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Page 24 • MNFL003 ║ Issue/Rev. 0.1 (9/13)
4.3
Master/Slave Communications
The master communicates with the microFlow.net Liquid by sending messages con-
taining function codes. Function codes indicate the actions the microFlow.net Liquid is
to perform.
The microFlow.net Liquid’s response to the master uses the function code field to
report on the status of the task it was assigned. The two possible reports are (1) a
normal, error-free response or (2) an exception response, indicating an error. A normal
response repeats the original function code. An exception response returns a code that
corresponds to the original function code, with its most significant bit set to a logic 1.
For example, a master directs an microFlow.net Liquid to read a group of holding
registers by sending the following function code:
0000 0011 (Hexadecimal 03)
If the microFlow.net Liquid completes the action without error, its response echoes
the original command. If an error occurs, the microFlow.net Liquid returns the
following message:
1000 0011 (Hexadecimal 83)
The microFlow.net Liquid augments its exception response by adding a code in the
data field that indicates what type of error occurred. The exception response is handled
according to the parameters of the application program controlling the master device.
For example, if the relay address is absent in the microFlow.net Liquid device, the
microFlow.net Liquid will return the exception response with the exception code shown
(02). This response indicates an invalid data addressfor the microFlow.net Liquid.
A listing of the exception codes appears below:
Code
Name
Meaning
01
Illegal Function
The function code received in the query is not an
allowable action for the slave. If a Poll Program
Complete command was issued, this code indi-
cates that no program function preceded it.
02
Illegal Data Address The data address received in the query is not an
allowable value for the microFlow.net Liquid.
03
Illegal Data Value
A value contained in the query data field is not
an allowable value for the microFlow.net Liquid.
04
Command Error
An unrecoverable error occurred while the
microFlow.net Liquid was attempting to perform
the requested action.
4.4
Contents of the Data Field
The data field consists of sets of two hexadecimal digits, in the range of 00 to FF
hexadecimal.
The microFlow.net Liquid reads the data field sent by the master to perform the actions
indicated by the function code. The data field contains information such as discrete and
register addresses, the number of items to be handled, and the count of actual data
bytes in the field.
If, for example, the master directs an microFlow.net Liquid to read a group of holding
registers (function code 03), the data field sent by the master must also indicate the
starting register and the number of registers to be read. If the master writes to a group of
registers in the slave (function code 10 hexadecimal), the data field sent by the master
must also indicate the starting register, the number of registers to be written, the count
of data bytes to follow in the data field, and the data to be written into the registers.
microFlow.net Liquid Modbus Communications Manual
Appendix