Data Translation DT9840 Series, User Manual

Page: 31 / 130

Principles of Operation
31
External C/T Clock
An external C/T clock is useful when you want to pace counter/timer operations at rates not
available with the internal C/T clock or if you want to pace at uneven intervals.
Counter/timer operations start on the rising edge of the clock input signal.
Table 2 lists the pin assignments of connector J17 on the module that correspond to the
external C/T clock input signals.
Note: For the DT9841-VIB, the clock input signal of counter/timer 0 is also brought out to a
BNC on the face plate (CT0 Clock). If you want to use this signal for a tachometer input,
ensure that you condition your signal to a TTL input, and configure your counter/timer for
continuous measure mode.
Internally Cascaded Clock
You can use software to internally route (or cascade) the clock input signal from a specified
counter/timer to the clock output signal of the proceeding counter/timer (without externally
connecting two counters together).
For example, if you specify a cascaded clock source for counter/timer 2, the clock input of
counter/timer 2 is internally connected to the clock output of counter/timer 1. The C/T clock
input source for counter/timer 1 can be an internal or external C/T clock input source, or you
can cascade counter/timer 1 to counter/timer 0, if you wish.
The rising edge of the clock input signal is active.
Note: You cannot select a cascaded clock for counter/timer 0, since there is no lower
counter/timer with which to cascade.
If you cascade counters, you must read the value of each counter/timer separately and add
the values together. For example, if counter/timers 0 and 1 are cascaded, the cascaded count
= Ctr 0 + (Ctr1 * 2
32
).
Table 2: External C/T Clock Signals on the DT9840 Series Modules
Counter/Timer Signal J17 Pin Number
User Clock Input 0 6
User Clock Input 1 4
User Clock Input 2 2