10 TROUBLESHOOTING

MELSEC-Q

10.3.3Troubleshooting when the Q series C24 does not return a response message even though an external device transmitted a message and the "RD" LED blinked

Symptom

Cause

 

 

Corrective action

MC

 

 

Check if the communication protocol is set

 

 

 

 

correctly.

 

 

 

• Check if the mode was switched without using

 

 

• Communication protocol

 

the buffer memory mode switching area.

 

 

 

Note

 

 

 

is set incorrectly.

 

• The communication protocol setting contents

 

 

 

 

 

 

 

 

and the communication protocol in the current

 

 

 

 

operation can be checked at buffer memory

 

 

 

 

addresses 250H to (CH1) and 260H (CH2).

 

 

• When "Check CD

Since the message is initialized (skipped reading)

 

 

 

when the CD terminal is turned OFF when "Check

 

 

terminal" was set, the CD

 

 

 

 

CD terminal" is set, change the wiring so that the

 

 

terminal was turned OFF.

 

 

 

 

CD terminal remains on all the time.

 

 

 

• Check if the external device sent a message

 

 

• The header byte of the

 

matched to the protocol and format.

 

 

 

Note

 

 

 

message is not data

 

 

 

 

 

• The Q series C24 skips all the messages

 

 

needed by the set

 

 

 

 

transmitted until the header byte set for each

 

 

protocol and format.

 

 

• An external device

 

protocol and format (for example, "ENQ

 

 

 

(05H)" for format 1) is received.

 

transmitted a message

 

 

 

 

• Check the Q series C24 station No. setting and

 

and "RD" blinked, but

• The Q series C24

 

 

message station No. designation.

 

the Q series C24 did

station No. switch

 

Note

 

 

not return a response

setting and message

 

• When the station No. designation is local

 

message. ("NEU"

station No. designation

 

station after the header byte set for each

 

remains on and "SD"

do not match.

 

protocol and format is received, the Q series

 

 

C24 performs receive processing.

 

does not blink)

 

 

 

 

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.

 

 

 

 

 

 

 

 

• The Q series C24 data

 

Note

 

 

communications

 

• Error detection by watchdog timers (timer 0 to

 

 

monitoring time is set to

 

timer 2) is possible in the following cases.

 

 

 

• When part of a message is skipped.

 

 

"infinite", or the

 

 

 

 

• When transmission from an external device

 

 

monitoring time is too

 

 

 

 

 

is interrupted.

 

 

long.

 

• When a link error was generated while

 

 

 

 

 

accessing another station over a

 

 

 

 

 

MELSECNET/H, MELSECNET/10.

 

 

 

 

See Chapter 6 of User's Manual (Application)

 

 

 

 

for a detailed description of the monitoring time.

 

 

• The path setting of the

 

Check the path of the PLC CPU to be accessed

 

 

PLC CPU to be

 

and set the path again.

 

 

accessed is incorrect.

 

 

 

 

Bidirectional Non procedure

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