I recently acquired a PCjr with a bunch of software. Machine's very clean.
I'm not really that interested in playing with this beast. Should I sit on
it as an investment? Is it worth anything now? Dnyone interested?
Thanks
P Manney
Is it illegal to yell "Movie!" in a fire station?
Thousands of discounted photo items at http://www.hmcltd.net/pgphoto
Every two years at this time the University of Queensland Alumni Association
holds a large sale of old books donated by individuals plus library
discards. I've found some good collectables there in the past, such as old
manuals, but this time (today) it took me five minutes to just find where
the computer books were - instead of a full trestle table top as in previous
years, there was just 4 inches of space reserved for them - maybe 20 books
in a hall of many many thousands.
I found the lady in charge and asked what had happened to them? She said "we
kept any computer books dated in the 50's or 60's (I didn't find any), and
anything fairly recent, but all donations from the 70's and 80's went out."
I asked "Did you send them to one of the charities to sell at their
bookfests?". Her response "oh no, they just go straight to the shredder! -
we can't get rid of them otherwise."
And sadly, I suspect she was correct. Besides the odd character like me
looking for manuals for long gone models, most of the stuff that went out
for sale was rubbish to 99.99+% of people and stayed on the table at the end
of the day. But I sure wish she had at least given me the chance to look
through that stuff before it hit the shredder.
The good news was that I found amongst the "sets" (encyclopedias etc) a full
set (I think, 22 volumes anyway) of the Time-Life series Understanding
Computers. Lots of good computer history and pictures - but I would be
interested if anyone has any criticism of the facts presented in any of the
articles in that series - they seem authoritative enough, but then some of
the people on this list know more than the usual sources that Time-Life
writers would have used.
Phil
in Brisbane, Australia.
Yesterday, at the KS10 move, I came away with two IBM PCs, but I
don't know enough about them to know if they are the AT or XT
models... they both have a 5.25" floppy and a hard drive, and
both have color monitors.
They are both desktop boxes... the monitors were sitting on top
of them.
So, how do I tell...
Megan Gentry
Former RT-11 Developer
+--------------------------------+-------------------------------------+
| Megan Gentry, EMT/B, PP-ASEL | Internet (work): gentry!zk3.dec.com |
| Unix Support Engineering Group | (home): mbg!world.std.com |
| Compaq Computer Corporation | addresses need '@' in place of '!' |
| 110 Spitbrook Rd. ZK03-2/T43 | URL: http://world.std.com/~mbg/ |
| Nashua, NH 03062 | "pdp-11 programmer - some assembler |
| (603) 884 1055 | required." - mbg |
+--------------------------------+-------------------------------------+
Hum.... Thanks for the tip on the seller! My M series is sick, so I was just
getting ready to fire off an email asking what cards were in the memory
backplane.... with a record like that, I don't think I'll bother (now if it
was a 7900A drive, I'd forge ahead anyways <grin>)!
Jay West
>> Hey, Sallam, couldn't this power thing be a good addition for
>> VCF 3.0? A speach about how to operate Eq. from a different
>> country (and power system) in an acceptable and safe fashion ?
>> Maybe if Philip and me (possible with a thirs, US guy) team up
>> to give an outline of the different circumstances to remember
>> when using foreign (old) computers ? There might be some need.
>
> Yes, I think this would make for a great workshop. I expect more people
> to start trading computers across the world and this will make for a very
> relevant discussion.
I think it's an excellent idea. In fact I have long thought a computer-buff's
guide to power electrics (the mains) would be worth writing.
But I doubt I'll manage VCF 3 this year. A holiday in CA is not something I do
every year, alas...
Philip.
>2. Phase.
>
>As I understand it the usual practice in the US is take _either_ a phase
and
>earth (neutral) _or_ two phases of the HV supply and feed a single phase
>transformer from this. The LV side of the transformer is (say) 240V with a
>centre tap. The centre tap is earthed and provides the neutral connection,
the
>outer two taps are then both hot at 120V, 180 degrees apart. This is true
_even
>if_ the primary (HV winding) is connected between two phases of the HV
3-phase
>system.
Close but not quite right. For single phase installations the power company
supplies 2 hots and neutral. This is a center tapped secondary and the
neutral is earthed at the service entrance. The primary of this transformer
is fed from only 1 high voltage phase.
>In the UK the usual practice is to connect all 3 phases of the HV (almost
>invariably 11kV phase to phase) to a delta-star 3-phase transformer. The
>secondary, the star-connected winding provides 3 phases and neutral.
Neutral is
>earthed at the transformer. Protective earth can be provided by _either_ a
>separate wire from the main earth at the transformer _or_ an earthing
electrode
>at each customer's site _or_ the neutral is earthed at a large number of
points
>on the system and the customer's protective earth is bonded to neutral at
the
>point of entry of the supply. In all cases the customer sees 240V line to
>neutral; if he gets more than one line (hot) connection, they will be 120
>degrees apart.
