YOKOGAWA STARDOM FCN-500, Инструкция по эксплуатации

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<2. Basic Design and Function Design>
15
TI 34P02K35-02E Jun. 6, 2018-00
2.2.4 Determining FCN-500, FCN-RTU Scan Cycle
As described in the previous section 2.2.3, “Checking FCN-500 and FCN-RTU
Performance,” it is recommended that CPU load be kept at 60% or lower.
Check that the estimated CPU load is 60% or lower, and there is no problem with
the scan cycle stated in the requirement specification if
applicable
. If the CPU
load exceeds 60%, investigate rectification measures.
● Setting Range for Scan Cycle
The scan cycle of the FCN-500 can be set to a value between 5 ms and 32760
ms in 5 ms increments. The scan cycle of the FCN-RTU can be set to a value
between 10 ms and 32760 ms in 10 ms increments. When setting the scan cycle
to 4 seconds or longer, note the precautions described later.
● When a Required Scan Cycle is Stated in Requirement Specification
If a scan cycle is stated in the requirement specification, use the stated value to
estimate the CPU load and check that it is 60% or lower. If the CPU load exceeds
60%, consider whether it can be reduced using the methods described later.
● When no required scan cycle is stated in requirement specification
If no scan cycle is stated in the requirement specification the engineer is given the
responsbility, determine the scan cycle from the execution time of the control
application estimated as described in Section 2.2.3, “Checking FCN-500 and
FCN-RTU Performance.”
● Example for Determining Scan Cycle
If engineer is asked to decide on the scan cycle, use Logic Designer’s default
scan cycle of 100 ms as a baseline consideration.
Example 1: When estimated control execution time is 20 ms
Estimated CPU load = 20 ms/100 ms = 20%
Based on the estimated CPU load of 20%, even considering the time for
accessing I/O modules, the CPU load is expected to be below the
recommended limit of 60%. Therefore, the scan cycle of 100 ms should be
fine.
Example 2: When estimated control execution time is 50 ms
Estimated CPU load = 50 ms/100 ms = 50%
The estimated CPU load of 50% is below the recommended limit of 60%.
However, if the time for accessing I/O modules is taken into consideration, the
CPU load is expected to approach 60%, or even exceed 60% if many I/O
modules are installed.
In this example, we should look into reducing the CPU load using the methods
described hereafter.
If the scan cycle is set to 100 ms, download the application early on in application
creation to check that there is indeed no CPU load problem.