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MELSEC-Q
10 TROUBLESHOOTING
10.3.9 Troubleshooting when communication error "SIO" generates
Symptom Cause Corrective action MC
Bidirectional
Non
procedure
• Data does not match the
stop bit setting.
• Check if the Q series C24 and external device
settings are the same.
• Transmission rate is too
fast and the next data is
transmitted before the Q
series C24 completes
processing of the
receive data.
• Decrease the transmission rate and restart data
communications.
• Data larger than the
receive buffer size was
received.
• Use DTR and DC control and interrupt
transmission before the buffer becomes full.
Perform RS and CS control when the modem
function is used.
• Increase the transmission interval and provide
an ample PLC CPU read processing time.
Note
• The non procedure protocol stores the data
received up to first the end code received, or
fixed length data, to the non procedure
receive buffer memory and turns on the Read
Request signal to the PLC CPU.
If the next data is sent while the Read
Request signal is ON, the data is temporarily
stored to the OS receive buffer. When the OS
receive buffer becomes full, the Q series C24
skips the rest of the data and turns on the
"SIO" LED.
• Communication error
signal "SIO" turned on.
• 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
devices so that cannot transmit simultaneously.