National Instruments 653 Series User Manual page 113

Note A purely resistive termination scheme is not recommended because of the current
drawn by the termination resistors. For example, a 90 Ω terminating resistor works well to
dampen reflections, but sinks 27 mA even at 2.4 V. NI 653X devices are rated to sink
only 24 mA.
How Much Current Can I Sink or Source?
Most NI digital I/O products have similar source and sink currents.
Note
Note If you are using the NI DAQCard-6533 for PCMCIA, your PCMCIA socket may not
provide sufficient power to drive all outputs at 24 mA.
© National Instruments Corporation
There is no specific cutoff frequency at which termination becomes
necessary.
Follow these signal conditioning recommendations for optimum use:
• Separate NI 653X signal lines from high-current or high-voltage lines.
These lines are capable of inducing currents in or voltages on the
NI 653X signal lines if they run in parallel paths at a close distance. To
reduce the magnetic coupling between lines, separate them by a
reasonable distance if they run in parallel, or run the lines at right
angles to each other.
• Do not run signal lines through conduits that also contain power lines.
Make sure the sink current does not exceed 24 mA at 0.4 V to guarantee
that TTL low voltage specifications are met. The sink current is the amount
of current that flows into the NI 653X when it asserts a TTL low signal
(often denoted by I
OUT
Also, it is important to make sure the source current does not exceed
–24 mA at 2.4 V to guarantee TTL high voltage specifications. The source
current is the amount of current that flows out of the NI 653X when it
asserts a TTL high signal (often denoted in an Output High Voltage
specification by I
OUT
Table D-2. Sink and Source Current for the NI 653X
Sink Current
24 mA at 0.4 V
Appendix D
or I in an Output Low Voltage specification).
OL
or I ).
OH
D-9
Hardware Considerations
Source Current
–24 mA at 2.4 V
NI 653X User Manual