AutomationDirect DL105, User Manual

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CPU
S
pecifications
and
Operation
4--15
CPU Specifications and Operation
DL105 PLC User Manual, 3rd Edition
I/O Response Time
I/O response time is the amount of time required for the control system to sense a
change in an input point and update a corresponding output point. In the majority of
applications, the CPU performs this task in such a short period of time that you may
never have to concern yourself with the aspects of system timing. However, some
applications do require extremely fast update times. In these cases, you may need to
know how to to determine the amount of time spent during the various segments of
operation.
There are four things that can affect the I/O response time.
S
The point in the scan cycle when the field input changes states
S
Input Off to On delay time
S
CPU scan time
S
Output Off to On delay time
The next paragraphs show how these items interact to affect the response time.
The I/O response time is shortest when the input changes just before the Read
Inputs portion of the execution cycle. In this case the input status is read, the
application program is solved, and the output point gets updated. The following
diagram shows an example of the timing for this situation.
Solve
Program
Read
Inputs
Write
Outputs
Solve
Program
Scan
Solve
Program
Field Input
Input
Off/On Delay
CPU Reads
Inputs
Output
Off/On Delay
I/O Response Time
Scan
Solve
Program
CPU Writes
Outputs
In this case, you can calculate the response time by simply adding the following
items:
Input Delay + Scan Time + Output Delay = Response Time
Is Timing Important
for Your
Application?
Normal Minimum
I/O Response