Description of Operation
Section
3-2
107
Example
Data exchange of analog input, PID operation and analog output
Analog Input Unit
Loop Control Unit
CPU Unit
I/O refreshing
I/O memory
Log input
Function block
Analog Input
block
A/D
conversion
Analog
input
converted
value
Execution
cycle 0.2 s
Analog Output
Unit
Log input
D/A
conversion
I/O refreshing
I/O memory
Analog
input
converted
value
PID block
Execution
cycle 0.2 s
Analog output
block
Execution
cycle 0.2 s
Operation
cycle
Operation
cycle
"External I/O refresh cycle on the overall system" here (simply called the
"external I/O refresh cycle" from here on) refers to the time up to when PID
operation is performed and the Analog Input/Output Unit outputs the analog
output values after the Analog Input Unit starts to read analog input values.
(This fresh cycle indicates the response of the overall system.)
Note
This "external I/O refresh cycle" is equivalent to the I/O refresh cycle (or
operation cycle) on a general controller. For this reason, when designing the
system, calculate this "external I/O refresh cycle" according to the calculation
formula shown below, and study whether or not there will be any problem in
controllability on the target application. In most cases, the maximum "external
I/O refresh cycle" is as follows depending on the timing of operations:
"about five times the cycle time of the CPU Unit" + "twice the operation cycle
of the function blocks on the Loop Control Unit"
In particular, study whether or not there will be any problem in controllability
of PID control in the case of fast-response control targets such as pressure or
flowrate.
Calculate the external I/O refresh cycle by the following calculation formula:
3-2-11-1
Maximum external I/O refresh cycle
The maximum external I/O refresh cycle is calculated as follows:
A/D conversion time + (4 x CY) + (
x CY) + (2 x T) + (
x T) + 20 + D/A
conversion time
where,
CY: Cycle time of the CPU Unit
T:
Max. operation time (
1)
:
Coefficient of the total number of function blocks exchanging data with
the CPU Unit (
2)
:
Smallest integer satisfying formula 1(
3)