Rockwell Automation 1794-VHSC 1794 FLEX I/O Very High Speed Counter Module User Manual - Manual (Page 16)

8 Rockwell Automation Publication 1794-UM010D-EN-E - July 2013
Overview of the Very High Speed Counter Module
is available in the Read Data file. The stored count remains in the Read Data file
until it is overwritten with new data.
Store-Reset/Start
The figures show the store count feature operating on the rising edge of the gate/
reset pulse. The user has the option of electing these same features using the
falling edge of the gate/reset pulse. The gate invert bit is active in the store count,
continuous/rate and period/rate modes.
Period/Rate Mode Use the period/rate mode to determine the frequency of input pulses by counting
the number of internal 5MHz clock pulses over a user-specified number of input
signal pulses. At the end of the specified number of pulses, the module returns the
frequency and the total number of pulses received.
A channel configured for period/rate mode acts as a period rate counter. An
internal 5MHz clock is used as a frequency reference. This clock is gated by the
incoming pulse train at the gate/reset input. The results of this gating action are
the number of pulses or a frequency. The frequency is returned in input file words
0 (LSW) and 1 (MSW) for channel 0 and word 2 (LSW) and 3 (MSW) for
channel 1. The total pulses received is stored in input file words 4 (LSW) and 5
(MSW) for channel 0 and words 6 (LSW) and 7 (MSW) for channel 1. Select
the period/rate mode by setting the appropriate bits in word 0 of the
configuration block.
1794-VHSC modules count the total number of pulses occurring at the Z (gate/
reset) pin. This function is frequency-limited to 200Hz X the scaler value. You
can reset this count by setting the VR bit.
Figure Period/Rate Mode on page 9 shows a diagram of the module used in the
period/rate mode.
Store Count,
reset to zero,
start counting
Rising edge
Store Count,
reset to zero,
start counting
Falling edge