I/O Response Time
Is Timing Important for Your Application?
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 practically instantaneously. However, some applications do require
extremely fast update times. There are four things that can affect the I/O response time:
• The point in the scan period when the field input changes states
• Input module Off to On delay time
• CPU scan time
• Output module Off to On delay time
Normal Minimum I/O Response
The I/O response time is shortest when the module senses the input change 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.
In this case, you can calculate the response time by simply adding the following items:
Input Delay + Scan Time + Output Delay = Response Time
Normal Maximum I/O Response
The I/O response time is longest when the module senses the input change after the Read
Inputs portion of the execution cycle. In this case the new input status does not get read until
the following scan. The following diagram shows an example of the timing for this situation.
In this case, you can calculate the response time by simply adding the following items:
Input Delay +(2 x Scan Time) + Output Delay = Response Time
DL205 User Manual, 4th Edition, Rev. B
3–27
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