Siemens SIMATIC S7-300 FM 350-1 Manual page 136

Operating Modes, parameters and commands
7.4 Measuring modes
Measurement sequence
FM 350-1 measures continuously. You define a specific update time in the parameter
settings.
The module returns the value "-1" until the first update time has expired. The first update
time starts when the gate has opened.
When the gate has opened, continuous measurement starts at the first pulse of the pulse
sequence to measure. The first measured value can not be calculated until the second pulse
was received.
On each expiration of the update time, a measured value is output at the checkback
interface (frequency, period or rpm.) The end of a measurement is reported at the
STS_COMP1 status bit. This bit is reset by the RES_ZERO and STS_RES_ZERO bits
according to the acknowledgement principle.
If the rotary direction is reversed within an update time, the measured value remains
indefinite for this measuring period. You can respond to any disturbance in the process by
evaluating the STS_DIR checkback bits (evaluation of the direction.)
136
FM 350-1 Counter module
Manual, 05/2011, A5E03648639-01