miket@bnrmtv.UUCP (Michael Thompson) (04/12/89)
Hello again everyone, I now have the "Unix PC Device Driver Development Guide" (thanks goes to Alan Lundin) and the "Unix PC Tech. Reference Manual" (thanks goes to John Wilkes) in my hands. Looking over the schematics to the Unix PC, I see that the expansion bus on the computer is pretty complicated. Each expansion card is expected to have its own DMA controller, but the CPU signals are hidden behind a lot of logic so hooking up a DMA chip such as the MC68840 or MC68850 doesn't look to easy or even possible. Any DMA chip is expected to go through the Bus Arbitration Circuit using the correct hardware protocal. Coming up with a simple circuit to implement this protocal seems like it would be pretty complicated. On the Floppy Tape Interface Board there is a good example of how to implement a DMA controller from an expansion slot and the circuit looks like it would be pretty easy to adapt to a SCSI controller. Pretty much along the lines of yanking out the WD2797 Floppy/Tape controller chip along with its glue logic and put the NCR 5380 or NCR 5390 SCSI controller chip in its place. If we did this, 75% of the design work is done for us. The problem is that the DMA is implemented with two AT&T proprietary DMA chips which I feel would be next to impossible to get a hold of. Am I right about this, or do we just have to order them from where AT&T gets them. Any help in this area would be greatly appreciated. If we could use these chips, problems such as conversion of the 16 bit bus to an 8 bit bus go away. Another idea which Tim Hogard suggested to me was to have a single chip microcomputer such as the 6811 control the SCSI chip and have it talk with the 68010 through a shared memory buffer. This would move a lot of the complication in writing a SCSI device driver from software to firmware controlling the 6811. This method would also take longer to implement. This would be the best route to take if we cannot get normal DMA working since it would still offload responsibility of transferring information from the 68010 to other circuitry. I really would like to borrow as much of the circuitry for the SCSI controller from the floppy tape controller since it is an existing design which works and has DMA. Can anyone with a technical manual for the Unix PC verify my ideas and get back to me with some thoughts. I would really like to hear from someone that the DMA chips (labeled 32-00379-00 and 32-00380-00 on the floppy/tape schematic) are easily available from AT&T. This would reduce design time of the circuit to a week or two. Also, does anyone have the AT&T publication "UNIX PC Expansion Bus Specification" since the technical reference manual doesn't really describe the expansion bus in great detail. It instead refers people developing hardware enhancements to that document. Thats enough for now. Keep in touch. Mike Thompson ----------------------------------------------------------------------------- | Michael P. Thompson, Member Scientific Staff | ### | BNR/Northern Telcom, Dept. 4Z15 | #### ##### ######### | 685A E. Middlefield Road | ############ ########### | Mountain View, CA 94039-7277 | #### #### #### | PH. (415) 940-2575 FAX. (415) 966-1067 | #### ####### ######## | amdahl! --\ | #### ##### ###### | UUCP. ames! ----->-- bnrmtv!miket | | hplabs! --/ | -----------------------------------------------------------------------------