3 Feb
2023
3 Feb
'23
5:23 a.m.
The actual ferrite core doughnuts do not break down with continued use, BUT
moisture or mechanical impact or vibration will damage or degrade the
ferrite cores. Otherwise the ferrite doughnut will live and maintain its
properties "forever".
The main cause of core memory mat faults is mechanical strain on the X/Y,
sense and inhibit wires.
This mechanical strain is caused by use and can be made worse by particular
usage patterns causing the core memory mat to vibrate producing an audible
ring. CDC's Extended Core Storage (ECS) cabinets had a characteristic sound
when you opened the doors.
Early on CDC 6000 series machines had a lot of reliability problems with
core memory, but eventually there was an ECO where the core memory modules
were immersed in potting compound to make the core mat rigid to prevent any
vibration. This made the once fragile core perform perfectly for the
remaining life of the machine.
The core memory in my PDP-8/e machines (H212 board) has the ferrite donuts
glued to the PCB via some white adhesive presumably to reduce vibrations.
Encasing the core mat in potting resin would have been the better solution
as it binds the wires to the ferrite doughnuts.
Tom
On Thu, Feb 2, 2023 at 11:17 PM Jon Elson via cctalk <cctalk(a)classiccmp.org>
wrote:
On 2/1/23 22:10, Will Cooke via cctalk wrote:
ot sure about that. What sort of numbers are we talking about?
> If all else fails there's core memory, which as far as I remember is
pretty much unlimited for both read and write.
suspect that core memory would wear out over time, as well. My reasoning
for this is the because in principle it works the same as FRAM. I usually
refer to FRAM as "core on a chip." Over time, the magnetic domains in FRAM
tend to stay in one polarization or another. I see no reason why the
magnetic domains in core wouldn't do the same. However, a single core is
probably bigger than the entire FRAM chip so there are a LOT more domains.
That means it would take a proportional amount of writes to wear out --
let's just say a million times. In addition, core access was in
microseconds, whereas FRAM and other modern memories are in nanoseconds.
So it takes something like 1000 times longer on the clock on the wall to
perform the same number of writes. So in the end something like a billion
times longer on the calendar to wear it out.
> On 02/01/2023 3:51 PM CST Paul Koning via cctalk <cctalk(a)classiccmp.org>
wrote:
paul
I don't know for sure and can't find any references, but I strongly
I would be very interested if anyone actually knows and especially if
there are references available.
I have extreme doubts that this is true. Memory cores are
just tiny versions of pulse transformers, and similar square
loop transformer core materials are used in switching power
supplies that run for decades at high switching
frequencies. Really, FRAM does not work much similarly to
core. The ferroelectric material is usually lead zirconate
titanate, not an actual ferromagnetic material. It is
written by an electric field, not magnetic, and the electric
field is sensed by a field effect transistor. I have NEVER
heard of core wear-out in magnetic core memories. The
flipping of the magnetic polarization in ferrite materials
does not break down the crystal structure.
Jon
Jon