Number of DMA Channel on System

[Colin hughes]

I need to log 16 (my be 32 later) simultaneous channel at the same time.
Is the limit of DMA chanels a windows system problem or just a card problem?
I understand that there is a limited number depending on the operating system.
So I cannot use 4 6601E cards (4 chanels each)at the same time on a PC.

I am told my NI that I could us PXI rack with a number of 6115 cards and an embeded controller running Window but I can not see that this would resolve the DMA problem unless it was a modified window system.

The other problem I see is that if Window 2000 or XP configers the cards to use he same chanel, Window will not let you change anything youself as all seeting are allway grayed out.
Judging my the large number of post on DMA perha
ps NI should write a white paper detaiing all the problems.

Thanks in advance fr your help.
Colin

[Russell]

Hello Colin,

Back with the ISA boards the limit of DMA channels was due to the computer itself. Now with PCI, DMA is handled by the PCI board itself. There has to be separate DMA controller hardware for each DMA channel. Our controller chip suppports up to 3 DMA channels. So I wouldn't say that this is a problem really. It's simply what the card offers, similar to the number of counters that is on the board.

There are fewer DMA channels than counters because 1) more DMA controllers would be more expensive, 2) Most applications probably would not require DMA channels for every counter. Interrupts may be sufficient for slower acquisitions or the the counters may be used for pulse generation which also wouldn't require DMA.

I guess I don't know enough abo
ut your application to comment on the 6115 suggestion. The 6115 is an analog simultaneous sampling board, but they only have 2 counters each. Perhaps it was suggested that you acquire this data with analog sampling. The advantage to this is that you can use the same DMA channel for all analog channels being scanned which would probably be a good alternative for your application.

Russell
Applications Engineer
National Instruments
http://www.ni.com/support