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Appendix 4.3 Reducing processing time
(1) Multiple CPU system processing
CPU modules access I/O modules and intelligent function modules through a bus (base unit pattern or extension
cable). Note that only one CPU module can use the bus at a time.
If more than one CPU module attempts to use the bus simultaneously, the CPU module attempted access later is
placed in Standby status until the processing of the first CPU module is completed.
In a multiple CPU system, this waiting time will cause delay in input and output, and consequently the scan time
increases.
(2) Maximum waiting time
In a multiple CPU system, the waiting time of the host CPU will reach the maximum when:
• The maximum number of CPU modules allowed in the system is used.
• An extension base unit is used.
• An intelligent function module on an extension base unit has high volume of data.
• All the CPU modules (the maximum number allowed) in the system simultaneously access a module on the
extension base unit.
(3) How to reduce processing time
The following methods can be taken for reducing the processing time in a multiple CPU system.
• Mount modules with a large number of access points (such as CC-Link IE, MELSECNET/H, and CC-Link
modules) together on the main base unit.
• Set one QCPU as a control CPU for all modules with a large number of access points (such as CC-Link IE,
MELSECNET/H, and CC-Link modules) to prevent simultaneous access.
• Reduce the number of refresh points of the CC-Link IE, MELSECNET/H, and CC-Link modules.
• Reduce the number of auto refresh points between CPU modules.
The scan time can be reduced by changing the following PLC parameter settings:( User's Manual (Function
Explanation, Program Fundamentals) for the CPU module used)
• "A-PLC Compatibility Setting" (except the Basic model QCPU)
• "Floating Point Arithmetic Processing" (High Performance model QCPU only)