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The PC takes longest to respond when it receives the input signal just after the
I/O refresh phase of the cycle. In this case the CPU Unit does not recognize the
input signal until the end of the next cycle. The maximum response time is thus
one cycle longer than the minimum I/O response time, except that the I/O refresh
time would not need to be added in because the input comes just after it rather
than before it.
Cycle time
Input
signal
Output
signal
Cycle
Cycle time
I/O refresh
I/O response time
Input
ON
delay
Instruction
execution
Instruction
execution
Instruction
execution
CPU Unit reads
input signal
Output
ON delay
Cycle time
Maximum I/O response time =
Input ON delay + (Cycle time × 2) + Output ON delay
Calculation Example The data in the following table would produce the minimum and maximum cycle
times shown calculated below.
Item Time
Input ON-delay 1.5 ms
Output ON-delay 15 ms
Cycle time 20 ms
Minimum I/O response time = 1.5 + 20 + 15 = 36.5 ms
Maximum I/O response time = 1.5 + (20 x 2) +15 = 56.5 ms
Note In this example the I/O refresh time is negligible has not been included in the
minimum I/O response time.
6-4-2 Remote I/O Systems
With Remote I/O Systems, only the cycle time of the PC needs to be considered
in computing the I/O response times as long as the remote I/O transmission time
is negligible and smaller than the cycle time. The cycle time, however, is in-
creased by the presence of the Remote I/O System.
The processing that determines and the methods for calculating maximum and
minimum response times from input to output are provided in this section. Cal-
culations assume that both the input and the output are located on Slave Racks
in a Remote I/O System, but the calculations are the same for I/O points on Opti-
cal I/O Units, I/O Link Units, I/O Terminals, etc.
X
Input on
Slave Rack
Output on
Slave Rack
Although more precise equations are possible if required, equations used for the
following calculations do not consider fractions of a scan.
Maximum I/O Response
Time
I/O Response Time Section 6-4