Mitsubishi Electric FR-D720S SC EC, Instruction Manual

Page: 377 / 512

Parameter Communication operation and settings
FR-D700 SC EC 6 - 227
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.
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
I001228E
Fig. 6-117: Data transmission
Start Address Function Data CRC Check End
T1 8 bits 8 bits n
×
8 bits
L
8 bits
H
8 bits
T1
Message Field Description
Address field
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 station number" is the slave address.
Function field
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.
Data field
The format changes depending on the function code (refer to page 6-228). Data includes the
byte count, number of bytes, description of access to the holding register, etc.
CRC check
field
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.
Tab. 6-68: Protocol details
Query message from Master
Device Address Device Address
Function Code Function Code
Eight-Bit
Data Bytes
Eight-Bit
Data Bytes
Error Check Error Check
Response message from slave
Code Function Name Outline
Broadcast
Communica-
tion
H03 Read Holding Register Reads the holding register data. Disallowed
H06 Preset Single Register Writes data to the holding register. Allowed
H08 Diagnostics
Function diagnosis.
(communication check only)
Disallowed
H10 Preset Multiple Registers
Writes data to multiple
consecutive holding registers.
Allowed
H46
Read Holding Register
Access Log
Reads the number of registers that
succeeded in communication last
time.
Disallowed