National Instruments PCI-6601, User Manual

Page: 34 / 52

Chapter 3 Signal Connections
© National Instruments 3-9 NI 660x User Manual
Note For all NI 660 x devices, o
u
tp
u
t freq
u
ency on any of the pins sho
u
ld not exceed
40 MHz. The maxim
u
m freq
u
ency yo
u
can drive at the I/O connector is affected by the
capacitive load yo
u
r cable presents. Yo
u
can achieve 40 MHz o
u
tp
u
t with a National
Instr
u
ments 1 m SH68-68-D1 shielded cable (capacitive load = 80 pF). At larger loads,
yo
u
r maxim
u
m o
u
tp
u
t freq
u
ency may be lower.
Counters
The co
u
nters on TIO devices are a s
u
perset of the DAQ system timing
controller (DAQ-STC) general-p
u
rpose co
u
nters developed by National
Instr
u
ments. These co
u
nters are backward compatible with the DAQ-STC
in f
u
nctionality and software programming. The same software API and
f
u
nctions are
u
sed to program the DAQ-STC general-p
u
rpose co
u
nters and
the co
u
nters on TIO devices.
The co
u
nters on TIO devices have two internal timebases: 100 kHz and
20 MHz. The co
u
nters on the NI 6602 and NI 6608 also have an 80 MHz
timebase. Each co
u
nter has a gate, a
u
xiliary, and so
u
rce inp
u
t. Each of
these inp
u
ts can be an internal or external signal that connects to the
I/O connector. Each co
u
nter also has an o
u
tp
u
t signal.
Counter
n
Source Signal
Yo
u
can select any PFI as well as many other internal signals as the
Co
u
nter n So
u
rce (Ctr n So
u
rce) signal. The Ctr n So
u
rce signal is config
u
red
in edge-detection mode on either the rising or falling edge. The selected
edge of the Ctr
n So
u
rce signal increments and decrements the co
u
nter val
u
e
depending on the application the co
u
nter is performing.
Yo
u
can export the Ctr n So
u
rce signal to the I/O connector’s defa
u
lt PFI
inp
u
t for each Ctr n So
u
rce. For example, yo
u
can export the Ctr0So
u
rce
signal to the PFI 39/CTR 0 SRC pin, even if another PFI is inp
u
tting
the Ctr0So
u
rce signal. This o
u
tp
u
t is set to high-impedance at start
u
p.
For most applications,
u
nless yo
u
select an external so
u
rce, the
80MHzTimebase signal (if available), 20MHzTimebase signal,
or 100kHzTimebase signal generates the Ctr
n So
u
rce signal.