Chapter 4 CPU Function
4 - 9
4.3 Program
4.3.1 Classifications of Program
All functional elements need to execute a certain control process are called as a ‘program’. In PLC
option card, a program is stored in the EEPROM mounted on a CPU module or flash memory of a
external memory module. The following table shows the classification of the program.
Program type Description
Scan program The scan program is executed regularly in every scan
Time-driven
interrupt program
(TDI)
The TDI programs are executed with a constant time interval specified
with parameter setting as below case.
If process time is needed faster than average 1 scan time, ▶
If process time is needed longer than average 1 scan time, ▶
If process time is need▶ ed with constant time interval,
Process driven
interrupt program
(PDI)
• The PDI programs are executed when external interrupt input is
applied.
High speed counter
driven interrupt
program (HSCDI)
• This interrupt programs are executed when comparison task signal is
applied.
Subroutine
program
• The subroutine programs are executed when they are called by the
scan program with a CALL instruction.
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