National Instruments 653X Manuale Utente

© National Instruments Corporation

This chapter contains timing diagrams for the handshaking and pattern I/O 
modes. You can use these diagrams to get a detailed understanding about 
what happens in hardware when using these modes.

All timing diagrams are in nanoseconds.

Use pattern I/O to transfer data at a timed interval upon the rising or falling 
edge of the REQ signal. The REQ signal can be generated internally by the 
653X device or supplied externally via the I/O connector.

Your transfer rate is limited by the minimum available bus bandwidth in your 

computer system, unless you are using the PCI/PXI-6534 device, which has onboard 
memory. Otherwise, you are limited by the number of other devices utilizing the bus and 
your application software, both of which can lower your transfer rate. For more 
information about transfer rates, see Appendix E, 

.

The 653X can internally generate a signal (REQ) with which to strobe data. 
To program the frequency of this signal, specify the timebase and interval 
as shown in the 

 section of Chapter 2, 

. The device captures data on the rising (active low) or falling 

edge (active high) of this signal. You can select the polarity of the REQ 
signal through software, as described in the 

section in Chapter 2, 

When generating an internal REQ signal, the asserted time of the resulting 
clock will be one period of the timebase used to generate the REQ. The 
exception is if you use a 20 MHz timebase (50 ns) and select an interval 
of 1. The REQ pulse is then asserted for 20–30 ns.

If you are using a version of NI-DAQ prior to version 6.8, the minimum value for 

the interval parameter is 2.