<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
Received on Mon Apr 26 1999 - 18:19:07 BST