Siemens SIMATIC S5-100U System Manual page 142

Introduction to STEP 5 S5-100U
Special Features of the Normal Mode
Significance of the Memory Submodule
Normal mode is only possible if the memory submodule is plugged in.
This submodule contains only the STEP 5 program.
The CPU RAM contains the STEP 5 program and the compiled program to be processed.
Program Change
You can enter, modify, or erase PBs, OBs and FBs only in the test mode.
You can read out the STEP 5 program with the programmer.
Signal Status Display
You can monitor and control signals states with the "STATUS VAR" and "FORCE VAR" functions.
The "STATUS" function can be used only in the test mode.
Diagnostics
The "BSTACK" diagnostics function cannot be activated.
Fault Analysis
The ISTACK bytes 23 to 27 are not valid. Therefore, you cannot determine the point in a program
where an interruption took place (programmable controller in STOP, e.g., programmed loop with
timeout). However, when compiling the program, errors (e.g., illegal operations and parameters) are
detected and displayed by the STEP address counter in the ISTACK. This counter points to the
error in the STEP 5 program.
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EWA 4NEB 812 6120-02b