Structure Of The Master Request - Eaton PowerXL DA1 Communications Manual

2 Modbus RTU
2.6 Operating mode Modbus RTU

2.6.1 Structure of the master request

2.6.1.1 Address
2.6.1.2 Function Code
2.6.1.3 Data
2.6.1.4 Cyclic redundancy check (CRC)
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• Parameter P5-01 (DA1), P-36 (DC1), or P-34 (DE1) is used to enter the
address (1 to 63) of the variable frequency drive/variable speed starter to
which the request is being sent. Only the variable frequency drive/
variable speed starter with this address will be able to respond to the
request.
• Address 0 is used for what is referred to as broadcasts (message to all
bus nodes) from the master. In this mode, individual nodes cannot be
addressed and data cannot be output by the slaves.
The function code defines the type of message.
The following actions can be performed:
Function code Designation
[hex]
03 Read Holding Registers
06 Write Single Register
The length of the data block (data: N x 1 bytes) depends on the function
code. The function code is made up of two hexadecimal values and has a
range of 00 to FF. The data block contains additional information for the slave
that will enable the latter to perform the operation specified by the master in
the function code (this information may specify the parameters that need to
be processed, for example).
Frames in the Modbus RTU operating mode include a cyclic redundancy
check (CRC). The CRC field is made up of two bytes that contain a binary
16-bit value. The CRC is always performed independently of the parity check
for the frame's individual values. The CRC result is appended to the frame by
the master. While receiving the frame, the slave performs a new calculation
and compares the calculated value to the actual value in the CRC field. If both
values are not identical, an error is set.
Modbus RTU for variable frequency drive DA1, DC1, DE1 09/16 MN040018EN www.eaton.com
Description
Used to read the holding registers (process data, parameters,
configuration) in the slave.
A master request enables up to 11 registers to be read.
Writing of a holding register in the slave.
With a general telegram (Broadcast) the appropriate holding
registers are written in all slaves. The register is read back
for comparison.