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4-11 Program Execution
When program execution is started, the CPU Unit cycles the program from top to
bottom, checking all conditions and executing all instructions accordingly as it
moves down the bus bar. It is important that instructions be placed in the proper
order so that, for example, the desired data is moved to a word before that word
is used as the operand for an instruction. Remember that an instruction line is
completed to the terminal instruction at the right before executing an instruction
lines branching from the first instruction line to other terminal instructions at the
right.
Program execution is only one of the tasks carried out by the CPU Unit as part of
the cycle time. Refer to Section 6 Program Execution Timing for details.
4-12 Special I/O Unit Interface Programs
This section provides programming methods and precautions for Special I/O
Unit operation.
4-12-1 Restarting Special I/O Units
When a Special I/O Unit is restarted, execution of IORF(97) is disabled until Spe-
cial I/O Unit initialization is completed.
SR 28100
(Unit #0 Restart Bit)
SR 27400
(Unit #0 Restart Flag)
ON
OFF
ON
OFF
Special I/O Unit
Initialization
Execution of IORF(97)
for Unit #0
Enabled Disabled Enabled
While the Restart Flag (SR 27400) is ON, normal END refreshing is performed
and the Special I/O Unit is initialized. This processing occurs regardless of the
settings in DM 6620, DM 6621, and DM 6623, which relate to Special I/O Unit
refreshing. IORF(97) instructions in the program won’t be executed for the ini-
tializing Unit until initialization is completed.
Special I/O Unit data that was to be refreshed might be lost during initialization.
When writing a program to restart Special I/O Units, disable programming that
depends on data from the initializing Special I/O Unit, such as data used in cal-
culations, while its Restart Flag (SR 27400 to SR 27415) is ON. Normal program
operations can continue for Units that aren’t initializing.
The Restart Flag won’t turn ON for Special I/O Units mounted on Slave Racks.
The standard Special I/O Unit restart time is (20 × the cycle time).
Special I/O Unit Interface Programs Section 4-12