Rockwell Automation 1771-VHSC , D17716.5.74 Very High S - Manual (Page 20)

1–12 Overview of the Very High Speed Counter Module
The continuous/rate mode is similar to the period/rate mode
previously described except the outputs in this mode are dynamic
outputs. Use this mode to determine the frequency of input pulses by
counting the number of internal 4MHz clock pulses over a
user-specified number of input signal pulses. Each output is turned
on as soon as the turn-on count is reached, and turned off as soon as
the turn-off count is reached. As the internal 4MHz clock is counted,
the outputs dynamically track the 4MHz count. This allows you to
turn an output on a certain number of 4MHz counts after the
gate/reset pin goes active, and turn it off a certain number of 4MHz
counts later.
1771-VHSC revision B and later modules count the total number of
pulses occurring at the gate/reset pin. This function is
frequency-limited. This total count is returned when you request
words 19 through 26 in your BTR. You can reset this count by
resetting the reset bit (bits 0-4 in BTW
word 1). Rollover and preset are inactive. Refer to appendix E for
additional information.
Figure 1.9
Period/Rate and Continuous/Rate Output Operation with
Scaler of 1
Incoming pulse train
at gate/reset terminal
4MHz internal clock
Sampled pulses
10684ĆI
Period/rate
Continuous/rate
Output on/off presets
active only on scaler
number pulse.
Output on/off presets
active during entire
pulse.
Use the rate measurement mode to count incoming pulses for a
user-specified time interval. At the end of the interval, the module
returns a value representing the sampled number of pulses and a
value indicating the incoming frequency. When the count and
frequency are updated, any associated outputs are checked against
their associated presets.
Continuous/Rate Mode
Operation in Rate
Measurement Mode