hp-41 Cards

Out of curiosity, what was the typical longevity of the data written to the HP-41 magnetic cards? If the cards were stored reasonably, how long can one expect them to be readable?


30 years and counting...


A couple of years ago I've read back some cards I wrote in 1980/1.


It must be a sensitive reading head/circuit and/or a strong write coupled with a very good error correction system and I suppose there's not much data per card anyway ,.... but you should really re-write that data from decade to decade to re-form a strong copy on the card.
Though I suppose you run the risk of buggering up the data in the actual process of re-writing it with the potential of muck partially blocking the head or some other mechanical glitch


There is no error correction and rather weak error detection. There is a simple checksum for each card side.


I suppose it comes down to tiny amounts of data on the strip in todays terms at least , so nicely spread out and distinct

Edited: 6 Nov 2013, 4:54 a.m.


HP-67/97/41 magnetic cards are very reliable in generals and this even after 30 years.

The only problems that I experienced with the cards in all those years, was something like
an oxidation of the magnetic side, easily correctable with a Stadtler white eraser gum.



The magnetic particles on the cards probably have similar characteristics to those used in ferrite permanent magnets. The demagnetization of the latter has been stated that 1% of the field decays within milliseconds of being magnetized. The next 1% decays in another second. The next 1% decays around a century later(!).

The limit to the lifetime of the data on these cards is likely to be the physical limits - that the oxide layer doesn't scrape off, etc. Keeping them in a cool, dry and intert place should keep them far longer than we will last.

The actual recording technique is simple. There are two tracks per side. A pulse on one track is a binary one; on the other is a binary zero. As such, it is self clocking. The differential nature makes it very noise and level immune. While terribly crude compared to modern magnetic data recording, it is simple, inexpensive and robust.

In the early 1980s, the PPC-UCI Newsletter published a spoof about an HP-41 module with 3141592654 bytes of storage using an incredibly small hard disk drive (thanks for a great article, Joe!). Amazing that micro-SD cards are more than an order of magnitude smaller in size and larger in storage. The old magnetic cards were crude but they were an important step to where we are now.


I expect they would last nearly indefinitely as long as the coating survives. I have an audio tape reel of Frank Sinatra from the 1950's that still sounds outstanding!

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