Appendix E Modbus Rtu Details; Message Framing - Michell Instruments Condumax II User Manual

Condumax II User's Manual
Appendix E Modbus RTU Details
E.1

Message Framing

START
3.5t
Start and End
The message begins and ends with a silent delay of 3.5 character times at the baud
rate of the network.
Address
The fi rst byte transmitted is the address of the Condumax instrument, which has an
address range of 1 to 247 or 01H to F7H. The master addresses the Condumax by
placing an address in the address byte and, if matched by the Condumax own address,
it will response to the message, otherwise it will be ignored. See Appendix B on setting
the address.
Function Code
The function code tells the Condumax which operation is to be performed on the data
in the following data bytes. The only valid codes are 03 (Read Holding Registers), or
06 (Write To Single Register) as these are the only two implemented in the Condumax.
An exception can occur if the message contains an unsupported function code, an illegal
data address or an illegal data value. If this occurs, the function code is incremented
by 80H and the data bytes returned are set to a value that describes the error. See the
section on exception responses.
Data Bytes
The data bytes within the message from the master contain additional information that
the Condumax must use to perform the action defi ned in the function code, such as the
starting register address and the number of registers to be retrieved.
CRC
The CRC is a 2 byte error check value from the result of a Cyclical Redundancy Check
calculation performed on the message contents. The CRC is appended to the message
as the last fi eld in the message, whereby the low-order byte is appended fi rst, followed
by the high-order byte.
Michell Instruments
FUNCTION
ADDRESS
CODE
1 byte 1 byte
DATA CRC
n x bytes 2 bytes
APPENDIX E
END
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