Siemens SIMATIC S7-300 FM 350-1 Manual page 138

Operating Modes, parameters and commands
7.4 Measuring modes
Limit monitoring
On expiration of the update time, the module compares the measured value (frequency,
speed or period) with the set low and high limits.
If the module detects an underflow of the actual measured value (measured value < low
limit), it sets the status bit STS_UFLW = 1. A hardware interrupt may also be generated.
If the module detects an overflow of the actual measured value (measured value > high
limit), it sets the status bit STS_OFLW = 1. A hardware interrupt may also be generated.
Figure 7-31
Reset the STS_OFLW and STS_UFLW bits by setting the RES_ZERO and STS_RES_ZERO
bits according to the acknowledgement principle. The module sets the status bits again if it
detects overflow of the measured value after the acknowledgement bits were set.
If programmed accordingly, you also can use the limit monitoring function to set output DO0.
Gate control
You can control, i.e. start and stop, measurements at FM 350-1 using the HW and SW
gates.
Start values after programming
Table 7- 30
Value
Low limit
High limit
Refresh time
138
Limit monitoring in measuring mode
Start value
Start value
programmed value
programmed value
programmed value
Manual, 05/2011, A5E03648639-01
FM 350-1 Counter module