Serial Interface (Modbus RTU)
134 M-Max Series Adjustable Frequency Drive MN04020003E—October 2013 www.eaton.com
Structure of the Master Request
Address:
●
The address (1 to 255) of the frequency inverter to be sent
the request is entered in parameter S2.3. Only the
frequency inverter with this address can respond to the
request
●
Address 0 is used as a so-called Broadcast (message to all
slaves) from the master. In this mode, individual slaves
cannot be addressed and data cannot be output from the
slaves
Function Code:
The function code defines the type of message. The
following operations can be executed on the M-Max:
Data
The length of the data block (Data: N x 1 Byte) depends on
the function code. This consists of two hexadecimal
character sets between 00 and FF. The data block contains
additional information for the slave in order to carry out the
operation defined by the master in the function code.
Example: number of parameters to be processed.
CRC
The telegrams in the Modbus RTU have a CRC (Cyclical
Redundancy Check). This CRC field consists 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 attached to the
end of the telegram by the master. The slave recalculates
while receiving the telegram and compares the calculated
value with the actual value in the CRC field. If both values are
not identical, an error is set.
Structure of the Slave Response
Required Transfer Time
●
The time between receiving a request from the master
and the response of the frequency inverter is at least
3.5 characters (rest time)
●
Once the master has received a response from the
frequency inverter, it must wait for at least the rest time
before it can send a new request
Normal Slave Response
●
If the master request contains a write register function
(function code 06 or 16), the frequency inverter
immediately returns the request as a response
●
If the master request contains a read register function
(function code 03 or 04), the frequency inverter returns the
read data with the slave address and the function code as
a response
Slave Response in the Event of an Error
If a request contains an error (for example, incorrect data
address or incorrect data value) other than a transfer error,
the frequency inverter returns an exception message without
executing anything. You can evaluate the exception message
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
04 Read Input Registers Reading of the input 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
10 Write Multiple Register Writing of several holding registers in the slave
With a general telegram (Broadcast) the appropriate holding registers are written in all slaves
A master request enables up to 11 registers to be read
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