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6
MODBUS PROTOCOL
Basic Procedures of Communications and Precautions
CAUTION
1. When you set the parameter (writing), set the key lock at first.
LT830 controller is always ready for communication. It responds at anytime to data requests
from the personal computer. However, when you set the parameter or control the instrument
from the personal computer, you need to set key lock (lock 3) on LT830 at first. Key lock will
be set at the front key on LT830 or through personal computer communication. If you try to set
the parameter or operate instrument from the personal computer at Non-lock, Lock1, Lock 2
LT830 will show error code 12H.
（
See paragraph 6.6
）
2. Take care of command re-transmission as there is no control signal line in use.
Since the LT830 controllers’ serial interfaces communicate freely without using any control line,
a reception failure may occur under some conditions. Exercise care when resending a command.
3. Don't disconnect or short any cables or instruments constituting the serial
interface, or turn the power on or off during communications.
Don't disconnect or short any cables or instruments constituting the serial interface, or turn
the power on or off during communications, or the operation may stop or lead to a malfunction.
When this happens, all the components of the serial interface must be reset to repeat the
operation from the beginning.
4. Send the next command after making sure that the communications drive has
been turned off.
For RS-485 communications interface, multiple instruments are connected to the
same communications line, only one instrument, of which instrument No. is specified by the
PC, drives the communications line. The communications drive is turned off at a certain
time (approx. 5 msec) after sending the last character so that all the characters are safely
received by the personal computer. If the PC sends a command to the next unit before
the communications drive is turned off, signals interfere with each other resulting in some
communication failure. Exercise caution when you use a high-speed PC.
