Program Errors - Siemens SIMATIC S5-100U System Manual

Diagnostics and Troubleshooting
5.3

Program Errors

5.3.1 Locating the Error Address
The SAZ (STEP address counter) in the ISTACK (bytes 25 and 26) contains the absolute address
of the STEP 5 statement in the programmable controller before which the CPU went into the STOP
mode.
Use the "DIR PC" programmer function to determine the associated block start address.
Example: You have entered a control program consisting of OB1, PB0 and PB7. An illegal
statement has been programmed in PB7.
Figure 5-1. Structured Program with an Illegal Statement
When it reaches the illegal statement, the CPU interrupts program scanning and enters the STOP
mode with the "NNN" message. The STEP address counter is at the absolute address of the next
(but not yet scanned) statement in the program memory.
5-8
OB1
JU PB0
JU PB7
BE
BE
PB7
PB0
L PB 0
BE
S5-100U
Illegal
statement
EWA 4NEB 812 6120-02b