PCI/PXI/PCIe-6509 User Guide and Specifications
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© National Instruments
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13
Figure 6.
NI 6509 Signal Connections
Digital output applications include sending TTL signals and driving external devices such as the
LED shown in the figure. Digital input applications include receiving TTL signals and sensing
external device states such as the state of the switch in the figure.
Protecting Inductive Loads
When inductive loads are connected to output lines, a large counter-electromotive force may
occur at switching time because of the energy stored in the inductive load. These flyback
voltages can damage the outputs and/or the power supply.
To limit these flyback voltages at the inductive load, install a flyback diode across the inductive
load. For best results, mount the flyback diode within 18 inches of the load. Figure 7 shows an
example of using an external flyback diode to protect inductive loads.
Figure 7.
Limiting Flyback Voltages at the Inductive Load
41
4
3
45
47
67
69
71
7
3
50, 100
NI 6509
+5 V
+5 V
LED
Port 7
P7<7..4>
Port 0
P0<
3
..0>
GND
TTL
S
ign
a
l
P
X
.
Y
GND
Lo
a
d
NI 6509
Fly
ba
ck Diode for
Ind
u
ctive Lo
a
d
s
