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8 MULTITASK PROCESSING
8.5.2 Message ports
8 - 16
It is possible to exchange messages, as if sending letters, between the programs, and
synchronize the executions by making (defining) a message transfer location
(message port) in memory in AD51H-BASIC.
To synchronize the executions via a message port, it is not necessary to consider the
interlocking of the wait status as required by the event control. The wait status is
created automatically.
Message port
(1) To use message ports
In order to use message ports, the following should first be understood:
• In order to use a message port, it is necessary to open the message port in one
of the BASIC programs in advance. A message port can be opened in any
program. Moreover, the program that opened the message port does not have
to reside in memory.
• The maximum length of a message is 256 bytes.
• Up to a maximum of 32 message ports can be opened in the communication
module. To distinguish each message port, a number from 0 to 31 is assigned
to each message port.
• Each message port can store up to 16 messages asynchronously. Message
data in a message port can be retrieved in the order of transmission (FIFO
method) or in the order of the program priority (priority method). Whether a
message port uses the FIFO method or priority is predefined by the message
port number.
Priority method • • • Message port numbers from 0 to 15
IN
FIFO method • • • Message port numbers from 16 to 31
Program 2
B
Message port
B
A
Program 2
Program 1
A
(16)
(15)
MELSEC-Q
Are there any
letters for me?
Program 1
Low priority
High priority
Message port buffer
B
(14)
(3)
(2)
B
A
A
OUT
(1)
8 - 16
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