strange opening at back of an HP 46


I just got an HP 46 (#1338S01941, no display option).
Someone has made an additional big rectangular opening
and two screw holes in the back panel by extending the
ventilation slots on the right side.

I've put two pictures at and

I'd like to know what the reason for this might have been.
My guess is, that the calculator was connected to some
laboratory equipment in order to collect data from experiments. Is this possible with an HP 46?


I used to have an HP46.

It was really an HP45 architecturally. As I recall, but came out of the Loveland, CO (USA) computing instrument (?) group instead of the calculator group.

Since the HP45/46 weren't programmable, they really wouldn't be useful as controllers. I have no idea why the cutouts were made.

BTW, it seems some HP46s were shipped without LED displays - the LED apparently was an "option" and the HP46 could be used with the (noisy) printer as its only "display".

Bill Wiese

San Jose CA USA


If the stopwatch function (officialy unsupported but available on the HP45) was available in the HP46 (may it be or not); then some switches, wired to the individual key contacts may have been used to capture the timing of up to 10 events.

When the HP45 was used as a stopwatch, and the clock was running, you can press any of the numeric keys and the time will be stored on the corresponding register (R0-R9). When the clock was stopped, pressing the same key will show the time previously stored on that register.

Please note that these comments are based on 30-years old memories (I never had an HP45), so there may be some inaccuracies...


I am pretty sure the stopwatch function is not available on the HP46. I think the ROM space was taken over for some of the printer fuctions. And, unlike the HP45, where there was an unused key position that was used to access the stopwatch, there are no unused key positions on the HP46 (or HP81 for that matter) -- all 40 possible positions in the matrix are used.

My guess is that if this 'hole' was used for a connector, then it was wired to the keyboard connector so that some external device could simulate keypresses. It wouldn't matter that the HP46 was not programmable, the external device could incorporate a sequencer (effectively a fixed program) to 'press' the right keys in the right order.

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