Magnet Cards


Hi everybody,

I have a small question:

Are there different formats (physical and locical) for the different HP calculators with strip reader, e.g. HP41, 67, 97 ... or are they all the same ?




HP-65 cards can only be read by another HP-65, so that alone would indicate that it uses a different format. Since the 65 used 6-bit words (while the 67, for instance, used 8-bit words), I'm not surprised they are incompatible.

HP-41 calculators (augmented with a plug-in card reader) can read cards written by a 67 or 97, although the reverse is not necessarily true. Finally, any HP-67 can read a 97 card, and vice-versa.



The cards are all physically the same, the different machines write different formats of data on them. The HP41 reader can read and execute MOST HP67/HP97 formatted cards.


The lowest level of the physical format is the same on all 3 classes of machine (65, 67/97 and 41). It's 2 physical tracks in each direction. A magnetic transition on
one track is a 0 bit, a transition on the other track is a 1 bit. A simple self-clocking system.
Apart from that, there are differences. The 65 is the real oddball, I think (it records fewer bits on the track than the others IIRC). The 67/97 and 41 schemes are similar (not too suprising), and 41 card readers can read (although not write, at least not with the standard software) 67/97 cards


Does anyone know where the format was documented? I thought it was in the PPC journal, but I went through the indeces and didn't find it. I'm fairly certain it was not in the HP Journal -- I do recall that the article involved the author having used their dentist's X-ray machine to take images of the magnetic domains!

I assume some magna-ease (or whatever it's called) and a low-powered microscope would be sufficient to analyze the format.



I remember reading an article in either PPC journal or CHHU Cronicle that described the format of the
HP67 cards. It refered to an earlier article covering the HP65 format that I've never seen.
I can try to find it again, but it'll take some time. IIRC, the HP67 article described the use of Magnasee to find the pulses on the card.


The HP97 service manual (on the Museum CDs) says to use Magna-See to check the speed of the card reader motor. Then it has a long list of resistor values to substitute in one location depending on the measured results. I saw Magna-See in operation once, a long time ago. It might have been an HP salesman who was showing it to some people in the factory where I worked. One thing I've always wondered about it, and the 97 S/M doesn't address this at all: after putting the fluid on a piece of media, which leaves a modulated film of magnetic particles on the surface, is it safe to run the same media past the head? Also, does anybody know where to find Magna-See? If it hasn't been banned due to the solvent it used.


As to whether you can re-use magnetic media coated with Magnasee, opinion seems to be divided on the matter. Some service manuals say 'On no account re-use the piece of tape which has been coated with Magnasee' or words to that effect. Others
specifically suggest wiping off the Magnasee and running the tape through again for the next test. IIRC the HP97 service manual implies you can do the latter.
It may depend on the type of head -- the HP card readers are not exactly high data density devices (even for the time), so you can probably get away with rather more damage to the heads than on a more
precision device. Oh, and if anyone knows a source of Magnasee or equivalent in the UK, I am looking for a can of it.


I don't know the specific product Magnasee, but have used a similar product in the past for audio head alignment. It is made by The Geneva Group, and is called MagView.

As I was using it only for viewing the track and not individual domains, I am not sure whether it is the same type of product, or what you will see under a high-power microscope. I am fairly sure that you can see no-amplitude to full-amplitude transitions... IF they are strong, well-defined and spaced out just a bit. It may also depend a little on your spraying technique. I only needed to "puff" my tapes, holding them vertically, to see the tracks themselves, and I had a graticule-slide to make measurements.

Since the iron particles are only magnetically stuck on the tape, careful wiping or washing of the card should remove it all with no ill effect. You don't want to run a card or tape past a magnetic playback head with anything magnetic and sheddable on it-- it is the tiny Gap in the head that can be corrupted with debris, and affect reading. But water and cotton balls will wash a tape or card off very well.

My can of Magview says: MAGVIEW PF-603, Geneva (c)1993, Minneapolis MN 55427-2848 USA, Toll-Free 1-800-358-5600. All it lists as ingredients are:

1,1, dichloro-1-flouroethane; iron powder; carbon dioxide.

I suspect you get your best viewing with a virgin card and a fresh recording.


Try (818-364-1800) for their Sprague-Mag product. I think they took over the MagView product line. They have three or four types of the stuff with particle sizes down to 0.05 micron if you just gotta see them teeny tiny bits.

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