Measurement Computing Data Translation DT3010 Series, Руководство пользователя, Страница: 133 / 232

Chapter 6
132
Internal C/T Clock
The internal C/T clock uses a 20 MHz time base. Counter/timer operations start on the rising
edge of the clock input signal.
Through software, specify the clock source as internal and the frequency at which to pace the
counter/timer operation (this is the frequency of the clock output signal). The maximum
frequency that you can specify for the clock output signal is 10 MHz. The minimum frequency
that you can specify for the clock output signal is 305.18 Hz.
External C/T Clock
The 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. The rising
edge of the external C/T clock input signal is the active edge.
Using software, specify the clock source as external and the clock divider used to determine
the frequency at which to pace the operation (this is the frequency of the clock output signal).
The minimum clock divider that you can specify is 2.0; the maximum clock divider that you
can specify is 65,536. For example, if you supply an external C/T clock with a frequency of 5
MHz and specify a clock divider of 5, the resulting frequency of the external C/T clock output
signal is 1 MHz. The resulting frequency of the external C/T clock output signal must not
exceed 2.5 MHz.
Connect the external C/T clock to the board through the DT740, STP268, or STP268-EC screw
terminal panel.
Table 11 lists the screw terminals that correspond to the external C/T clock
signals of each counter/timer.
Internally Cascaded Clock
You can also internally route the clock output signal from one user counter to the clock input
signal of the next user counter to internally cascade the counters. In this way, you can create a
32-bit counter without externally connecting two counters together. DT3010 Series boards
support software cascading on counters 0 and 1, 1 and 2, and 2 and 3.
Table 11: External C/T Clock Input Signals
Counter/Timer
Screw Terminal
on the DT740
Screw Terminal
on the STP268
Screw Terminal
on STP268-EC
0 TB58 TB54
a
a. STP268 must be attached to connector J1.
TB5
1 TB62 TB58
a
TB38
2 TB66 TB54
b
b. STP268 must be attached to connector J2.
–
c
c. To access counter/timers 2 or 3, connect an STP268 screw terminal panel to connector J2
on the DT3010-268 board.
3 TB70 TB58
b
−
c