HP-80 CHS Exponent Curiosity


My one and only classic HP is an HP-80, like many with the 1247... serial number but the ROM chip shows a date of 1/73.

It exhibits a strage characteristic when in exponential display mode with a number less than 1, and CHS is pressed. Instead of only inserting a negative sign in the left-most position, it also changes the exponential value to -99.

Toggling CHS leaves the exponential value at -99 but toggles the left-most negative sign as expected. However, pressing SAVE resets the exponential to the original value, switching back to decimal display shows the correct value and calculations are correct even when the exponential value shows -99.

For an example keystrokes and display:

Power on " 0.00 "
.1 SAVE " 0.10 "
Shift 7 " 1. -01"
CHS "-1. -99"
Shift 2 "-0.10 "

So it seems the CHS induced exponential -99 display is just a display curiosity, but I cannot find any reference to it anywhere. Is this something other people have seen on the HP-80?

Kind regards, MAX


HP's original serial number system used the four digit date code as the first week of production of machines of a particular revision level, rather than the week a particular unit was actually manufactured. For calculators the production volume quickly ramped to a high enough rate to make that scheme unusable, so at some point before 1976, for calculators only, the scheme was changed to have the date code reflect the actual production week, without regard to the revision level of the product.


I don't remember coming across this before, but my early HP-80 and later one (with a "Hewlett Packard 80" nameplate on the front) both exhibit this same display problem. The HP-81 I have doesn't have this issue, but the display system is somewhat different.


I'm fairly certain that it's a firmware bug, and they probably fixed it before releasing the HP-81. It's possible that late production HP-80 units that used the quad ROMs (two HP-45/55/70 style DIPs rather than a hybrid) might have it fixed, but I don't have such a unit to try.


Thanks guys for the feedback. Maybe it should be added under the 'Bugs' section?

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