ADLINK ACL-7120A/6 - Manual (Page 37)

Programming
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Mode 3: Square Wave Rate Generator.
Similar to Mode 2 except that the output will remain high until one
half of the count has been completed (or for even counts) and go
low for the other half of the count. This is accomplished by
decrementing the counter by two on the falling edge of each clock
pulse. When the counter reaches terminal count, the state of the
output is changed and the counter is reloaded with the full count
and the whole process is repeated.
If the count is odd and the output is high, the first clock pulse (after
the count is loaded) decrements the count by 1. Subsequent clock
pulses decrement the clock by 2. After time-out, the output goes
low and the full count is reloaded. The first clock pulse (following
the reload) decrements the counter by 3. Subsequent clock pulses
decrement the count by 2 until time-out. Then the whole process is
repeated. In this way, if the count is odd, the output will be high for
(N + 1)/2 counts and low for (N - 1)/2 counts.
In Modes 2 and 3, if a CLK source other then the system clock is
used, GATE should be pulsed immediately following Way Rate of a
new count value.
Mode 4: Software Triggered Strobe.
After the mode is set, the output will be high. When the count is
loaded, the counter will begin counting. On the terminal count, the
output will go low for one input clock period, then high again.
If the count register is reloaded during counting, the new count will
be loaded on the next CLK pulse. The count will be inhibited while
the GATE input is low.
Mode 5: Hardware Triggered Strobe.
The counter will start counting after the rising edge of the trigger
input and will go low for one clock period when the terminal count
is reached. The counter is re-triggerable. The output will not go low
until the full count after the rising edge of any trigger.