PDP-8: (and others) anyone tried NVRAM simulation of hard drives?

From: Ethan Dicks <erd_6502_at_yahoo.com>
Date: Thu Jan 30 10:43:01 2003

--- Frank Arnold <fm.arnold_at_inn-salzach.de> wrote:
> >Will Jennings:
> > Omnibus or Posibus? Why not Negibus? Neither Omnibus nor Posibus would
> > help me on my 8/i, since mine is Negibus...

> Hi,
> one way of doing this would be the use of a TU-58 tape drive. This device
> is internally using a disc-like structure, and it will connect to any
> serial port.

Presuming you _have_ a spare serial port... (read on)

> There should be a PDP8 OS8 driver for this.

Is there? I don't recall ever seeing one.

> The real TU58 is sweet, but terribly slow. On top of that, the tapes are
> expensive. But there are cool alternatives:
> You will find a semiconductor-emulated TU58 kit on:
> http://www.SpareTimeGizmos.com/Hardware/TU58_Emulator.htm
> Also there is a PC-Emulation on:
> http://www.not-compatible.org/PDP-11/programs/tu58sim.html

Yep. Both great alternatives.

> I havn't yet used any of these, (have a real TU58)...

I have several real TU58s (embedded in each 11/750, and one removed
from an 11/725 that gave its life 14 years ago to keep an 11/730
running). I use the ex-11/725 drive with DOS software to backup
my cartons and cartons of carts (with about an 80% success rate,
owing to the age of the tapes).

As for using them with a PDP-8, there are several issues that
come to mind...

Will (and I) want to hang a "disk" off of a negibus PDP-8/i (I
wouldn't mind hanging one off of a posibus PDP-8/L, but that's
a similar problem to solve). First, there are no spare serial
ports on hardware that old - you get a console TTY port
(implemented in a couple of dual-height cards - M706/M707, that
plug into pre-wired sections of the backplane) and that's it.
For external devices, there is a place to take the I/O bus off
to a different cabinet over a wad of cable the diameter of your
lower arm - the posi/negi-bus. Second, a "disk" type device
from that era was typically a data break device (DMA) - I _think_
the first PIO disk that came out for the PDP-8 was the RX01.
The throughput of a serial-port-attached device would be awful,
even eliminating the seek time of a real TU58 tape.

So... for an OMNIBUS or later machine (DECmate, SBC6120...), it
might be a feasible option, once you have a spare serial port
(for example, on my PDP-8/a, I have the console port from the
DKC8AA and a KL8JA (4 port) that I've never used, but it should
be possible, if there's a driver).

As to Will's original question, I myself have wondered what it
would take to design/build a compatible device. I would think
that for maximum compatibility, it would have to resemble something
like an RK8, RF08 or DF32. The IOTs and behavior are well known,
and AFAIK, the engineering drawings are available from the
usual places. One thing that has been slowing me down is
deciding how to attach a modern device to an -8/L or -8/i -
one way is to use a blank backplane and do it the traditional
way, with the multitude of DEC cables. Another way would be to
manufacture modern cables with DEC paddle cards on one end, and
modern connectors on the other end and use a modern enclosure
and connector arrangement to get to the signals. Still another
would be to wire-wrap (or use individual connector pins) to
tap the backside of the backplane, and route that to a modern
box. The third option is most likely to be the least expensive,
but a hassle to install/de-install. At a few $$$ per paddle card,
the second option is only slightly more expensive (probably less
than $100 for the project extra).

Thoughts? Suggestions? Has anyone else been kicking this around
for years?


P.S. - One of my many long-term projects has been to find a way to
attach / replicate an RX8E for use with an -8/i or -8/L. Perhaps
that might be a good first step. After all, many of us with
the CPUs probably already have the drives and diskettes.

P.P.S. - However the attachment is made, whatever the IOTs, I would
think that it might be a nice design goal to make the media compatible
with that from an SBC6120 - several of us have been using IDE
adapter frames with great success. I have an 8MB CF card with three
OS/8 partitions on it (2MB each). I was contemplating going with
a 20MB card and 6 partitions next. They are, I believe, easy enough
to load from a modern PC (but I did mine through the SBC6120 monitor).
Received on Thu Jan 30 2003 - 10:43:01 GMT

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