MSCP SCSI controller speed

9000 VAX vax9000 at
Tue Nov 8 09:14:14 CST 2005

On 11/8/05, Tim  Shoppa <tshoppa at> wrote:
> >From a post I made to the PDP-11 newsgroups in the previous
> millenium:
> Here are the peak data rates measured for read and write 64
> blocks-at-a-time:
>                   Read           Write
>                  ----------     ----------
> Andromeda SCDC    2.298 MB/s     1.131 MB/s
> CMD CQD440        2.397 MB/s     1.525 MB/s
> CMD CQD220        1.418 MB/s     0.882 MB/s
> CMD CQD220A       2.088 MB/s     1.409 MB/s
> DEC RQZX1         1.379 MB/s     1.097 MB/s
> Viking QDT        0.846 MB/s     0.704 MB/s
> DEC RQDX3         0.164 MB/s     0.161 MB/s

Thank you. After analyzing the Qbus activity diagram I conclude that
my controller won't be fast. There are several reasons.
1. The overhead of 53C90A chip is significant. I may try 53C94/96 later.
2. The ISA DMA is slow for the getpkt(), putpkt(), getdesc() and
putdesc() functions. 80188XL won't be faster.
3. The 80386-16 is slow in executing the MSCP code written in C.
80188XL won't be faster.

To improve 1, I need a faster SCSI chip, or need to integrate it into
the CPLD. To improve 2, I need a small cache to speed up the DMA, or
need a faster microprocessor. To improve 3, I need a faster
microprocessor, and I may need to rewrite the code in assembly

So the conclusion is, with the corrent design, the controller won't be
fast. I am reluctant to rewrite it in assembly language because I
don't want to divert too much away from the SIMH code. I may improve
the code to speed it up a little bit, maybe to 400kB/s. I am happy
that it can beat RQDX3.

vax, 9000

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