D Special Memory (Sm) Bits - Siemens Simatic S7-200 System Manual

Special Memory (SM) Bits
Special memory bits provide a variety of status and control functions, and also serve as a
means of communicating information between the CPU and your program. Special memory
bits can be used as bits, bytes, words, or double words.
SMB0: Status Bits
As described in Table D-1, SMB0 contains eight status bits that are updated by the S7-200
CPU at the end of each scan cycle.
Table D-1
SM Bits
SM0.0 This bit is always on.
SM0.1 This bit is on for the first scan. One use is to call an initialization subroutine.
SM0.2 This bit is turned on for one scan if retentive data was lost. This bit can be used as
either an error memory bit or as a mechanism to invoke a special startup sequence.
SM0.3 This bit is turned on for one scan when RUN mode is entered from a power-up
condition. This bit can be used to provide machine warm-up time before starting an
operation.
SM0.4 This bit provides a clock pulse that is on for 30 seconds and off for 30 seconds, for a
cycle time of 1 minute. It provides an easy-to-use delay, or a 1-minute clock pulse.
SM0.5 This bit provides a clock pulse that is on for 0.5 seconds and then off for 0.5 seconds,
for a cycle time of 1 second. It provides an easy-to-use delay or a 1-second clock pulse.
SM0.6 This bit is a scan clock which is on for one scan and then off for the next scan. This bit
can be used as a scan counter input.
SM0.7 This bit reflects the position of the Mode switch (off is TERM position, and on is RUN
position). If you use this bit to enable Freeport mode when the switch is in the RUN
position, normal communication with the programming device can be enabled by
switching to the TERM position.
S7-200 Programmable Controller System Manual
C79000-G7076-C230-02
Special Memory Byte SMB0 (SM0.0 to SM0.7)
Description
D
D-1