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Causes of Error and Reactions of the CPU
5.6.6
OB 27: (Substitution
Error SUF)
5.6.7
OB 30: Parity Error and
Timeout Error in the User
Memory (PARE)
PARE accessing the
operating system RAM
5 - 28
If an operation with a formal operand is to be carried out in a function
block, the CPU replaces (substitutes) this formal operand with the
actual operand in the block when the block is called during user
program processing.
If the CPU detects an illegal substitution, it interrupts the user
program and calls OB 27, if this is loaded. If OB 27 is not loaded, the
CPU changes to the STOP mode.
Apart from an illegal substitution, SUF is also indicated in the
following situations:
• • illegal operation code,
• • special situation:
you cannot open data blocks DB 0 and DB 1. The CPU handles the
operations "C DB 0" and "C DB 1" like substitution errors. A zero
is entered in the DBA and DBL registers.
The user memory is protected by a parity bit. The system program
checks whether the parity bit is correct each time the user memory is
accessed. If the parity bit is incorrectly set, a parity error is indicated.
The system program calls OB 30. If OB 30 is not loaded, the CPU
changes to the STOP mode.
The same reaction takes place if a timeout error occurs in the user
memory.
If a parity error occurs when accessing the operating system RAM, the
system program does not call OB 30, but changes to a HARD STOP.
CPU 948 Programming Guide
C79000-G8576-C848-04
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