－13－
5.2 Data Time Interval
In RTU mode: Below 28 bit-time (2.8 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 scanner)
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.
5.3 Message Configuration
The MODBUS message has the following configuration in both RTU and ASCII modes.
Slave address
Function code
Data
Error check
5.3.1 Slave address
The slave address can be programmed in advance by key operation within a range between 1 and 31. 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.
In case of the RS-232C, send "1" as a slave address. This recorder also sends a slave address of "1".
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.
5.3.2 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:
Parameters are mainly for changing the functions.
(2) Digital input data:
Parameters are such as input data status and alarm 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). If the data cannot be expressed with 16-bit numbers, use floating
data for reading and writing.
(4) Analog input data:
Information on measured data and instrument specifications. Numerical
values within the range of 16-bits are delivered as an output. If the data
cannot be expressed with 16-bit numbers, use floating data (floating-point
data) for reading.
(5) Floating data:
If the data cannot be expressed with the numerical values within the 16-bit
range (between –32768 and 32464), use floating data for its expression.
This expression is not available in the standard MODBUS.
