S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
831
Often-used special memory bits
The complete list of pre-defined special memory program symbols is available to your
project, in the System Symbol table.
Table E- 1 Often-used special memory bits
SM0.0 Always_On This bit is always TRUE.
SM0.1 First_Scan_On The CPU sets this bit to TRUE, for the first scan cycle, and sets it to FALSE
thereafter. One use for this bit is to call an initialization subroutine.
SM0.2 Retentive_Lost TRUE for one scan cycle if retentive data is lost
TRUE for one scan cycle when RUN mode is entered from a power-up condition
SM0.4 Clock_60s Clock pulse that is TRUE for 30 s, OFF for 30 s, for a cycle time of 1 min.
Clock pulse that is TRUE for 0.5 s, OFF for 0.5 s, for a cycle time of 1 s.
SM0.6 Clock_Scan Scan cycle clock that is TRUE for one scan cycle and OFF for the next scan
SM0.7 RTC_Lost For CPU models that have a real-time clock, the CPU sets this bit TRUE for one
scan cycle if the time on the real-time clock device was reset or lost at power up.
The program can use this bit as either an error memory bit or to invoke a special
SM1.0 Result_0 Set to TRUE by the execution of certain instructions when the result of the op-
SM1.1 Overflow_Illegal Set to TRUE by the execution of certain instructions on overflow or illegal numer-
Set to TRUE when a math operation produces a negative result
Set to TRUE when an attempt is made to divide by zero
Set to TRUE when the Add to Table instruction attempts to overfill the table
SM1.5 Table_Empty Set to TRUE when a LIFO or FIFO instruction attempts to read from an empty
table
SM1.6 Not_BCD Set to TRUE when an attempt is made to convert a non-BCD value to a binary
SM1.7 Not_Hex Set to TRUE when an ASCII value cannot be converted to a valid hexadecimal
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