Campbell Scientific CR7 Measurement and Control System - Manual (Page 92)

SECTION 9. INPUT/OUTPUT INSTRUCTIONS
9-2
PAR. DATA
NO. TYPE DESCRIPTION
01: 2 Repetitions
02: 2 Range code (Table 9-1)
03: 2 Card number for first
measurement
04: 2 Differential channel number
for first measurement
05: 4 Input location number for first
measurement
06: FP Multiplier
07: FP Offset
Input locations altered: 1 per repetition
*** 3 PULSE COUNT ***
INPUT RANGE - - - 32767 Counts per input
interval
There are three input configurations which may
be measured with the Pulse Count Instruction.
HIGH FREQUENCY PULSE
In this configuration the minimum pulse width is
2 microseconds. The maximum input
frequency is 250 kilohertz. The count is
incremented when the input voltage changes
from below 1.5 volts to above 3.5 volts. The
maximum input voltage is ±20 volts.
LOW LEVEL AC
This configuration is used for counting the
frequency of AC signals from magnetic pulse
flow transducers or other low voltage, sine wave
inputs. The minimum input voltage is 6 mV
RMS. Input hysteresis is 11 mV. The maximum
AC input voltage is 20 volts RMS. The
maximum input frequency ranges from 100Hz
at 6mV RMS to 10,000Hz at 50mV or greater.
Consult the factory if higher frequencies are
desired.
SWITCH CLOSURE
In this configuration the minimum switch closed
time is 1 millisecond. The minimum switch
open time is 4 milliseconds. The maximum
bounce time is 1.4 milliseconds open without
being counted.
All pulse counters in one I/O Module are reset
at the same time. The reset time interval is
equal to the execution interval of the program
table in which the Pulse Count Instruction(s)
occur. When programs are compiled, the CR7
will set the reset time interval to the execution
interval of the first program table in which a
Pulse Count Instruction occurs. The execution
intervals of subsequent program tables
containing Pulse Count Instructions will have no
effect on the reset time interval. (The maximum
input frequency is 250KHz. The maximum
number that can be stored in an accumulation
register is 65,535.)
When datalogger time is changed, whether
through the keyboard or with a
telecommunications program, a partial
recompile is automatically done to
resynchronize program execution with real time.
The resynchronization process resets the pulse
accumulation interval resulting in an interval
whose length can be anywhere between one
second too short to almost twice as long.
Pulses are not lost during resynchronization so
totalized values are correct but pulse rate
information such as wind speed can be up to
almost twice the correct value.
The options of discarding counts from long
intervals and pulse input type are selected by
the code entered for the 4th parameter (Table
9-2).
All Pulse count instructions for the same I/O
module and output instructions for the pulse
count data should be kept in the same program
table, preferably Table 1. If the pulse count
instruction is contained in a subroutine, that
subroutine must be called from Table 2.
When the system is interrupted for a task of
sufficient priority to allow the pulse counters to
exceed the programmed reset time interval, the
resulting larger count can either be discarded
leaving the value in the input location
unchanged from the previous value or it can be
used. If pulse counts are being totalized, a
missing count could be significant; the value
from the erroneously long interval should be
used. If the pulse count is being processed a
way in which the resultant value is dependent
upon the sampling interval, such as sample,
average, maximum, or minimum, it should be
discarded. The option of discarding counts
from long intervals and the input configuration
are determined by the 4th parameter according
to the following table.