Chapter 3
Digital I/O Functionality
©
National Instruments Corporation
3-15
NI CVS-1450 Series User Manual
Transmission Line Effects
Transmission line effects can degrade the signals on the I/O cables and
cause instability. To minimize transmission line effects, use twisted-pair
wires with a characteristic impedance of 118
Ω
to connect external signals
to the 44-pin I/O DSUB connector. Use a 75
Ω
coaxial cable, such as
RG-179, to connect to the SMB connectors.
Figure 3-8 shows connections to the 44-pin DSUB connector and the
TRIG 0 SMB connector that minimize transmission line effects.
Figure 3-8.
Example Connections
When connecting to TTL inputs on the NI CVS-1450 device, match the
output impedance of the transmitting device to the characteristic impedance
of the cable. For example, if the cable characteristic impedance is 118
Ω
,
make R
s
equal to 118
Ω
, as shown in Figure 3-8.
TTL OUT(0)
TTL IN(0)
118
Ω
44-Pin
DSUB
3
2
16
17
62 k
Ω
Transmitting
Equipment
R
S
TRIG 0
75
Ω
TRIG 0
SMB
+5 V
NI CVS-1450 device
Receiving
Equipment
Receiving
Equipment
RG-179
Coaxial Cable
