Mitsubishi MELSEC LJ71C24, User Manual

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15 TROUBLESHOOTING
15.3.3 No response message is returned even though the external device transmitted a
message and the "RD" LED blinked
Symptom Cause Corrective action
MC
B
idi
rec
tio
na
l
Non-
proc
edur
al
P
redef
ined
• 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 communication protocol status
(address: 250
H
/260
H
).
• 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.
• Use the communication monitoring function to check
the message from the external device.
Note
• The 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 C24 station No.
setting and message
station No. designation do
not match.
• Check the C24 station No. setting and message
station No. designation.
• Use the communication monitoring function to check
the message from the external device.
Note
• When the station No. designation is host station
after the header byte set for each protocol and
format is received, the C24 performs receive
processing.
• The C24 data
communications
monitoring time is set to
"Infinite wait", 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 CC-Link IE Controller
Network
Refer to User's Manual (Application) for a detailed
description of the monitoring time.
• An external device
transmitted a
message and "RD"
blinked, but the C24
did not return a
response message.
("NEU." remains on
and "SD" does not
blink)
• The path setting of the
CPU module to be
accessed is incorrect.
Check the path of the CPU module to be accessed
and set the path again.