how fast i can read DI port in PCI6601

[G Sudheer]

Hi,

I want to read positions of my encoder on raising edge of 6-12 digital inputs. Encoder gives 3.6 - 300kHz pulses. Digital values change at the rate of 1-80 Hz.(12.5 milliseconds). Can I collect all the changes in the Digital lines without loss? Normal rates ar far below the max rates mentioned. What would be the max rate at which I can sense the dis in PCI 6601?

[Filipe A.]

Hello;

The only way to asure that no data will be lost, at each time that any one of the 12 digital lines gives a rising or falling edge, is to have 12 counters doing buffered position measurement, and have each one of the digital lines gating each counter.

To accomplish that, you will need an extra 6602 board, that has 8 counters in it, so both boards will give the 12 needed counters.

If you try to do that by Software (sensing the state change of the digital lines throug the easy I/O digital VIs, you will definitely get wrong results, since there is a Software overhead that is completely system dependent).

Hope this helps.
Filipe A.
Applications Engineer
National Instruments

[Kevin_Price]

Another option to consider is to pair your counter/timer board with one of the 653x high-speed digital devices, such as the PCI-DIO-32HS. The digital board can be set up in "change detection" mode which will save the digital pattern only when one of the bits changes.

The digital board can further fire a pulse over RTSI with each bit change. Your counter board can then be programmed to buffer the instantaneous encoder position with each pulse.


At your change rate of ~80 Hz or less, you could also program a second counter to track cumulative time of the pulses. (With your 6601, you will need to program the "data transfer mode" to be "interrupts" using "Set DAQ Device Information.vi" because you only have one DMA channel available for buffered c
ounting operations.)