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MELSEC-Q
11 TROUBLESHOOTING
11.3.13 Communication is intermittent.
Symptom Cause Corrective action
MC
Bidirectional
Non
procedure
Pre-defined
• With a multidrop link, two or
more devices transmitted
data at the same time.
• Connect the external devices and the Q series C24 in
a 1:1 configuration and perform a communications
test. Since data is transmitted simultaneously from
two or more devices with a multidrop link when
normal communications among all the devices is
possible, interlock the external devices so that they do
not transmit simultaneously.
• The signal cable wiring
connection is faulty.
• Replace the cable, or make the connections firm.
• When "Check CD terminal" is
set, the CD(DCD) signal is
turned on/off repeatedly.
• Change the wiring so that the CD(DCD) signal
remains ON all the time, or change the setting to "Do
not check CD terminal".
• When half-duplex
communications is selected,
the ON/OFF timing of each
signal is not matched.
• Control the external device so that the ON/OFF
timing of each signal is matched.
See User's Manual (Application).
• Requested processing of the
next transmission before the
processing of the current
transmission is completed.
• Requested processing of the
next reception before the
processing of the current
reception is completed.
• Make sure to obtain handshake in the sequence
program.
• Communication via modem
is disrupted.
• Since communication via modem may be disrupted
due to low line quality or instantaneous interruption,
create a sequence program for reconnecting the line
in case of unexpected line disruption.
If this symptom occurs frequently, reduce the
communication speed of the modem, and retry the
communication.
• Communications are
intermittently
established and lost.
(
1)
• The data sent from the Q
series C24 are received
back on the RS-422/485
interface (CH2) side.
• Disable the echo back in the Echo back
enable/disable setting before communication.
1 The following explains how to take corrective actions when a normal exit response
message cannot be received upon sending of a command message.
(a) When a response message of abnormal end is received
1) When an external device sent a command message and received an
abnormal end response message, take corrective action with the error
according to the error code in the response message (see Section 11.2).
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