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MELSEC-Q
7 DATA COMMUNICATIONS USING BIDIRECTIONAL PROTOCOL
POINT
(1) Perform error processing according to the error code received immediately
after the NAK message at the device that received NAK as the response
message after data transmission.
Chapter 10 shows the error codes that are transmitted from the Q series C24.
(2) If the Q series C24 receives an NAK response while transmitting data to an
external device, it completes data transmission, then reads the NAK, perform
abnormal completion.
(3) If the Q series C24 detects an error while receiving data, it ignores the receive
data corresponding to the data length.
If the data length is abnormal, the Q series C24 ignores all the data up to the
receive message head data (ENQ, etc.) received thereafter.
(5) External device time-out check
When checking time-out up to reception of the response message at the external
device during data transmission from external device to the Q series C24, make
the time-out time the time shown below, or longer.
(PLC CPU maximum scan time
2) + 100 ms
(6) External device framing error
When nothing is transmitted from the Q series C24 to the external device though
the RS-422 or RS-422/485 interface, a framing error may be generated in the
external device.
Make the external device skip the data up to transmission of the head data (ENQ,
NAK, etc.) of the message from the Q series C24.
Before communicating data through the RS-422/485 interface, check Q series
C24 specifications given in Section 3.3.
(7) Data bit settings
When adding a sum check code to a message, set the data bits to 8 bits using the
transmission setting of the GX Developer.
For more details on the data bit settings, see Section 4.5.