Allen-Bradley POINT I/O 1734-VHSC24, User Manual, Page: 38 / 64

Publication 1734-UM003A-EN-P - August 2000
3-10 Very High Speed Counter Module Input and Output Data
Rollover (Configuration Word 10)
This long word sets the number of counts the counter will accumulate
before rolling over. For example, a value of 1000 will produce a count
sequence of: 998, 999, 0, 1, 2
…
while incrementing or 2, 1, 0, 999,
998
…
while decrementing. Rollover is a 32 bit number with a useable
range of 1
≤
value
≤
0x01000000 (16,777,216). In PWM [3]
configuration, this value should be zero; in
count [0], x1 encoder [1],
x2 encoder [2] and x4 encoder [4] configurations, it should be
specified to some non-zero value; and in
period/rate [5], continuous/
rate [6] and rate measurement [7] configurations is a ‘don’t care’.
Preset (Configuration Word 11)
This long word sets the preset value the counter will be loaded with,
when a Counter Preset, CP, command is issued. Preset is a 32 bit
number with a range of 0
≤
value
≤
0x00FFFFFF (16,777,215).
Counter ON and OFF Windows (Configuration Words 12 to 19)
T hese long words program the four compare window's ON and OFF
values. The first compare window for each counter is utilized in
PWM
[3] configuration and, when PWM is programmed for a channel, the
associated compare window should remain at 0. The range of each
entry is 0
≤
value
≤
0x00FFFFFF (16,777,215). When a tie is connected
to a compare window, that window must be specified (i.e. ON value
≠
OFF value
≠
0). These Windows are always interpreted as counts,
regardless of the configuration setting and may be computed as
follows for frequency modes
5, 6 & 7 :
Period/Rate (5) and Continuous Rate
counts = ( scalar x 2.5E6 ) / desired_frequency
Rate Measurement
counts = time_base [sec] x gate_interval x desired_frequency