Interesting part

From: Gary Oliver <go_at_ao.com>
Date: Sat Jun 12 18:48:27 2004

Tony Duell wrote:
...

> UK PAL video equipment used to use a glass delay line as part of the PAL
> decoder. This was a 'mechanical' device -- piezo-electric transducers
> sent a pulse through the glass which was picked up about 64us (a little
> less -- just under 1 line time) later by a similar transducers. Old VCRS
> -- the better ones anyway -- used a 64us (exactly 1 line time) delay line
> to store a video line to be used if there was a dropout on the tape.
> Modern ones don't seem to bother :-(
>
> I have often wondered about using such delay lines for data storage...

Back in the early 70s at Oregon State University, there sat a "home
built" (using Naval Research funding) computer called "Nebula." (You can
look it up - it's in many of the online histories/chronologies) Its
memory consisted of 4k by 34 bit (yes, 34 bit) words made from Corning
Electronic Devices glass delay lines (part number 853302.) These delay
lines were 100us devices and, at about 27mhz bit rate, we stored around
2k bits per device. This resulted in 64 words of storage per card, so
with 64 cards we achieved a whopping 4k words (16 kbytes) of storage.

It was all DTL logic driven (as was most of the computer at that time)
built from your basic 2n3406 and 2n3407 transistors, if memory serves.
I fooled around with this machine from 1970 through the mid 70s, getting
its small drum offline storage working and adding a few instructions,
general maintenance etc. as well as writing a bunch of software. When
computer time on this machine was "free" relative to the $300 / hr on
the "big iron" it was an easy choice.

The memory was quite reliable, not temperature sensative and would
retain its contents indefinitely (as long as power wasn't removed, of
course.)

The 34 bit word layout consisted of 32 "numeric" data bits, plus a S
("spare") and P ("parity") bits. The 34 bits were all part of the
programmer model. The "parity" bit was misnamed, since it really never
was involved in a word 'parity' check and the "spare" bit wasn't really
a "spare." Both were testable and setable but not involved in numeric
or logical operations. The only changed on store and explicit set/clear
operations.

The machine also had 2k similar size words, made from basically the same
glass delay lines, but organized as a content-addressable memory. The
CAM was not usable as program storage, but you could store data there,
after a fashion.

As I said, the memory worked well and including the drivers and timing
controls, was a comparable volume to core memory of the time. Of
course, it was slow and serial (imagine a drum with a 100 uS rotation
rate.) But since the machine itself (at the outset) had a 100uS cycle
time, it was still "relatively" fast.

We later (mid 70s) replaced the glass memory with a core storage module
from a Stretch compute (OSU received a Stretch - minus CPU alas) and
Nebula got one of it's core modules. This resulted in 32k words of
storage with a cycle time of a few microseconds. Unfortunately Nebulas
core control logic wasn't up to that speed (it was a serial computer by
nature) so the best we ever got was about 30 uS word time (and that was
pushing it a bit.)

Short answer, glass was quite reliable. I still have one of the modules
from the machine (after it was "decomissioned" for the core changeout)
and want to build a small demo memory with it some day. Only wish I had
enough to build a reasonable size machine...

-Gary

>
> -tony
Received on Sat Jun 12 2004 - 18:48:27 BST