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MELSEC-Q
10 TROUBLESHOOTING
10.3.2 Troubleshooting when "RD" LED does not blink even though an external device is
transmitting a message
Symptom Cause Corrective action MC
Bidirectional
Non
procedure
• Signal lines are not
connected correctly.
• Check if the Q series C24 and external RD and
SD signal lines are cross connected.
• Use the external device OPEN statement to
check if the opened port and the cable
connection port are matched.
• External device
transmission control
signals are not turned
ON.
• Connect the wiring so that the "DSR", "CS", and
other transmission control signals are ready.
Note
• Since the control signal specifications depend
on the device, connect the wiring by seeing
the instruction manual of the device used.
• "RD" does not blink
even through an
external device is
transmitting a message.
• If the message passes
through an RS-232
RS-422 converter,
modem, etc., the signal
is interrupted at one of
the intervening devices.
• Check the specifications of the converter and
modem and recheck the settings and wiring
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