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5-12
H Timing of Message Response Reading
Responses are read when the Message Communications Enabled Flag turns ON in the next cycle. The
Message Communications Enabled Flag will turn OFF when a Master Unit is executing message com-
munications. If the message response is faster than the PC ladder program cycle time, the Message
Communications Enabled Flag will remain ON and response processing will not be possible. Execute
the read response processing before IOWR, as shown in the following diagram, and produce a Mes-
sage Communications Status Flag (d) using bit A.
Read response
processing
Execution
condition
Message
Communications
Enabled Flag
B
B
A
25506
DIFU C
A
SET D
C
RSET D
D
Message
Communications
Enabled Flag
Equals Flag
(@) IOWR
C
S
D
A
Note If the read response processing is executed after IOWR, there can be no Message Communica-
tions Enabled Flag response by means of IOWR, so an attempt could be made to read the re-
sponse even though the message communications have not been completed.
H Communications Flags
Flag Functions
Equals Flag (SR 25506) The Equals Flag turns OFF when an error occurs in writing a command
from the CPU Unit to the Master Unit.
This Flag turns ON after a command has been written normally from the
CPU Unit to the Master Unit.
Error Flag (SR25503) The Error Flag is OFF when all operands and the control code are legal.
This Flag turns ON when an illegal operand or control code is set or
when there is an error in instruction execution.
Message Communications
Enabled Flag in the Master Unit
status area (bit 12 in IR 101 +
10 x unit No.)
The Communications Enabled Flag turns OFF during messages
communications or when message communications are not possible.
This Flag is ON when message communications are possible.
CompoBus/D Communications Card Operations Chapter 5