7
PROGRAMMING CAUTIONS
7.2 Cautions on Fixed-Scan Clocks and Fixed Scan Execution Type Programs
7
- 11
1
OV
ER
VIE
W
2
SYS
TEM
CONFIGURA
TI
ON
3
TR
AC
KI
N
G
C
ABL
E
4
P
R
OCE
DURE
FO
R
S
TAR
TING
UP
A
RE
DUN
DAN
T
S
Y
S
T
E
M
5
RE
DUNDA
NT S
Y
S
T
E
M
FUNCTIONS
6
RE
DUN
DAN
T
S
Y
S
T
E
M
NE
TW
O
R
K
S
7
PR
OG
R
A
MM
IN
G
CA
UTIO
N
S
8
TR
O
U
BLE
S
HOOTING
(4) Interrupt by the Internal Timer (I28 to I31)
When system switching occurs, execution of interrupt is suspended until system
switching is complete.
Also, interrupt from the module mounted on the extension base unit cannot be made.
When system switching occurs, the new control system CPU module starts counting
the interrupt interval time from 0.
Therefore, the fixed scan execution interval is increased (T+ ') as shown in Diagram
7.9.
The maximum value for the extension time is (T+ ) when = '.
Diagram 7.9 Operations of Interrupts by Internal Timers at System Switching
Begin system
switching
System Switching complete
T : System switching time
System switching activated
Control system
Control system
New standby system
New standby system
New control system
Standby system
Standby system
New control system
Scan execution type
Program
Interrupt type
Program
Scan execution type
Program
Interrupt type
Program
T
Extension of interrupt interval
: Interrupt interval
: Period between starting scan
execution type program and
system switching