21 Sep
2015
21 Sep
'15
5:31 p.m.
tony duell wrote:
If it was
possible to perform a LLF using the same RX50 drive on
the Rainbow, what was the reason why an LLF could not also be
It is. Remember the RX50 is just a drive, it does not include any of
the controller electronics.
performed on a PDP-11? There seems to be a number
of possibilities:
(a) There was some hardware that the Rainbow had which was missing
on the PDP-11 systems
It's more the reverse!. The Rainbow just has a standard controller chip on the
processor bus (I forget which processor, I can look at the schematics if you
want). The controller chip can do what is needed for a LLF, and there is
nothing in the way to prevent software from sending the commands to do
that.
On the PDP11 there is a lot more stuff between the
processor and the disk
controller chip. Even the Pro 300 series has a microcontroller (8051?) on
the floppy controller board. Therefore the processor you can program
(PDP11) can't do arbitrary things to the disk controller chip, it is very likely
that sending the right commands to do an LLF is one of the things you
can't do.
HOWEVER, while the PDP-11 is still unable to perform an
LLF on an RX50 when an RQDX3 is present, it is possible
to perform an LLF on a floppy in an RX33. Does that still
seem compatible with your explanation?
I would not attempt a trick question with you since I don't
know enough to set one up. Again, I am just trying to
understand what DEC did and how DEC managed to
still avoid being able to perform an LLF on an RX50
using an RQDX3, yet supported an LLF right from
RT-11 on the RX33 using an RQDX3?
The one time I watched it being done, I nearly fell off
my chair in astonishment. After all those decades using
the PDP-11, I was finally allowed to FORMAT a floppy
and it actually worked. Of course, the floppy in question
was identical to a HD 5 1/4" PC floppy which any PC
at that time would also be able to FORMAT. And in
any case, the floppies were usually sold with a FORMAT.
So all that would normally be needed would be the
INITIALIZE command. But it would be nice to be
able to understand how the RQDX3 can FORMAT
an RX33, but not an RX50. I also have an RX23
drive on a CQD 220/TM which I have never tried.
But since that is a non-DEC host adapter, I don't
think it counts,
Jerome Fine