[HECnet] RT-11 filesystems, was Re: DECnet-RT?

[Clem Cole]

I've forgotten to be honest -- IIRC, no formatting was needed like a
traditional tape drive.   But it is possible we may have had a program to
do the formatting and I've just forgotten it. I do remember that it was a
very simple, saturation scheme. We had a scope on the read electronics of
the TA11 and decoded it all.   NRZ style, 8-bit bytes, no parity, fixed
blocks with a prefix and suffix - serial encoding (unlike a 7/9-track) -
which is why the capacity is so low.  Again, I've forgotten, we had the
prints for the TA11 controller and its very possible the format info was in
there -- I really don't remember.   It was just a stream of bits down the
tape, not serpentine like a QIC tape (which has a head that switches
'tracks' and writes a long stream turns the tape around and then switches
to the next head and writes it backward.   I've forgotten if you got one to
two passes on TA11.  Traditional Phillips tape could be flipped over (side
1 and 2).   I don't remember if TA11's worked that way.

FWIW: Phil Karn (*a.k.a. *KA9Q) and I as lab partners, built an 8085 based
controller for our project for the RT course that could read/write them
using a standard cheap radio shack style music drive but with different
'tronics that we created for the read/write.   Phil had built up his custom
S100 Z80 system (the machine he used to write the KA9Q TCP) and we
interfaced it that system.   He took the drive with him when we graduated
(remember that  8" floppy drives in those days were still pretty expensive
for a student).

I don't remember much more than that to be honest.  Our 8085 work was done
in C and Assembler on a UNIX box at Mellon Institute.  My memory is that we
prototyped our cassette system was on a Multibus 8085 originally - one of
the distributed Front-End prototypes we had Mellon, which I had access.  We
later moved it to Phil's machine after I made a custom PC board just for
the controller and the r/w amps.

Clem

On Tue, Mar 31, 2020 at 5:18 PM Thomas DeBellis <tommytimesharing at gmail.com>
wrote:

> How did you format the tape?  A TU55 DECtape could be formatted, but I
> thought that the cassette tapes had to be bought preformated.
> On 3/31/20 5:14 PM, Clem Cole wrote:
>
> exactly.   You could use some COTS tapes if you carefully cut the case
> with a box cutter to make the notch, which we as students got pretty good
> at. IIRC Memorex worked best.  You could not use the really cheap tape, but
> DEC used saturation recording.   The official DECcassettes and compatible
> from the aftermarket sellers cost more than what you got at the music store
> in Oakland.
>
> On Tue, Mar 31, 2020 at 5:04 PM Dave McGuire <mcguire at neurotica.com>
> wrote:
>
>>
>>   He's talking about DECcassette, not DECtape.  The DECcassette looks
>> like a standard Philips-style audio tape (of Walkman fame) but has a
>> notch in the top.
>>
>>             -Dave
>>
>> On 3/31/20 5:01 PM, Thomas DeBellis wrote:
>> > Are you referring to DECtape II?  That was a cassette.
>> >
>> > I was referring to the (nearly indestructible) earlier format: simply
>> > called DECtape or DECtape I.  It's the same media as LINCtape (a small
>> > reel), but with a very different controller.  These could store a little
>> > over 70K (36 bit) words.
>> >
>> > On 3/31/20 4:40 PM, Clem Cole wrote:
>> >> Dave the TA11 (DEC proprietary Phillps Cassettes) were 150 ft long.
>> >>  I just looked in my 1976 Peripherals Handbook -- Tape capacity of
>> >> 92,000 bytes (not kbytes mind you).   Two tapes per TA11; one for the
>> >> OS and the other the user.   We had a couple at CMU on 11/20's running
>> >> RT-11 in the EE Digital lab for the RT system's course - in fact, the
>> >> famous "110v non-maskable interrupt" occurred on one of those machines
>> >>
>> >> On Tue, Mar 31, 2020 at 3:14 PM Dave McGuire <mcguire at neurotica.com
>> >> <mailto:mcguire at neurotica.com>> wrote:
>> >>
>> >>     On 3/31/20 2:33 PM, Thomas DeBellis wrote:
>> >>     > 32MB really was ginormous back in the day; our labs had RT on
>> >>     RK05's,
>> >>     > which held about 2.5MB.  Way more than a DECtape.
>> >>
>> >>       When I started out, I had it on RL01s.  But I suspect you have a
>> few
>> >>     years on me. ;)  That was quite a bit of space at the time.
>> >>
>> >>                 -Dave
>> >>
>> >>     --
>> >>     Dave McGuire, AK4HZ
>> >>     New Kensington, PA
>> >>
>>
>>
>> --
>> Dave McGuire, AK4HZ
>> New Kensington, PA
>>
>
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