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MELSEC-Q
10 TROUBLESHOOTING
10.3.3 Troubleshooting when the Q series C24 does not return a response message even
though an external device transmitted a message and the "RD" LED blinked
Symptom Cause Corrective action MC
Bidirectional
Non
procedure
• Communication protocol
is set incorrectly.
• Check if the communication protocol is set
correctly.
• Check if the mode was switched without using
the buffer memory mode switching area.
Note
• The communication protocol setting contents
and the communication protocol in the current
operation can be checked at buffer memory
addresses 250
H
to (CH1) and 260
H
(CH2).
• When "Check CD
terminal" was set, the CD
terminal was turned OFF.
• Since the message is initialized (skipped reading)
when the CD terminal is turned OFF when "Check
CD terminal" is set, change the wiring so that the
CD terminal remains on all the time.
• The header byte of the
message is not data
needed by the set
protocol and format.
• Check if the external device sent a message
matched to the protocol and format.
Note
• The Q series C24 skips all the messages
transmitted until the header byte set for each
protocol and format (for example, "ENQ
(05
H
)" for format 1) is received.
• The Q series C24
station No. switch
setting and message
station No. designation
do not match.
• Check the Q series C24 station No. setting and
message station No. designation.
Note
• When the station No. designation is local
station after the header byte set for each
protocol and format is received, the Q series
C24 performs receive processing.
• The Q series C24 data
communications
monitoring time is set to
"infinite", or the
monitoring time is too
long.
• Set, or shorten, the monitoring time and
retransmit the message from the external
device and determine the trouble from the
contents of the time-out error.
Note
• Error detection by watchdog timers (timer 0 to
timer 2) is possible in the following cases.
• When part of a message is skipped.
• When transmission from an external device
is interrupted.
• When a link error was generated while
accessing another station over a
MELSECNET/H, MELSECNET/10.
See Chapter 6 of User's Manual (Application)
for a detailed description of the monitoring time.
• An external device
transmitted a message
and "RD" blinked, but
the Q series C24 did
not return a response
message. ("NEU"
remains on and "SD"
does not blink)
• The path setting of the
PLC CPU to be
accessed is incorrect.
Check the path of the PLC CPU to be accessed
and set the path again.