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MELSEC-Q
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 full-duplex communications through the RS-232 interface.
(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 Configurator-SC word/byte units
designation.
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 (0022
H
to 005F
H
) immediately after the NAK
response.