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MELSEC-Q
10 TROUBLESHOOTING
10.3.11 Troubleshooting when communications is intermittently established and lost
Symptom Cause Corrective action MC
Bidirectional
Non
procedure
• 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 signal is
turned on/off repeatedly.
• Change the wiring so that the CD 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 Chapter 8 of User's Manual (Application).
• Communications are
intermittently established
and lost.
(
1)
• 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.
1 The following explains how to take corrective actions when a normal exit response
message cannot be received upon sending of a command message.
1) When a response message of abnormal end is received
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 10.2).
2) When response messages cannot be received
Change the setting value for the response monitoring time (timer 1, default
value is 5 seconds).
(See Section 6.2 of the User's Manual (Application).)
If response messages still cannot be received after changing the setting value,
check the communication protocol setting of the corresponding interface, the
ON status of the displayed LED, and the connection cable.
3) When the first part of a response message cannot be received
Increase the setting value of the message wait time (the default value is 0 ms).
(See Section 6.4 of the User's Manual (Application).)
If response messages still cannot be received after increasing the setting value,
it is necessary to reduce the processing time from the end of transmission
processing to the start of reception processing on the external device side.