7 DATA COMMUNICATIONS USING BIDIRECTIONAL PROTOCOL
7.4 Data Communications Precautions
The following shows the precautions when performing data communications using the bidirectional protocol.
(1)When the transmission sequence is in the initial status, it indicates that data transmission and reception processing has not been started.
The Q series C24 transmission sequence is initialized in the following cases.
•The power is turned on, the CPU is reset.
•When operating, or the mode is switched.
•When a response message (ACK, NAK, etc.) was received in reply to data transmission.
•When a response message (ACK, NAK, etc.) was transmitted in reply to data reception.
•When the CD signal was turned off during data communications using Check CD terminal in
(2)As a data transmission procedure, data transmission from the external device or the Q series C24 should be performed after an response for the immediately preceding data transmission has been received.
(3)The external device and PLC CPU must agree so that the units of the data length (word count/byte count) in the message to be transmitted and received is the same. The PLC CPU units can be set using the GX
Make the length of the data area in the message to be transmitted and received the size of the Q series C24 buffer memory transmission data designation area and receive data storage area, or less.
(4)NAK code response
1) Response from the Q series C24 to external device
After the error detection message reception complete, it transmits the NAK code to the external device.
2) Response from external device to the Q series C24
Transmit the error code (0022H to 005FH) immediately after the NAK response.
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