> 
> note that it works OK if size is a multiple of 512 <= to 64512
> (which is 65536 - 512*2).
> 
By some reasom MAXPHYS has been 63k on VAX since day one (I think it
comes from b_bcount being a short, but I really don't know). I changed
it to 64k some year ago, but obviously the asc driver have it hardcoded:

        sc->sc_maxxfer = 63 * 1024;

You should try to change that to MAXPHYS instead and see if it works,
if it do not work then it may be the DMA engine or something that do not
like >=64k transfers, but that's not likely.

Please tell how it worked.

-- Ragge