Trash or Treasure?

From: Jerome Fine <jhfine_at_idirect.com>
Date: Sat Sep 23 00:21:13 2000

>Chuck McManis wrote:

> It is a PDP-11/23. (essentially) And yes, I think it is a bit of treasure
> since it has a non-DEC combined floppy/hard drive controller so you can
> probably make it into a very nice RT-11 system. I'm thinking about building
> one of those in one of the standalone DEC backplanes. It would be very cool.

Jerome Fine replies:

Based on the information that the box is an SMS Qbus system with a combined
hard/floppy disk controller, I thought that I might mention the following
information. I also have one which I pulled the boards from. Originally,
it also contained a dual CPU (probably an 11/73 as opposed to the 11/23)
along with some memory. The KEY point is that the 8" floppy was connected
to the same controller emulation as the hard drive. HOWEVER, the emulation
for the hard drive, AS WELL AS THE FLOPPY, is MSCP or DU.
And since the floppy drive is also DOUBLE-SIDED, the use of such
8" floppy media is allowed as well. Now I can't seem to remember using
single-density media in the 8" floppy drive, but standard DEC 8" double-density
media compatible with a DEC RX02 floppy drive are what the 8" floppy
drive in my SMS system use and read and write. So, if your system is the
same, YOU WILL NOT BE ABLE TO BOOT A STANDARD
DEC RX02 FLOPPY on the SMS system unless the boot code uses
the MSCP device driver interface.

Under RT-11, that is easy to set up even on the DEC RX02 floppy
drive. The RT-11 command:
"COPY/BOOT:DU DY0:RT11FB.SYS DY0:"
will change the BOOT DEVICE DRIVER from "DY:" to "DU:". Note
the use of the argument ":DU" after the BOOT switch. Note that the
floppy drive was: SET DU1: PORT=0,UNIT=1,PART=0".

Another point. Assuming that you even have a driver for an RX03 for
RT-11 and an RX03 floppy drive that is 3rd party compatible, it is likely
that under DY(X).SYS that a double sided RX03 media will use the
full first side of the media followed by the full second side - at least this
is what DEC seemed to intend doing under V4.0 of RT-11 before the
optional code was removed by V5.0 of RT-11 for the DY.MAC file.
(Perhaps Megan can verify that aspect from V4.0 to V5.0 of RT-11.)
However, DEC never produced an RX03 floppy and RT-11 never
distributed correct code for a double-sided floppy, although there were
a number of 3rd party RX03 drives produced and a number of 3rd
party enhancements for the DY(X).SYS device driver which allowed
double-sided use of the 8" RX03 media based on the above usage
of first one complete side followed by the other. Indeed, I am
presently using a version of DYX.SYS under V5.4G of RT-11 for
the DSD 880/30 which includes a single 8" double-sided floppy
drive. This version of DYX.SYS not only allows the use of double-
sided media, but also includes a bounce buffer when the user
buffer is in physical memory above the 256 KByte boundary.
The bounce buffer is required since the controller for the DSD 880/30
is only an 18 bit controller and V5.4G of RT-11 allows up to the
full 4 MBytes of physical memory under RT-11.

The final point is that assuming someone is able to produce a double-
sided 8" floppy with the DYX.SYS noted above, the SMS emulator
for the MSCP implementation of the 8" floppy drive is quite happy
with single-sided floppy media from a DEC RX02 or 3rd party
compatible. BUT on the SMS MSCP emulation on the built-in
controller, the 8" floppy drive for double-sided media for the
system that I have reads BOTH tracks on both sides of the
double-sided floppy media BEFORE switching cylinders and
reading the next pair of tracks (on alternate sides of the media).
For this reason, double sided media from an RX03 floppy
drive using the DY(X).SYS device driver under RT-11 may
(in my case was not) be compatible with the use of 8" double-sided
floppy media on the SMS box with the MSCP emulation since
the data on the media is not in sync after the first track (17 blocks?).

If anyone has any questions because I did not explain the problem
with double-sided media correctly, please ask again.

Sincerely yours,

Jerome Fine
Received on Sat Sep 23 2000 - 00:21:13 BST