[Simh] Limits on MSCP controllers

[Tim Shoppa]

A related oddball question I always had about the M9058 breakout board.

It would appear that it could support four hard drives and one floppy. BUT
the RQDX3 manual says that RQDX3 itself maxes out at 4 drives.

If anyone has ever tried RQDX3 + M9058 + Four Drives + Floppy it would be
something to hear about! I could barely keep two RD series hard drives
spinning.

Tim N3QE

On Sun, Jun 23, 2019 at 1:30 PM Bob Supnik <bob at supnik.org> wrote:

> The four ports is not arbitrary. SimH simulates actual hardware. DEC
> never built a backplane MSCP controller with more than four ports.
>
> If you want to extend the current RQ simulator to include third party
> boards (either SMD-based emulators or SCSI-based emulators), feel free
> to add an appropriate mode switch. I don't know what controller ID these
> third party boards returned, though, nor do I know how VMS determined
> the number of ports per controller.
>
> I think it would be better to understand why VMS is waiting to mount
> additional discs. Alternately, just create bigger discs and have fewer
> of them.
>
> /Bob
>
> On 6/23/2019 12:00 PM, simh-request at trailing-edge.com wrote:
> > This is, though, another slight silliness in simh. The limit to 4 disks
> > per controller is very artificial. The UDA50, KDA50, and maybe some
> > other controllers only have four actual, physical ports, which limited
> > them to only have four disks per controller. However, MSCP itself do not
> > have such a limitation, and common SCSI controllers for these machines
> > which use MSCP allows more than four disks on a controller, and most
> > software (at least RSX and VMS) also do not limit themselves to only
> > configure max four disks on a controller. I wish simh didn't put such
> > arbitrary limitations in. simh also decides on unit numbers by itself,
> > which could also have been nice to be able to choose.
>
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