Rockwell Automation 1771-VHSC , D17716.5.74 Very High S - Manual (Page 18)

1–10 Overview of the Very High Speed Counter Module
Figure 1.8
Period/Rate Mode
scaler
From internal 4MHz clock
From user's encoder/pulse generator
Gate/Reset Terminal
Incoming pulse train
at gate/reset terminal
4MHz internal clock
Sampled pulses
1771ĆVHSC
A Not used
B Not used
10684ĆI
In 1.8, the incoming pulse train from the gate/reset terminal is used
to sample pulses from the 4 MHz internal clock. As the frequency of
the incoming pulse train at the gate/reset terminal increases, the
number of sampled pulses from the 4 Mhz clock decreases. This
relationship is shown in Table 1.A. Since accuracy is related to the
number of pulses received over the sample period, the accuracy will
decrease with increasing input frequencies at the Gate/Reset
terminal. To some extent, the decrease in accuracy can be lessened
by scaling the input frequency through the use of a scaler. A scaler
value of 1 will only return an accurate input frequency if incoming
pulses have a 50% duty cycle. If frequency exceeds 500KHz, the
number 999,999 is returned.