5.5 Message Configuration
5.5.1 ASCII Mode
ASCII mode message is configured to start by Header [: (colon) (3AH)] and end by Delimiter
[CR (carriage return) (0DH) + LF (Line feed) (0AH)].
Data section: Max. 2 x 252 characters
Header
(:)
Slave
address
Function
code
Data
Error check
LRC
Delimiter
(CR)
Delimiter
(LF)
5.5.2 RTU Mode
RTU mode is configured to start after idle time is processed for more than 3.5 character
transmissions, and end after idle time is processed for more than 3.5 character transmissions.
(Communication speed 9600 bps, 19200 bps: 1.5 character transmission times,
Communication speed 38400 bps: 1.75 ms)
Data section: Max. 252 bytes
3.5 idle
characters
Slave
address
Function
code
Data
Error check
CRC-16
3.5 idle
characters
(1) Slave Address Field
Slave address is an individual instrument number on the slave side, and is set within the range
1 to 95 (01H to 5FH). 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 0 (00H, Broadcast address) can identify all the slaves connected. However, slaves
do not respond.
(2) Function Code Field
The function code is the command code for the slave to undertake one of the following actions.
Type
Function
Code
Sub-Function
Code
Contents
03 (03H) Reads a single or multiple piece(s) of data
from slave(s).
04 (04H) Reads information from slave(s).
06 (06H) Writes a single piece of data to slave(s).
Data access
16 (10H) Writes multiple pieces of data to slave(s).
08 (08H) 00 Echoes back the request message.
Diagnostics
43 (2BH) 14 Reads device identification information.
The 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, if the master sends request message setting 13H to the function code by mistake,
slave returns 93H by setting the MSB to 1, because the former is an illegal function.)
For negative acknowledgement, the exception codes below are set to the data of the response
message, and returned to the master in order to inform it of what kind of error has occurred.
Exception 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)
Status unable to be written. [(e.g.) AT is performing.]
18 (12H) During setting mode by keypad operation
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