On Sun, Jan 15, 2006 at 07:42:09PM +0200, Peter wrote:
> >> I'm also thinking perhaps of some of these spacecraft (eg Voyager)
> >> that might have a very long life. Do they not use EPROMs of the
> >> day ? If so, what's to stop their programming just fading away - or
> >> are they not expected to retain it anyway and are simply space junk
> >> with some transmitting capability ? Do they have special long-
> >> retention NASA cells ?
> 
> They have special 'long retention' wire memory or plated memory. The 
> code is effectively burned into what amounts to a printed circuit board 
> or weaved through memory cores. Unless someone unthreads the wire or 
> cuts some board traces the code stays the way it was made.

I did that once actually for a "burned-in" serial number in a project.
On a two-sided circuit board I layed out 8 vertical tracks in parallel
on the front-side, and another 8 on the back-side, horizontally. Bits
were defined by adding or subtracting vias. Very simple really, but
tremendously wastefull of IO lines.

The advantage was the serial number was "visible" to both the user, by
simple inspection of the circuit board, and the PIC. A 40-pin PIC vs.
the 28 I would have otherwise used was not that important for a design
that was being built maybe 2 or 3 times.

Yeah, this was for an art project...


One thing I'd like to do is automate this. Write a little script that
would take a serial number and automatically make a symbol representing
it electrically. That or make a grid of solder jumpers and fill the
jumpers by hand so each board can be different.

Of course these days with the densities seen in multilayer circuit
boards, you could easilly store a few kilobytes I think... Maybe even
make sections with thick traces followed by thin traces, acting as
fuses, then it'd be programmable. We could called it a Programmable
Read-Only Memory...

I'll give you a shout if I ever am silly enough to actually build one!

-- 
pete@petertodd.ca http://www.petertodd.ca
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