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3. Function
This is a code to designate the function executed at a slave station. When a
message is sent from a master to a slave device the function code field tells the
slave what kind of action to perform. When the slave responds to the master, it uses
the function code field to indicate either a normal response or that some kind of
error occurred. For normal response, the slave simply echoes the original function
code. For an exception response, the slave returns a code that is equivalent to the
original function code with its most-signification bit set to a logic 1.
4. Data
Data are the data required for executing function codes. The composition of data
varies with function codes. Refer to chapter 6 for details.
A data address is assigned to each data in the temperature controller. For
reading/writing the data by communication, designate the data address.
5. CRC check
This is the code to detect message errors (change in bit) in the signal transmission.
On the MODBUS protocol (RTU mode), CRC-16 (Cyclical Redundancy Check) is
applied. For CRC calculation method, refer to section 5.5.
6. End
Following the last transmitted character, a similar interval of at least 3.5 character
times marks the end of message. A new message can begin after this interval.
5.3 Response of Slave Station
1. Response for normal command
To a relevant message, the slave station creates and sends back a response
message, which corresponds to the command message. The composition of
message in this case is the same as in section 5.2. Content of the data field depend
on the function code. For details, refer to Chapter 6.
2. Response for abnormal command
If contents of a command message have an abnormality (for example, non-actual
function code is designated) other than transmission error, the slave station does
not execute that command but creates and sends back a response message at error
detection. The composition of response message at error detection is shown on
below, the value used for function field is function code of command message plus
80H.
