GFK-1527A Chapter 4 Normal Operation 4-19
4
Background User Checksum and Background WindowTiming Instructions
Performing User program Checksu m verification an d Background Wind ow Diagnostics ad d s time
to the sweep; the more checksums an d d ia gnostics that are performed each s weep, 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 determin e th e n umber of words to checksum each sweep.
Progr am 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 str u cture.
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 pr ogram checksummed (see followin g table).
C: the total time in milliseconds needed to perform background diagnostics (see following
table).
CGR772 CGR935
Milliseconds pe r b yte of program
checksummed (F) .0064 ms/byte .0014 ms/byte
Time to perform Back gr ound
Diagnostics (C) 3479 ms 376 ms