
A vacuum tube ROM? - sbierwagen
http://tubetime.us/index.php/2018/06/04/a-vacuum-tube-rom/
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CamTin
Very cool! The more I read about early computing, the more fascinated I become
that the major problem innovators needed to solve was memory, not computing
per-se. There were precious few things that could be teased into working as
read-write memory at electronic computer speeds before semiconductor memory,
and they were all monstrously expensive (vacuum tube flip flops, Williams
tubes, core memory) or nasty and dangerous (mercury delay lines). Core,
despite its expense, was a major breakthrough in that it was solid state and
therefore much less error prone.

I can't find the reference now, but I remember reading about some early
computer engineers (possibly on Whirlind?) contemplating the use of a
microwave retransmitting station as a form of memory, essentially creating a
"mercury" delay line with microwaves in the atmosphere.

This is how desperate people were for anything even remotely affordable which
could be tortured to behave somewhat like memory! No wonder Intel made a
bundle when they started offering chip memory. The stuff it was replacing was
just totally inadequate for the purposes many people wanted to put it to.

~~~
xelxebar
There are some old giant calculators Friden EC-132s?) that propogated torsion
waves down coils of wire for a delay line.

This is way out of my knowledge domain, so I'm curious why mercury delay loops
were used in the first place.

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Intermernet
"Mercury was used because the acoustic impedance of mercury is almost exactly
the same as that of the piezoelectric quartz crystals; this minimized the
energy loss and the echoes when the signal was transmitted from crystal to
medium and back again. The high speed of sound in mercury (1450 m/s) meant
that the time needed to wait for a pulse to arrive at the receiving end was
less than it would have been with a slower medium, such as air (343.2 m/s),
but it also meant that the total number of pulses that could be stored in any
reasonably sized column of mercury was limited. Other technical drawbacks of
mercury included its weight, its cost, and its toxicity. Moreover, to get the
acoustic impedances to match as closely as possible, the mercury had to be
kept at a constant temperature. The system heated the mercury to a uniform
above-room temperature setting of 40 °C (104 °F), which made servicing the
tubes hot and uncomfortable work. (Alan Turing proposed the use of gin as an
ultrasonic delay medium, claiming that it had the necessary acoustic
properties.)"

[https://en.m.wikipedia.org/wiki/Delay_line_memory](https://en.m.wikipedia.org/wiki/Delay_line_memory)

~~~
CamTin
Also, they're not "loops" in the physical sense, though they do operate
logically as loops: pulses go in one end of the tube, travel through the
mercury, and are received at the other end, where they are repeated
elecrtonically back to the transducer at the starting end. "Reading" a bit
means waiting for the moment when it is hitting the pickup end, and pulling it
out of the loop at the same time as repeating to the transmitting end.
"Writing" means waiting for the same moment in the "loop" and replacing the
output of the pickup transducer with a signal that represents the data you
wish to write.

Thus, there is an inherent tension between making the tubes longer (more
storage per tube) versus keeping them short (lower access times). This tension
is inherent to all forms of "delay" memory, including the wire torsion memory
grandparent mentions.

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greenyoda
Cathode ray tubes were also used as RAM in early computers:

[https://en.wikipedia.org/wiki/Williams_tube](https://en.wikipedia.org/wiki/Williams_tube)

One of these computers was the machine at Manchester that Alan Turing worked
on:

[https://en.wikipedia.org/wiki/Manchester_Baby](https://en.wikipedia.org/wiki/Manchester_Baby)

~~~
bjackman
There's one of those* on display in the CS building at the University of
Manchester, it's the size of a forearm and IIRC stored 1 byte :)

* A CRT RAM thing. It's a refreshing RAM that takes advantage of the persistence of the the phosphor glow.

~~~
monocasa
Williams tubes don't rely on the phosphar coating (and some don't have a
phosphar coating at all). It's all about the static electricity on the tube.

~~~
bjackman
Oh yeah, that's true. Thanks for correction.

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NegativeLatency
Can anyone explain why the letters and characters in the grid are ordered they
way they are?

A-Z is non sequential, I’m guessing it’s something to do with making character
selection logic simpler, but by looking at the letters I couldn’t come up with
a definite pattern or rule.

~~~
Animats
It's some variation of BCD code, which was inherited from punched cards and
still lives on in some IBM mainframes.[1]

[1]
[https://en.wikipedia.org/wiki/BCD_(character_encoding)#48-ch...](https://en.wikipedia.org/wiki/BCD_\(character_encoding\)#48-character_BCD_code)

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CamperBob2
Very cool, especially the "SEM with a fixed target" analogy. I've often
wondered how the old SAGE terminals handled character generation, and this
post fills in the blanks nicely.

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sprash
Ok... Now I feel the sudden urge to build a working terminal with a
Charactron.

Why can't I have simple hobbies.

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kregasaurusrex
I'm usually too scared to work with anything over 120V- especially since parts
on the schematic linked require a 1000V power supply to work. Very neat, but
too dangerous for the average home-gamer like myself to be inspired enough to
go out and build.

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CamperBob2
1000V, at the current levels needed for this application, amounts to the juice
behind a good carpet shock.

~~~
kragen
Dunno about the Monoscope in particular, but a lot of tubes need several
milliamps, which is quite a bit more painful than a good carpet shock. Not
fatal, but not something I'm eager to experience. And I'm the guy who used a
one-henry inductor to administer electric shocks to people in high school —
including, repeatedly, myself.

The issue is that it's a lot easier to build or buy a kilovolt power supply
that doesn't have adequate current limiting than one that does. And even one
that has it _in theory_ may not have it in practice — that big capacitor
across the output? Make sure it isn't just wired directly to the output
terminals, because its ESR sure as hell isn't going to be adequate current
limiting.

So I think it's reasonable to be wary of kilovolt circuits. Dying is easy, but
you only get to try it once.

~~~
kragen
Lightedman's dead comment, in response to my, "The issue is that it's a lot
easier to build or buy a kilovolt power supply that doesn't have adequate
current limiting than one that does," said, "You can buy Van de Graaf
generators all day long. Quarter-million volts minimum for $200."

While van de Graaff generators are indeed relatively affordable, and indeed
some even cost less than US$200, a working microwave oven costs US$60, and a
broken one can be had for under US$10. Furthermore, a safe van de Graaff
generator is not actually capable of supplying enough current at the high
voltage to operate a vacuum tube, while a deadly microwave-oven transformer
is; and there are orders of magnitude more microwave ovens available.

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pjc50
> video amplifier I used had a gain-bandwidth product of only 4.5MHz.

That's .. not really what I'd call a video amplifier, unless the amplifier
itself is also made out of tubes.

~~~
tubetime
good enough for NTSC or PAL monochrome composite video.

~~~
dfox
Without any gain. What is typicaly meant today by video opamp has GBW in low
hundreds of MHz.

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sprash
Original catalog entry / advertisement can be found here on page 18:

[http://informationdisplay.org/Portals/InformationDisplay/Iss...](http://informationdisplay.org/Portals/InformationDisplay/IssuePDF/V05N05-1968%20SepOct.pdf)

