29
• A response message from the slave in exception (error) status (When a value
out of the setting range is set.)
The function code MSB is set to 1 for the response message in exception (error)
status (Value: 86H, ASCII: 38H 36H).
If an exception code (Value: 03H, ASCII: 30H 33H Value out of the setting range)
is returned, the error can be determined by reading this code.
(Fig.
8.2.2-9)
8.2.3
RTU mode
8-bit binary data in command is transmitted as it is.
Data format Start bit
: 1 bit
Data bit
: 8 bits
Parity
: No parity
Stop bit
: 1 bit
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
characters transmission and end after idle time is processed for more than 3.5
characters transmission. (See Fig. 8.2.3-1)
(Fig.
8.2.3-1)
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.]
Function code
The function code is the command code that makes the slave to undertake the
following action types (Table 8.2.3-1).
(Table 8.2.3-1)
Function code
Contents
03 (03H)
Reading the setting value and information from slaves
06 (06H)
Setting to slaves
A function code is used to discern whether the response is normal
(acknowledgement) or if any error (negative acknowledgement) has 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, exception code (Table 8.2.3-2) 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.
3.5 idle
characters
Slave
address
Function
code
Data
Error check
CRC
3.5 idle
characters
(3AH)
Slave
address
(30H 31H)
Exception
code
Error check
LRC
Delimiter
1
2
2
2
Number of characters
Header
(30H 33H)
(37H 36H)
Function
code
(38H 36H)
(0DH 0AH)
2
2
