MSCP SCSI controller speed

Considering the major Qbus SCSI controllers do not have near the speed of a slow 386 cpu, speed is not the issue. I'd have to look but I think none used anything faster than maybe a 10mhz 68000. While the 53C90 does have large overhead it's also not quite the bottleneck.

I would look first at the code. First place I'd peek at is to see what the code overhead is. Also Qbus transfers can be block mode DMA for best speed but watch the length as memory says 4words per block were a maximum to avoid bus timout errors. ISA does not have this problem but it's fairly slow for 8bit moves (1meg Byte/S). As to beating a RQDX thats easy consider that the MFM drives are 5mbits/S range(.625k bytes/S burst) rate and slow seeking combined with no local caching more than a sector or two. Also the CPU was a T-11 at a mere 7.5mhz. I've considered hacking a SCSI controller for Qbus once. The approach on paper was a PCI PC system board with a Adaptec2900 series controller with a parallel adaptor to Qbus. That was focused on building as little hardware as possible becuase the real task was software. The item worked out were software as in PC level code that didn't require a bootable OS (rom resident would be a winner). However the Qbus side was sorted down to a M7941 parallel line unit (PIO) or M7950 (DMA) with a driver based on one of the removable non-MSCP cartridge disks (RK, RL or RM). I quit after realizing that I didn't have a 290x SCSI board nor wished to program PCs. That and someone gave me a CMD SCSI adaptor. ;)