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 Special Memory Byte SMB0 (SM0.0 to SM0.7)
SM Bits Description (Read Only)
SM0.0 This bit is always on.
SM0.1 This bit is on for the first scan cycle. One use is to call an initialization subroutine.
SM0.2 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.
SM0.3 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.
SM0.4 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.
SM0.5 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.
SM0.6 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.
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
communications with the programming device can be enabled by switching to the TERM
position.

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 Special Memory Byte SMB1 (SM1.0 to SM1.7)
SM Bits Description (Read Only)
SM1.0 This bit is turned on by the execution of certain instructions when the result of the operation is
zero.
SM1.1 This bit is turned on by the execution of certain instructions either when an overflow results or
when an illegal numeric value is detected.
SM1.2 This bit is turned on when a negative result is produced by a math operation.
SM1.3 This bit is turned on when division by zero is attempted.
SM1.4 This bit is turned on when the Add to Table instruction attempts to overfill the table.
SM1.5 This bit is turned on when either LIFO or FIFO instructions attempt to read from an empty table.
SM1.6 This bit is turned on when an attempt to convert a non-BCD value to binary is made.
SM1.7 This bit is turned on when an ASCII value cannot be converted to a valid hexadecimal value.
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