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MELSEC-Q
8 MULTITASK PROCESSING
2) If program B reads a message before program A writes it, the execution is
synchronized as follows:
A
A
B
B
B
B
1) ZMESSAGE to
ZMESSAGE OPEN to
Prepare and open
the message port.
2) ZMESSAGE GET
Read the message.
ZMESSAGE OPEN to
Open the message
port.
3) ZMESSAGE PUT
Write the message.
A
Program B
Program A
If there is no message in the
message port, program B waits
until a message is written to
the message port.
When a message is written to
the message port, program
B reads the message.
B
An " error" occurs if a message cannot
be received within the specified
<timeout> period.
Unlike the event control, it is not necessary to interlock the programs when using a
message port because the program goes into the wait status automatically.
REMARK
In message exchange using message ports, the OS does not keep track of which
program writes which message.
If it is desired to keep track of which program writes a given massage, add the
program name or similar identifier to the message so that it can be identified on the
reading side.
