Mitsubishi Electric MELSEC iQ-R Series User Manual page 246

Important points regarding message sending and receiving handshakes
• When sending consecutive messages, make sure that the sending request trigger relay of the previous message is OFF
before switching ON the sending request trigger relay of the next message.
• Set a 20 ms trigger hold time for the transmission request trigger of the secondary message after receipt of the primary
message. This allows the SECS/GEM communication software to be notified that the primary message receiving
notification trigger relay is off before the notification that the transmission request trigger relay of the secondary message is
on.
• When sending has failed, the sending request trigger relay is switched OFF. The sending-failed relay switches ON. The
programmable controller CPU must not only switch OFF the sending request trigger relay, it must also monitor that the
sending failure relay is ON.
9 SECS/GEM COMMUNICATION SOFTWARE KNOWLEDGE
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9.4 Sending and Receiving Messages