jmm@skivs.UUCP (Joel M. Miller) (10/01/89)
It's too bad that discussions of Mac data acquisition and experimental control focus on who has the fastest boards or the glitziest interface, or whether there are more Mac or PC data acq products to choose from. I am no Mac expert (this will soon be apparent), but I know that there are basic problems, related to the Mac's architecture, principally, I think, to the fact that all NuBus cards share a single hardware interrupt. Without prioritized hardware interrupts it is, apparently, not possible for one card to get at the CPU while another card is using the bus. Thus, acq cards are subject to being detached from the CPU for significant and indeterminate times, resulting in missed samples. The main instance of this problem is caused by video cards, which grab the NuBus every 60th of a second to update the display. This can take a while. Thus, if you are not to miss samples, you have the choice of accepting a 60Hz maximum sampling rate (I need 1000Hz), or a "dead" display (I need to see the data as it comes in). If you are willing to do various non-standard things, like turning off interrupts & writing directly to video ram, you can cludge an "impoverished display"; but, then, why use a Mac? DMA (eg, National Instruments NB-DMA-8) does not help at all with the basic problem, and additionally, makes the incomming data inaccessible to the CPU; closed-loop control -- realtime response to incoming data -- becomes impossible. Does anyone know a way around these problems -- a way to get a KHz, closed- loop data rate with a live display? The new Mac IIci has on-board video; does this solve the problem? I also need to save the raw, incoming data to disk, and I think there is a problem with the way the Mac does disk writes, that it waits, idly, for completion. I could be wrong about this. Does anyone know? Perhaps there other circumstances that would prevent a sample from getting to the CPU and back into the outside world in a timely manner. Does anyone know of any "housekeeping" or other activities that can do this? -- Joel M Miller Internet: jmm@skivs.ski.org Smith-Kettlewell Institute Usenet: uunet!skivs!jmm 2232 Webster St Bitnet: jmm%skivs.ski.org@uunet.uu.net San Francisco, CA 94115 Voice: 415/561-1703 Fax: 415/561-1610