Cores, light bulb goes on ... - test-drb@ccmp.vtda.org

27 Apr 1999

...
 <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... 

I don't usually have trouble with American English, but I must confess this
baffles me.  Does it mean something like "Now you know why nuts don't work"?
:-)

...
 <     +--+
<     |  |
<Ch1 -+  +----+  +-----
<             |  |
<             +--+
<       +--+
<Ch2 ---+  +----+  +-----
<               +--+
<
<So I stared at it a bit and the little bulb went on between my ears. 

[snip]

...
  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... 

Hows this for an idea.  If you find that the toroidal ferrites that Siemens and
people make don't have enough hysteresis, why not go to the other extreme and
try the stuff magnets are made of.  Ring magnets as used for loudspeakers and
things are probably a bit large :-)  -  I wonder if it's possible to drill a
hole down a bar magnet and then cut slices off.  For mass production, I'm sure
the magnet manufacturers would sell you the stuff unmagnetised...

...
 <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. 

Now can someone enlighten me:  There will be a minor glitch-type delay between
read and sense pulses with no transition, and a much bigger delay with a
transition.  How do you tell the difference?  Is this one of the applications
where a monostable really is useful?

...
 <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. 

Perhaps now you have a test rig you can try different pulse widths as well...

...
 <   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. 

I think what he meant was:  Rather than sending write current through all core
planes and inhibit current back through those to which you don't want to write,
why not send write current only through those planes to which you want to write?

My initial guess was that you might connect the write wires of several planes in
series, and have a single driver for the lot.

But then I realised that this would be very difficult to drive - the voltage
required to drive a given current would depend on what you were writing - or
worse still, what you were reading!

So why is it done this way?

...
 <   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. 

The other property that affects this is size.  The smaller the core, the less
current it takes to magnetise it.

Philip.