
Big
1
(MSB first)
High byte first
Low byte second
High word first
Low word second
High byte first
Mid-high byte second
Mid-low byte third
Low byte fourth
Little
0
(LSB first)
Low byte first
High byte second
Low word first
High word second
Low byte first
Mid-low byte second
Mid-high byte third
High byte fourth
Compression of cyclic data enable
Pr 15.34
Default 0 (disabled)
Range 0 to 1
Access RW