−13−
6.2 Data Time Interval
In RTU mode: Below 28 bit-time (2.9 msec. at 9600 bps, 1.4 msec. at 19200 bps)
In ASCII mode: Below 1 second
When sending a message, keep the time interval of data constituting one message not longer than the
time specified above. When the time interval of data is longer than the above, the receiver unit (i.e.,
this controller) recognizes that the data transmission from the sending unit is complete, so that the
subsequent data is processed as an abnormally received message.
While the message characters must be consecutively sent in RTU mode, the ASCII modes allows for
a maximum interval of 1 second between characters, making it possible to use a master unit (PC) with
a relatively slow processing speed.
6.3 Message Configuration
The MODBUS message has the following configuration in both RTU and ASCII modes.
Slave address
Function code
Data
Error check
6.3.1 Slave address
The slave address can be programmed in advance by key operation within a range between 1 and 99.
The master unit usually communicates with one slave unit. While messages from the master unit are
received commonly by all the units in connection, only the slave unit corresponding to the slave
address included in the command message responds to the message sent.
The slave address “0” is used for a message from the master unit addressed to all the slave units
(broadcast message). The slave units do not send a response back to the master unit.
6.3.2 Function code
Function codes refer to the functions to be executed by the slave units. The data is generally classified
as follows. Refer to the reference table for details.
(1) Digital parameters
: AT Start
(2) Digital input data
: Event status.
(3) Analog parameters
: Information on various parameters. Numerical values should be :
kept within the 16-bit range between –32768 and 32767 (see the
reference table for details).
(4) Analog input data
: Information on measured data. Numerical values within the range
of 16-bits are delivered as an output.
