Mitsubishi Electric FR-A800 Series Instruction Manual page 689

 Outline
• The MODBUS communication protocol was developed by Modicon for programmable controllers.
• The MODBUS protocol uses exclusive message frames to perform serial communication between a master and slaves.
These exclusive message frames are provided with a feature called "functions" that allows data to be read or written. These
functions can be used to read or write parameters from the inverter, write input commands to the inverter or check the
inverter's operating status, for example. This product classifies the data of each inverter into holding register area (register
address 40001 to 49999). The master can communicate with inverters (slaves) by accessing pre-assigned holding register
addresses.
NOTE
• There are two serial transmission modes, the ASCII (American Standard Code for Information Interchange) mode and the RTU
(Remote Terminal Unit) mode. However, this product supports only the RTU mode, which transfers 1 byte data (8 bits) as it
is. Also, only communication protocol is defined by the MODBUS protocol. Physical layers are not stipulated.
 Message format
Programmable controller
(Master)
Programmable controller
(Master)
• Data check time
Item
Monitoring, operation command,
frequency setting (RAM)
Parameter read/write, frequency setting
(EEPROM)
Parameter clear / All parameter clear
Reset command
• Query
A message is sent to the slave (the inverter) having the address specified by the master.
• Normal response
After the query from the master is received, the slave executes the request function, and returns the corresponding normal
response to the master.
• Error Response
When an invalid function code, address or data is received by the slave, the error response is returned to the master.
This response is appended with an error code that indicates the reason why the request from the master could not be
executed.
This response cannot be returned for errors, detected by the hardware, frame error and CRC check error.
• Broadcast
The master can broadcast messages to all slaves by specifying address 0. All slaves that receive a message from the
master execute the requested function. With this type of communication, slaves do not return a response to the master.
NOTE
• During broadcast communication, functions are executed regarded of the set inverter station number (Pr.331).
688
5. PARAMETERS
5.15 (N) Communication operation parameters
Downloaded from ManualsNet.com
Query communication
Query Message
Inverter (slave)
Broadcast communication
Query Message
Inverter (slave)
Less than 12 ms
Less than 30 ms
< 5 s
No reply
search engine
Inverter response time
(Refer to the following table for the
data check time)
Response Message
Data absence time
(3.5 bytes or more)
No Response
Check time