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8. COMMUNICATIONS USING DEDICATED PROTOCOLS
MELSEC-A
8.2.3
Precautions during data communications
The conditions under which the AJ71C24 transmission sequence is
initialized are as follows:
*The power supply is turned ON or the PC CPU is reset with the reset
switch.
Data communications is completed normally.
*The control code EOT or CL is received.
*The NAK control code is received.
During full-duplex communications through the RS-232C interface,
the CD signal is turned OFF.
(The ONIOFF status of the CD signal is ignored if the CD terminal
check function is disabled.)
NULL code transmission from the AJ71 C24
A framing error might occur in the AJ71C24 if nothing is sent from the
computer to the AJ71C24 via the RS-422 interface. In this case, the
AJ71 C24 sends 'OOH' (NULL code) to the computer. These NULL codes
should be ignored by the computer.
The computer should also ignore all data sent from the AJ71C24 prior
to an STX, ACK, or NAK code.
NAK response from the AJ71 C24
The NAK response is given from the AJ71C24 to the computer using
the dedicated protocol if an error is detected.
Therefore, the NAK
response may be output even while the computer is sending data in the
full-duplex communications mode.
Data link error processing
The AJ71C24 enters the standby state (see Section 3.4
1 1 0
list for
programmable controller CPU) if a data link error occurs during data
communications with a PC CPU (the PC CPU number being other than
FFH) on MELSECNET.
If an error is detected by the computer when executing the time check,
send a clear command (EOT or CL, see Section
8.4.5
(1)) to initialize
the transmission sequence.
Sending a command from the computer
When sending a command from the computer to the AJ71 C24 using the
dedicated protocol, send the command only after the data communica-
tions called by the preceding command is completed.
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