Technical data

5.7 Control Bits and Interrupt Stack
Using the PLC INFO and OUTPUT ISTACK online programmer
functions, you can analyze the operating status, the characteristics of
the CPU and the user program and any 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 (addresses)
with the current condition codes, the accumulator contents and the
cause of the problem.
If several errors occurred, a
multiple level interrupt stack is
constructed as follows:
depth 01 = last cause of problem,
depth 02 = next to last cause of problem etc.
If an ISTACK overflow occurs (more than 13 entries) the CPU
goes into the STOP mode immediately. If this happens, you must
perform a POWER OFF/POWER ON and 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 used. It may differ from the screen represented here.
Nevertheless, the description of the individual positions on the
screen in these programming instructions is valid.
Control Bits and Interrupt Stack
CPU 928B Programming Guide
5 - 10 C79000-B8576-C898-01