From: lasner@watsun.cc.columbia.edu (Charles Lasner)
Subject: Re: Worst tape drives. Was Re: 9 track data storage
Reply-To: lasner@watsun.cc.columbia.edu (Charles Lasner)
Organization: Columbia University
Date: Wed, 22 Jul 1992 17:47:02 GMT

In article <1992Jul22.124305.1794@unipalm.co.uk> ian@unipalm.co.uk (Ian Phillipps) writes:
>peter@taronga.com (Peter da Silva) writes:
>
>>You don't want to know what a TK50 is. Probably the worst cartridge design
>>for tape transport ever.
>
>I agree, but worst ever - there's hot competition! I remember the
>old-style DEC tapes. 3" diameter open-reel jobs; no capstan motors. The
>drive just span the reel motors. I had to use a Fortran compiler which
>used them as backing store. Coffee while compiling? No - lunch!
>
>OK, they weren't "cartridge" drives, but they were close. ish.

[Flamethrower On]

That's about the worst maligning of DECtape I have heard this year!

For starters:

You are talking about a random access device that is serially implemented,
yet totally block-replaceable, and has no additional penalties for passing
the data as in streamers, and can even transfer data backwards if the 
software supports it.  (Not all systems did that, but the hardware does.)
In all probability you were using an underpowered system where the disk
device was the DECtape, and are blaming software that needs a real disk
for something as inefficient as you attempted.  When used as a backup
device or just serially, would it really be that slow?

Moreover, you misunderstand the basic principle of storage on this medium. 
The tape rides on an air bearing so it never touches the head.  The data
is recorded with Manchester phase-encoding timed against a dedicated
timing track written only when formatting, so tape speed is not a factor
in reading or writing the data.  Indeed there have been postings about
using the device with hand cranking of the reels when the motors died, etc.
All data is recorded redundantly on pairs of tracks; a 1/4" hole can be
punched in the tape and it will likely still be readable due to the 
redundancy.

The medium is sandwich tape, meaning the oxide is coated like audio lab
tape with a clear non-oxide layer, which dramatically lowers tape wear.

Besides the timing track there is a so-called mark track, which allows for
unique detailed information everywhere along the media.  All blocks are
numbered on both ends so searching can be both rapid and accurate.  There
are checksums at both ends of the block to allow the data to be read in
the direction reverse to how it was written.

PDP-10 support of the DECtape controller allowed for up to all 8 drives to
be spinning at once, by allowing "dead reckoning" on 7 of the drives.
Periodically, each tape can be selected to determine position relative to
desired block, until one achieved the exact position desired and then the
data is transferred.

This is perhaps the most reliable device in the entire industry for
magnetic storage of data.  Only fire or extreme magnetism can kill it.

I have DECtapes formatted in 1969 that read reliably today.

LINCtape is DECtapes forerunner; the same tape system except for a simpler
format that can't transfer backwards.  (It can search backwards.) I have
read tapes formatted in 1962 on it without problems whatsoever.

cjl

Newsgroups: alt.folklore.computers