Mitsubishi Electric FR-F740 EC Instruction Manual page 443

Parameter
Message frame (protocol)
Communication method
Basically, the master sends a query message (question) and the slave returns a response mes-
sage (response). When communication is normal, Device Address and Function Code are cop-
ied 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. 6-168: 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. 6-76:
FR-F700 EC
Query message from Master
Device Address
Function Code
Eight-Bit
Data Bytes
Error Check
Address Function
8 bits 8 bits
Description
Is 1 byte long (8 bits), and can be set to any of 0 to 247. 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. 331
"RS485 communication station" 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
function that it wants to request from the slave, and the slave performs the requested opera-
tion. The following 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 nor-
mal response, it returns the function code set by the master. When the slave returns an error
response, it returns H80 + function code.
Code Function Name
H03 Read Holding Register
H06 Preset Single Register
H08 Diagnostics
H10 Preset Multiple Registers
Read Holding Register
H46
Access Log
The format changes depending on the function code (refer to page 6-294). 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 mes-
sage 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.
Protocol details
Communication operation and setting
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.
Makes a function diagnosis.
(Communication check only)
Writes data to multiple
consecutive holding registers.
Reads the number of registers that
succeeded in communication last
time.
I001228E
End
H
T1
8 bits
Broadcast
Communica-
tion
Disallowed
Allowed
Disallowed
Allowed
Disallowed
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