National Instruments 653X - Manual (Page 36)

Chapter 2 Using Your 653X
653X User Manual 2-18 ni.com
Deciding Transfer Direction
You can choose to send data from your 653X device to the peripheral
device (output), or from the peripheral device to your 653X device (input).
Choosing an Internal or External REQ Source
In pattern I/O, the 653X device acquires/generates data on every falling or
rising edge (programmable) of the REQ signal. The REQ signal can be
generated internally or based on the clock of a peripheral device. An
example of using external REQ is sharing a sample clock of an analog input
device so you can synchronize the analog and digital operations.
Deciding the REQ Polarity
By default, data from an external REQ source is transferred on the rising
edge of the signal and on the falling edge of the internal REQ source. You
can reverse the REQ polarity by using the following functions:
• NI-DAQ C interface—Specify the REQ polarity in the
DIG_Group_Mode function before calling the
DIG_Block_PG_Config function.
• LabVIEW—Specify the REQ polarity in the Digital Mode Config VI
that is called by the DIO Config VI.
Note For more information on LabVIEW VIs and NI-DAQ functions, consult the
LabVIEW Help and the NI-DAQ Function Reference Help.
Refer to Table C-1, 653X I/O Connector 68-Pin Assignments , for an
overview of all control/timing trigger lines.
Deciding the Transfer Rate
If you are generating the REQ signal internally, you need to specify the rate
of data transfer. The transfer rate is specified in software by using two
parameters, the timebase frequency and timebase divisor:
where
timebase frequency = 20 MHz, 10 MHz, 1 MHz, 100 kHz, 10 kHz,
1 kHz, or 100 Hz, and
timebase divisor = an integer between 1 and 65,355.
transfer rate (Hz)
timebase frequency
timebase divisor
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