The delta-star is the same but with 120 phase to neutral and 208 phase to
phase. In the UK it is the same but 240 phase to neutral and 416 phase to
phase.
You can also commonly find 480 3 phase delta- star 277 phase to neutral.
This is then used for lighting (277 florescent is common) and large loads
like air conditioning. In these installations the customer has to provide
their own transformer to 208/120.
>In remote locations, only 2 phases of the 11kV system are taken to the
site.
>Small loads get a single phase 11kV/250V transformer with one end of the LV
>earthed; larger loads get a single phase 11kV/500V transformer with a
centre tap
>earthed. Some farm equipment (I am told - I have never seen this) is rated
for
>480V single phase.
I don't think this is avail. in the US.
>
>It is very unusual for a domestic installation to get more than one phase
and
>neutral. But 3-phase can be done - the supply company don't mind putting
it in
>if they think you'll buy a lot of electricity!
They will here also and I checked into getting it here but I am a few miles
>from the closest point that 3 phase is available for distribution. I would
either have to guarentee a large $ per month usage or pay them to add the
additional lines. Over $10,000 was mentioned.
Dan
Hi all,
I read a web page somewhere that indicated DEC put in the public domain
all the documentation they no longer publish.
Is this true? Can anyone point me back to the original web page?
Thanks,
clint
<Ok after spending much of the weekend trying to get my core driver working
<and feeling really good about having all the parts I needed in my spares
<box, I realized I had built half of an h-bridge. Since I had already
<designed a really nice MOSFET based h-bridge that could switch 10 - 15 amp
<for my robots I slapped my head and went "Doh!"
;) that will do.
<Anyway, so I wired up my #2 nut with sense wire to the h-bridge and start
<slapping current back and forth through my nuts. :-) Seriously though when
<viewed on 'scope it looks something like:
<
< +--+
< | |
<Ch1 -+ +----+ +-----
< | |
< +--+
< +--+
<Ch2 -+ +----+ +-----
< +--+
Transformer action prior to saturation.
<UNTIL you get to about 7.5 amps or so, and then it looks like:
No you know what nuts dont work...
< +--+
< | |
<Ch1 -+ +----+ +-----
< | |
< +--+
< +--+
<Ch2 ---+ +----+ +-----
< +--+
<
<So I stared at it a bit and the little bulb went on between my ears.
<
<The first pulse "writes" a zero to the core, if the core is either already
<magnetized with a zero, or has nothing in it, then the sense line sees the
<deta-B (change in magnetic field) and a current is induced in the wire.
<When the second pulse comes along it "writes" a one into the core, same
<effect on the sense wire. The clever part comes when your "write" a zero t
<a core that has a one already written to it.
Core is one of the few DESTRCUTIVE READ memories. You have to try and flip
it to see if it flips, if it does you had a (one or zero) and if didn't
you have a (zero or one).
Designing circuits the transformer and the "bistable" properties makes for
interesting logic.
<In this case the current in the wire induces its standard magnetic field,
<but that field is _cancelled_ by the field already in the core, thus for a
<period of time the sense line sees no change in magnetic flux, and so no
<current is induced. Then however the core switches to 0 and the sense wire
<sees that change in flux and out comes the induced pulse now delayed from
<the initial point by a time controlled by how long it took to saturate the
<core and switch it.
Yep. Watching that spring snap in interesting on a scope. Those nuts are
slow too! Try a bunch of other materials now that your set up...
<The gap between the pulse start and the sense pulse is used to tell whethe
<or not the core had a 1 in it. Now in the DEC design what happens after th
<read pulse (which is really a "write zeros" pulse, is they take the data
<they just read and re write with the write ones pulse. However this time
<since the sense lines aren't needed to figure out what the cores had in
<them, they use them for "inhibit" currents.
They used seperate wires (four wire cores) in some cases as it was easier
that all that switching.
<--------------------------
<Cool stuff, now it raises some new questions:
< 1) Do you want your pulses to be long enough to switch the
< core exactly, or longer? (eg does writing a zero just cancel
< a one or does it cancel the one and write a zero in its place?)
I forget.
< 2) Why not just gate the write one current pulse? That would save
< on the inhibit current stuff.
well you have to know what your writing back and to do that you have to
read it first. Hence the common write after read cycle on many machines
>from the era of core.
< 3) What properties of a material make it easier to switch at lower
< currents? I don't want to build a core plane with nuts if I need
< 8 amps to switch them.
Good magnetic conductors that hold their magnetizm. Some steels, ferrites,
cobalt alloys, alnico, a few rare earths. Try some of those ferrite beads
used for bypassing in RF work.
Allison
