National Instruments NI PCI-8254R - Manual (Page 26)

Chapter 2 Digital I/O
© National Instruments Corporation 2-15 NI PCI-8254R 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 D-SUB connector.
The following figure shows connections to the 44-pin D-SUB connector
that minimize transmission line effects.
When connecting to TTL inputs on the NI 8254R, 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 the figure above.
TTL OUT(0)
Receiving
Equipment
TTL IN(0)
118 Ω
44-Pin
DSUB
3
2
16
17
62 k Ω
Transmitting
Equipment
R
S
+5 V
NI 8254R