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Peripheral Servicing Priority Mode Section 6-9
6-9-1 Peripheral Servicing Priority Mode
If the Peripheral Servicing Priority Mode is set, program execution will be
interrupted at the specified time, the specified servicing will be performed, and
program execution will be resumed. This will be repeated through program
execution. Normal peripheral servicing will also be performed after the I/O
refresh period.
Peripheral Servicing Priority Mode can thus be used to execute periodic ser-
vicing for specified ports or Units along with the normal peripheral servicing.
This enables applications that require priority be given to peripheral servicing
over program execution, such as process control applications that require
rapid response for host monitoring.
• Up to five Units or ports can be specified for priority servicing. CPU Bus
Units and CS/CJ Special I/O Units are specified by unit number.
• Only one Unit or port is executed during each slice time for peripheral ser-
vicing. If servicing has been completed before the specified time expires,
program execution is resumed immediately and the next Unit or port is not
serviced until the next slice time for peripheral servicing. It is possible,
however, that the same Unit or port will be serviced more than once dur-
ing the same cycle.
• Unit or ports are serviced in the order in which they are detected by the
CPU Unit.
Note 1. Even though the following instructions use the communications ports, they
will be executed only once during the execution cycle even if Peripheral
Servicing Priority Mode is used:
RXD(235) (RECEIVE)
TXD(236) (TRANSMIT)
2. If more than one word is read via a communications command, the con-
currence of the read data cannot be guaranteed when Peripheral Servicing
Priority Mode is used.
3. The CPU Unit might exceed the maximum cycle time when Peripheral Ser-
vicing Priority Mode is used. The maximum cycle time is set in the PLC
Setup as the Watch Cycle Time setting. If the cycle time exceeds the
Watch Cycle Time setting, the Cycle Time Too Long Flag (A40108) will be
turned ON and PLC operation will be stopped. If the Peripheral Servicing
Priority Mode is used, the current cycle time in A264 and A265 should be
monitored and the Watch Cycle Time (address: +209) adjusted as re-
quired. (The setting range is 10 to 40,000 ms in 10-ms increments with a
default setting of 1 s.)
Peripheral
servicing
Normal per ipheral
service
Peripheral
servicing
Peripheral
servicing
Program
execution
Interrupted
Program
execution
Interrupted
Program
execution
Interrupted
Program
execution
I/O refresh
T
0
: Time slice for program execution
T
1
: Time slice for peripheral servicing
1 cycle
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