Mitsubishi Electric 800 Series Instruction Manual page 145

NOTE
• During broadcast communication, functions are executed regarded of the set inverter station number (Pr.117).
 Message frame (protocol)
• Communication method
Basically, the master sends a query message (inquiry), and slaves return a response message (response). At normal
communication, the device address and function code are copied as they are, and at erroneous communication (illegal
function code or data code), bit 7 (= H80) of the function code is turned ON, and the error code is set at data bytes.
Query message from Master
Device Address
Function Code
Eight-Bit
Data Bytes
Error Check
Message frames comprise the four message fields shown in the figures above.
A slave recognizes message data as one message when a 3.5 character long no-data time (T1: start/end) is added before
and after the data.
• Details of protocol
The following table explains the four message fields.
Start
T1
Message field
Address field
Function field
Data field
CRC check field
 Function code list
Read/
Function name
Read holding
Read
registers
144
3. RS-485 Communication
3.5 MODBUS RTU
Device Address
Function Code
Eight-Bit
Data Bytes
Error Check
Response message from slave
Address Function
8 bits 8 bits
"0 to 247" can be set in the single-byte (8-bit) length field. Set "0" when sending broadcast messages
(instructions to all addresses), and "1 to 247" to send messages to individual slaves.
The response from the slave also contains the address set by the master. The value set in Pr.117 PU
communication station number is the slave address.
"1 to 255" can be set as the function code in the single-byte (8-bit) length filed. The master sets the function to
be sent to the slave as the request, and the slave performs the requested operation. Refer to the function code
list for details of the supported function codes. An error response is generated when a function code other than
those in the function code list is set.
The normal response from the slave contains the function code set by the master. The error response contains
H80 and the function code.
The format changes according the function code. (Refer to
count, number of bytes, and accessing content of holding registers.
Errors in the received message frame are detected. Errors are detected in the CRC check, and the 2 bytes
length data is appended to the message. When the CRC is appended to the message, the lower bytes of the
CRC are appended first, followed by the upper bytes.
The CRC value is calculated by the sender that appends the CRC to the message. The receiver recalculates
the CRC while the message is being received, and compares the calculation result against the actual value
that was received in the error check field. If the two values do not match, the result is treated as an error.
Code
write
The data of the holding registers is read.
The various data of the inverter can be read from MODBUS
registers.
System environmental variable (Refer to
H03 Real time monitor (Refer to the FR-E800 Instruction Manual
(Function).)
Fault history (Refer to
Model information monitor (Refer to
Inverter parameters (Refer to
Data
L
n × 8 bits
8 bits
Description
page
Outline
page 150.)
page 152.)
page 152.)
page 151.)
CRC check
H
T1
8 bits
145.) The data, for example, includes the byte
Broadcast
communication
Not available
End
Message
format
reference
page
page 145