FP2 Serial Data Unit
Precautions Regarding Serial Data Unit Operation and Programs
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5.2 Precautions Concerning Communication
5.2 Precautions Concerning Communication
5.2.1 Precautions When a Start Code is Valid
When receiving data
When operating with a valid start code, be aware that the data between the most recent
start code received and the end code is processed as one frame.
STX STX ETX
STX: Start code
ETX: End code
When start code is valid: 1 frame
When start code is not valid
If no end code is received because of a communication problem or for another reason,
the serial data unit waits for an end code to be received.
To prevent the system from waiting endlessly,
• Send the data again from the RS232C device on the other side.
• If the RS232C device on the other side is not capable of resend
processing, the sequence program should be set up so that time
out and retry processing are carried out, and a request is sent to the
device on the other side to send the data again.
When the re−sent data is received from the RS232C device on the other side, the serial
data unit proceeds as follows.
STX STX ETX
STX STX ETX
When start code
is not valid:
Re−sent frame
Re−sent frame
To shared memory, as received frame
To shared memory,
as received frame
Interrupted frame
Interrupted frame
When start code
is valid:
This becomes invalid.
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