5.4 Control Bits and Interrupt Stack
You can use the online functions PLC INFO and OUTPUT ISTACK to
analyze the following: operating status, characteristics of the CPU,
characteristics of the user program, possible causes of errors and
interruptions.
Note
You can display the control bits in any mode. You can display
the ISTACK only in the STOP mode.
•• The control bits indicate the current and previous operating status
and the cause of the problem.
If several errors occurred, the control bits indicate all of them.
•• The ISTACK indicates the location of the interruption in question
(addresses) with the current condition codes, the accumulator con-
tents, and the cause of the problem.
If several interruptions occurred, a multiple level ISTACK is
constructed as follows (maximum 5 levels):
DEPTH (level) 01 = last cause of interruption
DEPTH (level) 02 = next to last cause of interruption, etc.
When an ISTACK overflow occurs, the CPU goes into the STOP
mode immediately (HARD STOP!). You must then turn the power
off and on again and perform a cold restart.
The meanings of the individual abbreviations in the control bits and in
the ISTACK are described below.
Note
The text on the screen of your programmer depends on the PG
software you are using. It may therefore differ from the display
shown here. The description of the screen information is nevertheless
relevant.
Control Bits and Interrupt Stack
CPU 948 Programming Guide
C79000-G8576-C848-04
5 - 9
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