7 DATA COMMUNICATIONS USING BIDIRECTIONAL PROTOCOL

MELSEC-Q

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.

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