Eaton PowerXL DC1 Series - Structure of the Master Request

8 Modbus RTU
8.3 Operating mode Modbus RTU
140 DC1 variable frequency drive 12/13 MN04020003Z-EN www.eaton.com
8.3.1 Structure of the master request
8.3.1.1 Address
• P-36 is used to enter the address (1 to 63) of the variable frequency drive
to which the request is being sent. Only the variable frequency drive
with this address can respond to the request.
• Address 0 is used as a so-called Broadcast (message to all bus users)
from the master. In this mode, individual modules cannot be addressed
and data cannot be output from the slaves.
8.3.1.2 Function code
The function code defines the type of message. The following actions can be
performed in the case of DC1 variable frequency drives:
8.3.1.3 Data
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).
8.3.1.4 Cyclic redundancy check (CRC)
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 check is always run independently of the parity check
for the individual characters of the telegram. 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.
Function code [hex] Designation Description
03 Read Holding Registers Reading of the holding registers in the slave (process
data, parameters, configuration).
A master request enables up to 11 registers to be read.
06
Write Single Register 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.