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69GFK-0827 Chapter 4 Operation
Background User Checksum and Background Window
Timing Instructions
The following information is provided to allow you to guarantee full coverage of the
number of program words verified by the User Program Checksum per sweep and the
Background Window Diagnostics within a certain amount of time. It is important to
understand that the more checksums performed and the larger the background
window, the longer the sweep will take. For example setting the number of words to
checksum to 176 will add 2.8 ms to each sweep.
First, you should determine the number of words to checksum per sweep.
Words per Sweep =
(program size ÷ 2) x (sweep time) x maximum completion time
[max. completion time – (program size x F)] x max. completion time – C) – (C x F x program size)
Where:
Words per Sweep: The number of words to set in the PLC Configuration to be
checksumed each sweep. This number must be a multiple of 8.
Program Size: The size of the user program in bytes. You can get this from the
Logicmaster 90 PLC Memory Usage screen, which is accessed by pressing the status
softkey (F3) from the Programmer Main menu, then the plcmem softkey (F5). Add
the ”USER PROGRAM” size on this screen to 10566 (this accounts for internal
memory usage not added to the user program memory.
Sweep Time: The sweep time in milliseconds of the user program with checksums and
background window turned off. You can get this from the Logicmaster 90 status line
on any screen.
Maximum Completion Time: The amount of time in milliseconds that you want to have
full coverage of background diagnostics.
F: A constant that represents milliseconds per byte of user program checksumed.
This constant is dependent on the type of CPU module (that is CPUs with 80386
microprocessors are faster than CPUs with 80186 microprocessors). This constant is
0.008 ms/byte for the CPU 780.
C: A constant with units in milliseconds, which is also dependent on the type of
CPU. For the CPU 780 it is 4480 ms.
Following is an example of calculating the Words per Sweep, using the following data:
User program Size = 89434 bytes
Program Size =User program Size + 10566 = 89434 + 10566 = 100000 bytes
Sweep Time = 100 ms
Max. Completion Time = 120000 ms (2 minutes)
Words per Sweep =
100000 ÷ 2 x 100 x 120000
[120000 – 100000 x 0.008] x (120000 – 4480) – (4480 x 0.008 x 100000)
174Words per Sweep =
You should round this answer (174) up to the nearest number divisible by 8 (8, 16, 24,
etc.) which is 176 since Logicmaster 90 only accepts numbers divisible by 8. Next use this
number in the following formula to determine how long to set the background window
time.
Background Window Time =
C x (sweep time + words per sweep x F x 2)
(maximum completion time – C)