
2 Basic principles of the counter module
Counter Module SIMATIC FM 350-2
V1.0, Item ID: 83146298
18
C
o
p
yr
ig
h
t
S
ie
m
e
n
s
A
G
2
0
1
3
A
ll
ri
g
h
ts
r
e
se
rv
ed
2.5.5
Frequency measurement
In this mode, the FM 350-2 counter module counts the pulses received within
a predetermined time window. This is then used to calculate the frequency.
It is important to note that the calculated value is only available and able to be
output after the first measuring cycle. During the first time window, the output is -1.
The reason for this feature is so that the first measuring cycle can be differentiated
from the real system values that are either "0" or greater.
The measured value is output in mHz at the driver block.
Figure 2–8: Time diagram for frequency measurement
2.5.6
Rotational speed measurement
In this mode, which is virtually identical to the "frequency measurement" mode,
the FM 350-2 counts the pulses received from a signal transmitter within
a predetermined time window and uses this to calculate the rotational speed.
Here, the measured value from the first cycle is once again only available in
the second time window.
The rotational speed is output in 10
-3
min
-1
at the driver block.