5.7.3
Settings
Calculating and setting the
number of time slices
The processing time for the self-functions is distributed on time slices
which are called once per cycle. The number of time slices can be
selected. This means that you can increase the time required for the
self-test functions per cycle.
Calculating the number
First, you must estimate the time you can leave for the self-test within
the cycle: the length of a time slice is approximately 500 µs, i.e. the
self-test requires 500 µs in each cycle.
Once you have estimated how much time is available, you can
calculate the number of time slices each taking approximately 500 µs.
Setting the number
You can set the number of time slices in system data word RS 136
(16-bits wide). The default is 1 time slice (minimum value). You can
set up to a maximum of 10 time slices (5 ms required in the cycle).
The number of time slices is derived from the value of system data
word RS 136 as follows:
RS 136 = 0 or 1: 1 time slice
RS 136 = 2: 2 time slices
RS 136 = 3: 3 time slices
etc.
Activating/deactivating the
tests
You can activate the individual tests, e.g. in a start-up block, by setting
the corresponding bits in RS 137 to ’1’ and deactivate the tests by
setting the bits to ’0’.
Note
After an OVERALL RESET on a newly inserted CPU, all the test
functions are switched off.
The next time you make an OVERALL RESET, only the test
functions to be run in the OVERALL RESET are activated. All
other test functions are deactivated.
This means that you can only check the user memory of a newly
inserted CPU by activating the test function in RS 137 following
an OVERALL RESET and then repeating the OVERALL RESET.
Self-Test
CPU 948 Programming Guide
C79000-G8576-C848-04
5 - 37
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