FPΣ User's Manual
8.8 Modbus RTU Communication
245
Modbus RTU slave function
After having received a command message from the master station, the slave stations send
back the response message based on the instructions received. Do not execute the
F145_WRITE and F146_READ instructions on slave stations.
1
Master
2
Slave
Modbus RTU command message frame
START ADDRESS FUNCTION DATA CRC CHECK END
Transmission time for
3.5 characters
8 bits 8 bits
n × 8 bits
16 bits
Transmission time for
3.5 characters
ADDRESS
(station no.)
8 bits, 0–99 (decimal)
1)
0 = broadcast address
FUNCTION
8 bits
DATA
Varies depending on the commands.
CRC
16 bits
END
Transmission time for 3.5 characters (differs depending on baud rate). Please refer to
"Reception done judgment time".
1)
FPWIN Pro does not support the address range from 0–247 of the Modbus RTU protocol.
Response in normal status
The same message as a command is returned for a single write command. A part of a
command message (6 bytes from the beginning) is returned for a multiple write command.
Response in abnormal status
If a parameter which is to be processed but is disabled is found in a command (except for a
transmission error):
ADDRESS FUNCTION + 80H ERROR CODE CRC
ERROR CODE
1: Illegal function
2: Illegal data address (no word address)
3: Illegal data value (not a multiple of 16)
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