!
322
Even
if the cycle time does not exceed the set value of the watchdog timer
, a long
cycle
time can adversely af
fect the accuracy of system operations
as shown in
the following table.
Cycle time (ms) Possible adverse affects
10 or greater TIMH(15) inaccurate when TC 016 through TC 511 are used.
(Accuracy when using TC 000 through TC 0015 not affected.)
20 or greater 0.02-second clock pulse (SR 25401) not accurately readable.
100 or greater 0.1-second clock pulse (SR 25500) not accurately readable and
Cycle Time Error Flag (SR 25309) turns ON.
200 or greater 0.2-second clock pulse (SR 25501) not accurately readable.
6,500 or greater FALS code 9F generated regardless of watchdog timer setting
and the system stops.
Online Editing When
online editing is executed from a Programming Device, operation will be
interrupted
for a maximum
of 260 ms and interrupts will be masked to rewrite the
user
program. No warnings will be given for long cycle times during this
interval.
Check the effects on I/O response time before editing the program online.
Caution Editing
the program
online can cause delays in I/O responses with no warnings being given from
the
system for the long cycle time produced by editing online. Before editing online, make sure that
delays in I/O responses will not create a dangerous situation in the controlled system.
6-2 Calculating Cycle Time
The
PC configuration, the program, and program execution conditions must be
taken
into consideration when calculating the cycle time. This means taking into
account
such things as the number of I/O points, the programming
instructions
used,
and whether or not peripheral devices are employed. This section shows
some basic cycle time calculation examples. To simplify the examples, the in-
structions
used in the programs have
been assumed to be all either LD or OUT
.
The
average execution time for the instructions is thus 0.47
µ
s. (Execution times
are given in the table in 6-3 Instruction Execution Times.)
6-2-1 PC with I/O Units Only
Here,
we’ll compute the
cycle time for a simple PC. The CPU controls only I/O
Units,
eight on the CPU Rack and five on a 5-slot Expansion I/O
Rack. The PC
configuration for this would be as shown below
. It is
assumed
that the program
contains 5,000 instructions requiring an average of 0.47 µs each to execute.
CPU Rack
Expansion I/O Rack
12-point Output Units
8-point Output Unit
16-point Input Units
8-point Input Units
8-point Output Units
Calculating Cycle Time Section 6-2
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