National Instruments 653X Manuale Utente

© National Instruments Corporation

There is no specific cutoff frequency at which termination becomes 
necessary.

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. The DIO-32HS is only rated to sink 
24 mA.

Follow these signal-conditioning recommendations for optimum use:

•

Separate 653X device signal lines from high-current or high-voltage 
lines. These lines are capable of inducing currents in or voltages on the 
653X device 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 653X device when it asserts a TTL low signal 
(often denoted by I

out

 or I

ol 

under Output Low Voltage specification).

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 653X device when it 
asserts a TTL high signal (often denoted by I

out

 or I

oh 

under output high 

voltage specification).

Most National Instruments digital I/O products have similar source and sink 

currents.

If you are using the DAQCard-6533 for PCMCIA, your PCMCIA socket may not 

provide sufficient power to drive all outputs at 24 mA.

Table D-2.  Sink and Source Current for the 653X Devices

24 mA at 0.4 V

–24 mA at 2.4 V