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GFK-1527A Chapter 4 Normal Operation 4-19
4
Background User Checksum and Background Window
Timing Instructions
Performing User program Checksum verification and Background Window Diagnostics adds time
to the sweep; the more checksums and diagnostics that are performed each sweep, the longer the
sweep will take. For example, setting the Words to Checksum to 216 adds about 0.6 ms to each
sweep in a CGR935 (216 words x 2 bytes/word x 0.0014 ms/byte = 0.6 ms).
For users wanting to compare Program Checksum verification and Background Window
Diagnostics within a set amount of time (for example, 60 seconds), the following formula can be
used to estimate the necessary settings for Words to Checksum and Background Window Time.
These calculations can be used for Normal Sweep mode or Constant Window mode. They are not
valid for Constant Sweep mode.
Finding the Words to Checksum Each Sweep
First, you should determine the number of words to checksum each sweep.
Program Size x Sweep Time
Words per Sweep = ------------------------------------------------------------------------
[Max. Completion Time - (Program Size x F) - C] x 2
Where:
Words per Sweep
: The number of words to set in the PLC Configuration to be checksummed
each sweep. The number calculated must be rounded up to the next number divisible by 8 (8,
16, 24, etc.).
Program Size
: The sum of the sizes of the user programs in bytes. If there is a ladder logic
program, add 11,000 bytes to account for internal memory usage that is not included in the
user program memory displayed by the programmer. The 11,000 bytes is an approximate
number typical for most LD programs. If a more accurate number is desired, use the file size
of the _main.dec file instead of the 11,000. The _main.dec file can be found on disk inside of
your folder’s directory structure.
Sweep Time
: The sweep time in milliseconds when the number of checksum words is set to
zero and the Background Window timer is set to zero.
Maximum Completion Time
: The amount of time in milliseconds that you want to have full
coverage of these diagnostics. For example, 1 minute is 60,000ms.
F
: the number of milliseconds per byte of program checksummed (see following table).
C
: the total time in milliseconds needed to perform background diagnostics (see following
table).
CGR772 CGR935
Milliseconds per byte of program
checksummed (F)
.0064 ms/byte .0014 ms/byte
Time to perform Background
Diagnostics (C)
3479 ms 376 ms