Mitsubishi Electric FR-E700 Instruction Manual page 422

Communication operation and setting
Message frame (protocol)
Communication method
Basically, the master sends a query message (question) and the slave returns a response message (re-
sponse). When communication is normal, Device Address and Function Code are copied as they are,
and when communication is abnormal (function code or data code is illegal), bit 7 (= 80h) of Function
Code is turned on and the error code is set to Data Bytes.
Fig. 6-133: Data transmission
The message frame consists of the four message fields as shown above. By adding the no-data time
(T1: Start, End) of 3.5 characters to the beginning and end of the message data, the slave recognizes
it as one message.
Protocol details
Start
T1
Message Field
Address field
Function field
Data field
CRC check
field
Tab. 6-89: Protocol details
6 - 268
Query message from Master
Device Address
Function Code
Eight-Bit
Data Bytes
Error Check
Address Function
8 bits 8 bits
Description
The address code is 1 byte long (8 bits) and any of 0 to 247 can be set. Set "0" to send a broadcast
message (all-address instruction) or any of 1 to 247 to send a message to each slave. When the slave
responds, it returns the address set from the master. The value set to Pr. 117 "PU communication sta-
tion number" is the slave address.
The function code is 1 byte long (8 bits) and can be set to any of 1 to 255. The master sets the func-
tion that it wants to request from the slave, and the slave performs the requested operation. The fol-
lowing table gives the supported function codes. An error response is returned if the set function
code is other than those in the following table. When the slave returns a normal response, it returns
the function code set by the master. When the slave returns an error response, it returns H80 + func-
tion code.
Code Function Name
H03 Read Holding Register
H06 Preset Single Register
H08 Diagnostics
H10 Preset Multiple Registers
Read Holding Register Access
H46
Log
The format changes depending on the function code (refer to page 6-269). Data includes the byte
count, number of bytes, description of access to the holding register, etc.
The received message frame is checked for error. CRC check is performed, and 2 byte long data is
added to the end of the message. When CRC is added to the message, the low-order byte is added
first and is followed by the high-order byte. The CRC value is calculated by the sending side that adds
CRC to the message. The receiving side recalculates CRC during message receiving, and compares
the result of that calculation and the actual value received in the CRC check field. If these two values
do not match, the result is defined as error.
Device Address
Function Code
Eight-Bit
Data Bytes
Error Check
Response message from slave
Data CRC Check
L
n 8 bits
8 bits
Outline
Reads the holding register data.
Writes data to the holding register.
Function diagnosis.
(communication check only)
Writes data to multiple consecutive
holding registers.
Reads the number of registers that
succeeded in communication last
time.
Parameter
I001228E
End
H
T1
8 bits
Broadcast
Communica-
tion
Disallowed
Allowed
Disallowed
Allowed
Disallowed