Modbus Rtu Details; Message Framing - Michell Instruments Liquidew EExd User Manual

Liquidew EExd User's Manual
Appendix F

Modbus RTU Details

F.1

Message Framing

START ADDRESS
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 first byte transmitted is the address of the instrument, which has a range of 1 to
247 or 01H to F7H. The master addresses an instrument by placing the address in the
address byte and, if matched, a Liquidew EExd will response to the message, otherwise
it will be ignored. See Appendix E on setting the address.
Function Code
The function code tells the Liquidew EExd which operation is to be performed on the
data in the following data bytes. The only valid codes are
or 06 (Write To Single Register)
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
Appendix F.2.
Data Bytes
The data bytes within the message from the master contain additional information that
the Liquidew EExd must use to perform the action defined 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 field in the message. The low-order byte is appended first, followed by the
high-order byte.
Michell Instruments
FUNCTION
CODE
1 byte 1 byte
as these are the only two implemented.
DATA CRC
n x bytes 2 bytes
03 (Read Holding Registers)
APPENDIX F
END
3.5t
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