Technical data

System Software for S7-300/400 System and Standard Functions - Volume 1/2
A5E00709327-01
11-1
11 SFCs for Handling Synchronous Errors
11.1 Masking Synchronous Errors
Introduction
Synchronous errors are programming and access errors. Such errors occur as a
result of programming with incorrect address areas, numbers, or incorrect
addresses. Masking these synchronous errors means the following:
• Masked synchronous errors do not trigger an error OB call and do not lead to a
programmed alternative reaction.
• The CPU "records" the masked errors that have occurred in an error register.
Masking is carried out by calling the SFC 36 "MSK_FLT".
Unmasking errors means canceling a previously set mask and clearing the
corresponding bit in the event status register of the current priority class. Masking
is canceled as follows:
• By calling SFC 37 "DMSK_FLT"
• When the current priority class has been completed (only for S7-400).
If an error occurs after it has been unmasked, the operating system starts the
corresponding error OB. You can program OB121 for a reaction to programming
errors and OB122 for a reaction to access errors.
You can use SFC 38 "READ_ERR" to read out the masked errors that have
occurred.
Note: With the S7-300 (except CPU 318), regardless of whether an error is masked
or unmasked, the error is entered in the diagnostic buffer and the group error LED
of the CPU is lit.