It’s always interesting to see what will come out of a hacker meet-up. At the Observe, Hack, Make festival earlier this month [Bertho] was talking to a guy named [Erik] about how flip-dot displays work. [Erik] mentioned that the control theory is the same as core memory. So when [Bertho] got back to his home workshop he started playing around with it to see if a flip dot display can be made to behave exactly like core memory.
We’re really glad a successor to core memory was found since it’s pretty slow. But the concept still makes for some fun exploration (here’s the obligatory Arduino implementation of core memory). It uses magnetic rings with two conductors running through them that pass at right angles to each other. Sound familiar? This is exactly how flip-dot displays work.
There are, of course, some differences. The biggest one being that the displays don’t have the sense wire present in core memory. That was an easy enough thing for [Bertho] to get around. He added the grey sense wire by threading it through the inside of the hardware. The other hurdle he had to overcome was to alter the controller firmware to match the destructive tendency of core memory (reading the state also resets it).
So far he’s just set this up as a proof of concept, reading the sense wire while repetitively reading and writing to the “memory”. But it’s engaging to see what was captured on the scope. We asked him about his future plans, specifically what he would use to automatically read from the sense wire. His response is found after the jump.
Mike Szczys wrote:
Just out of curiosity, if you are adding your own sense wires to the display, what kind of circuit do you think you’ll use to read from them?
I’m not going for a sense-wire. It is too difficult to amplify in a stable fashion (I only got 20mV with a lot of noise). Moving the wire will also change the flux-coupling and makes it even harder.
Real core-memory has a well-defined coupling and has an inductance that is orders of magnitude lower than flipdots. Therefore, the sense-wire will see well-defined pulses, whereas the flipdot version sees rather slow flux-buildup. The stored energy in the flipdot coils make rather nasty spikes where real core-memory has negligible energy stored.
It is much easier to do current-sensing on the H-bridge setup, which has a distinct shape based on the “seen” inductance. Simple amplification of the current-sense voltage combined with a comparator gives current-rise-time and that is indicative for what was stored.
The flipdots need to have 350mA for 450us to flip, so that is a fixed timing frame (see my other flipdot posts on my website for description how that works). The current-shape is then easily transformed into a memory-content analysis.