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5
MODBUS PROTOCOL
Basic Procedures of Communications and Precautions
Attention!
1. A data request immediately after turning the power on will lead to an error.
The SE3000 series field scanners are always ready for communications. They are at anytime
responsive to data requests from personal computers. However, immediately after turning the power on,
the recorders do not deliver a normal response until the data of all channels is ready. It takes, for
instance, about 8 seconds until all the data becomes ready for the SE3000 field series scanners for 48
points. When receiving a data request during this period, the SE3000 field series scanners return the
error message No.12 (set mode error).
2. Take care of command re-transmission as there is no control signal line in use.
Since the SE3000 series field scanners' 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 RS422A/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.
