Siemens SIMATIC S5-100U, System Manual

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S5-100U Diagnostics and Troubleshooting
5.3.2 Tracing the Program with the “BSTACK” Function
Program trace with “BSTACK” is not possible on the 605U programmer.
During program processing, the following information about jump operations is entered in the block
stack (BSTACK):
• The data block that was valid before program processing exited a block.
• The relative return address
- It specifies the address where program processing will continue after the return from the
called up block.
• The absolute return
- It specifies the memory address in the program memory where program processing will
continue after the return.
You can call up this information with the “BSTACK” programmer function in the STOP operating
mode if a fault caused the CPU to go into the STOP operating mode. “BSTACK” then reports the
status of the block stack at the time the interruption occurred.
Example:
Program scanning was interrupted at function block FB2. The CPU went into the STOP mode with
the error message “TRAF” (because of incorrect DB access, e.g., DB5 is two words long and DB3
is ten words long).
“BSTACK” lets you determine the path used to reach FB2 and lets you know which DB was open
at the time of call up. “BSTACK” contains the three (marked) return addresses.
Figure 5-4. Tracing the Program with “BSTACK”
06
OB1
04 JU PB2
02
00 JU PB1
PB1
xx BE
00
PB2
04
02 JU PB4
00 C DB5
xx BE
18 BE
PB3
16 JU FB2
00 C DB3
PB4
08 JC FB2
00
10
xx BE
FB2
00
2A L DW4
xx BE
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08 JC PB3
xx BE
10
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“TRAF” error
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EWA 4NEB 812 6120-02b
5-11