DFM-400P Rev 2.1 Installation and Appplication Guide Edition 1.6
Page 11 of 14
Counting Pulses
The Pulse Input Field Module has two sets of counting registers.
Pulse Inputs
Counter Type
Max. Pulses per Sample Time Max Pulse Rate
PP1 and PP2
16 bits
65535
2000 Hertz
PP3 and PP4
16 bits
(equivalent)
65535
200 Hertz
Pulses arriving on the inputs are fed into their appropriate counter registers. On every
sample interval time for each Pulse Input the counter contents are added into the Total
Pulses property.
Pulse rate is an important consideration. The objective is to count for a long enough
time to get a reasonable number of pulses to do calculations with, but not to overflow the
counters and ‘lose’ pulses and/or get erroneous values. The
Rule of Thumb
is to
multiply the expected maximum pulse rate by the sample interval time and verify that
the result remains within the Maximum Pulses per Sample Time as listed above. Notice
that only PP1 and PP2 are designed to handle the higher pulse rates. In any case, a fault
will indicate an overflow condition should it occur.
Generally a pulse counter is accurate to +/- one pulse due to timing considerations.
Reset
There are two types of resets that apply to the Pulse Input Field Module. Neither affects
the clock, but both will clear out a fault caused by an overflow condition.
•
Reset Command: All counting properties are set to 0 - including total pulses - when the
PI object is reset, which is typically initiated by a user (i.e., OWS command). However,
note that resetting the device does not affect counted pulses or object values.
•
Change to Sample Interval: The Total Pulse Count value is not affected. But since the
sample interval is fundamental to operation, all other properties are set to 0, at least
until new samples are counted (which depends on the sample interval).
