Deterministic Read/Write Operations with a 6508

[MrQuestion]

I have a PXI-6508 DIO card controlled by a PXIe-8108 Real Time controller. I’m trying to generate and capture accurate data in a deterministic fashion.

I am having two issues.

1.)    The data collected/generated is not deterministic. As seen in scope capture NonDeterministic.tif a clk trail is low for almost twice as long as it should be. Also the frequency is not as I would expect in a 1Mhz timed loop. I would have expected  a  500Khz clk. Instead I see about a 72Khz clk.

2.)    When I perform a data read on port0 the data out on port 3 is reset as seen in the scope capture WithReadEnabled.tif. Data port 3 goes into a tristate mode when performing a read operation on port0.

The image of the LabVIEW code is a simplified example that I made to help narrow the issue.  The scope captures are taken after active signal conditioning and not directly from the card. But anytime the port is in a tristate mode it floats to about 2 volts after conditioning.

After performing some research it appears that the PXI-6508 is a limited DIO card, and may not be able to perform the functions that I need.

I don’t necessarily need to have a 500Khz clock, but I would like to know what the 6508 is capable of.

I do need to be able to sample one port while maintaining the logic state of the other port.

I do need a deterministic generation and sampling of all signals.

I suspect that I need the card to generate a task clock/sync signal for my DIO tasks, but I don’t think the 6508 is capable of this. Could somebody please verify this?

 

[MrQuestion]

***UPDATE***

I have found additional information.

Ends up the issue with a port resetting is a known problem described in this link PCI-6508 Digital Ports cannot be controlled Independently.

It is caused by a port expander chip that is used on this card.

I also found out that NI does not recomend this card for new designs. The person who designed this station or purchased parts probably didn't understand the warning that was on the purchasing site.

This still doesn't explain why I can't get more deterministec results with this card.

[Marand]

Hi,

In order to use the PXI-6508 in a deterministic way, you need to have Farlab OS RT, do you have it?

Regards,

[MrQuestion]

I'm using LabVIEW RT on a 8108 PXI module. So, yes it is technically in Pharlap.

The 8108 is a high end controller that allows access to a 1Mhz clock line to clock the timed loops.

I'm discovering that it takes about 8us for a DAQmx task to do anything with this card.

I have used register based access before with other cards to speed things up, but the 6508 is a different beast. Ends up there are several flavors of the 82c55 I/O chip on the 6508. As a result there are different flavors of the PXI-6508 card.

The user manual for this card and the 6503 is decent, but it also describes the issues with this card because of the different I/O chips. Thus why NI now suggest that any new designs really shouldn’t use this card.

I wanted a card that can be accessed at 5us without buffering, and pre-queuing a sequencer in the card hardware. In any case this card can not physically perform the tasks I was wanting.