Smb0: Status Bits; Smb1: Status Bits - Siemens Simatic S7-200 System Manual

S7-200 Programmable Controller System Manual

SMB0: Status Bits

As described in Table D-1, SMB0 contains eight status bits that are updated by the S7-200 at the end of
each scan cycle.
Table D-1
SM Bits
SM0.0
SM0.1
SM0.2
SM0.3
SM0.4
SM0.5
SM0.6
SM0.7

SMB1: Status Bits

As described in Table D-2, SMB1 contains various potential error indicators. These bits are set and reset
by instructions at execution time.
Table D-2
SM Bits
SM1.0
SM1.1
SM1.2
SM1.3
SM1.4
SM1.5
SM1.6
SM1.7
D
408
Special Memory Byte SMB0 (SM0.0 to SM0.7)
Description (Read Only)
This bit is always on.
This bit is on for the first scan cycle. One use is to call an initialization subroutine.
This bit is turned on for one scan cycle 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.
This bit is turned on for one scan cycle 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.
This bit provides a clock pulse that is on for 30 seconds and off for 30 seconds, for a duty cycle time of 1
minute. It provides an easy-to-use delay, or a 1-minute clock pulse.
This bit provides a clock pulse that is on for 0.5 seconds and then off for 0.5 seconds, for a duty cycle
time of 1 second. It provides an easy-to-use delay or a 1-second clock pulse.
This bit is a scan cycle clock which is on for one scan cycle and then off for the next scan cycle. This bit
can be used as a scan counter input.
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 communications
with the programming device can be enabled by switching to the TERM position.
Special Memory Byte SMB1 (SM1.0 to SM1.7)
Description (Read Only)
This bit is turned on by the execution of certain instructions when the result of the operation is zero.
This bit is turned on by the execution of certain instructions either when an overflow results or when an
illegal numeric value is detected.
This bit is turned on when a negative result is produced by a math operation.
This bit is turned on when division by zero is attempted.
This bit is turned on when the Add to Table instruction attempts to overfill the table.
This bit is turned on when either LIFO or FIFO instructions attempt to read from an empty table.
This bit is turned on when an attempt to convert a non-BCD value to binary is made.
This bit is turned on when an ASCII value cannot be converted to a valid hexadecimal value.