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MELSEC-Q
14 SYSTEM CONFIGURATION OF MULTIPLE PLC SYSTEM
14.2.9 Reducing the time required for multiple PLC system processing
(1) Multiple PLC system processing
A bus (base unit pattern, extension cable) is used by the CPU module when
accessing the I/O module and intelligent function module, and this bus cannot be
used by plural CPU module at the same time.
The CPU modules that attempted buss access afterwards when plural CPU
module use the bus simultaneously will assume the "Standby" status until
processing for the CPU module that executed the procedure first has been
completed.
This "Standby" status (the amount of time the CPU module must wait) will cause
delays in input and output on the multiple PLC system, and result in extended
scan times.
See Chapter 18 for details on extended scan times.
(2) Maximum standby time
The host PLC will reach the maximum standby time in the following cases with a
multiple PLC system.
• When four CPU modules are used on the multiple PLC system.
• When additional base units are in use.
• When intelligent function modules that possess vast quantities of data are
mounted onto additional base units.
• When four CPU modules simultaneously access modules mounted onto
additional base units.
(3) Reducing the time required for multiple PLC system processing
The following methods are available for reducing the amount of time required for
multiple PLC system processing.
• Combine modules with many access points, such as MELSECNET/H and CC-
LINK refresh, etc., together into a main base unit.
• Set modules with many access points, such as MELSECNET/H and CC-LINK
refresh, etc., as control module on a single CPU module, and ensure that
simultaneous access does not occur.
• Reduce the number of MELSECNET/H and CC-LINK refresh access points.
• Reduce the number of automatic refresh points between CPU modules.
POINT
It is possible to reduce scan time by changing the following PLC parameter
settings:
• A Series CPU compatibility setting
• Floating point arithmetic processing
See Section 18.3 for details.
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