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• A response message from the slave in abnormal status (When a value out of the setting range

is set.)

(Fig.

6.2-7)

The function code MSB is set to 1 for the response message in abnormal status (86H).

The abnormal code (03H: Value out of the setting range) is returned as contents of error.

6.3

RTU mode

8-bit binary data in command is transmitted as it is.
Data format

Start bit

: 1 bit

Data bit

: 8 bits

Parity

: Even/No/Odd (Selectable)

Stop bit

: 1 bit/2 bits (Selectable)

Error detection : CRC-16 (Cyclic Redundancy Check)

Data interval : 3.5 characters transmission time or less

(1)

Message configuration

RTU mode is configured to start after idle time is processed for more than 3.5 character transmission

and end after idle time is processed for more than 3.5 character transmission. (See Fig. 6.3-1)

(Fig.

6.3-1)

(2) Slave address

Slave address is an individual instrument number on the slave side and is set within the range
00H to 5FH (0 to 95).
The master identifies slaves by the slave address of the requested message.
The slave informs the master which slave is responding to the master by placing its own address
in the response message.
[Slave address 00H (broadcast address) can identify all the slaves. However slaves do not respond.]

(3)

Function code

The function code is the command code for the slave to undertake the following action types (Table
6.3-1).
(Table 6.3-1)

Function code

Contents

03 (03H)

Reading the setting value and information from slaves

06 (06H)

Setting to slaves

Function code is used to discern whether the response is normal (acknowledgement) or if any error

(negative acknowledgement) is occurred when the slave returns the response message to the master.

When acknowledgement is returned, the slave simply returns the original function code.
When negative acknowledgement is returned, the MSB of the original function code is set as 1
for the response.
(For example, when the master sends request message setting 10H to function code by mistake,
slave returns 90H by setting the MSB to 1, because the former is an illegal function.)
For negative acknowledgement, abnormal code (Table 6.3-2) below is set to the data of response
message and returned to the master in order to inform it that what kind of error has occurred.

(Table 6.3-2)

Abnormal code

Contents

1 (01H)

Illegal function (Non-existent function)

2 (02H)

Illegal data address (Non-existent data address)

3 (03H)

Illegal data value (Value out of the setting range)

17 (11H)

Illegal setting (Unsettable status)

18 (12H)

Illegal setting (During setting mode by keypad operation, etc)

(4) Data

Data depends on the function code.
A request message from the master side is composed of data item, number of data and setting data.
A response message from the slave side is composed of number of bytes, data and abnormal code
in negative acknowledgement. Effective range of data is –32768 to 32767 (8000H to 7FFFH).

3.5 idle

characters

Slave

address

Function

code

Data

Error check

CRC

3.5 idle

characters

Slave