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3 COMMUNICATION PROCEDURE
3.1 Features of Communication Process
3 COMMUNICATION PROCEDURE
This chapter explains the considerations when performing communication from an external devices to the programmable
controller system using MC protocol.
3.1 Features of Communication Process
The following shows the features of data communication using MC protocol.
Request messages and response messages
There are two types of messages in MC protocol; request message and response message.
The request message sent from the external device is processed in the CPU module via MC protocol supported devices. The
processing result is returned as a response message to the external device.
Half-duplex communication
Data communication by MC protocol is performed with half-duplex communication.
*1
When accessing CPU module, send the next command message after receiving the response message against the
previously sent command message from the CPU module. (The command messages cannot be sent until the reception of the
response message is completed.)
When the system between external devices and CPU module is configured with an m:n connection, the next command
message cannot be sent until data communication between either of the external devices and CPU modules is completed.
*1 When using the on-demand function using C24, full-duplex communication can be performed.
External device
Supported device
Execution of the processing.
Sending a request message
According to the message sent from the external
device, performs the requested processing such as
reading or writing.
Sends a request message to the supported device.
Repeated as needed.
Returning a response message
Receiving the response message
Receives the response message from the supported
device, and confirms the processing result.
Upon completion of the processing, sends a
response message to the external device.
End of the communication
The external device performs end processing to
terminate the communication.
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