# HP Forums

Full Version: Mystery Calculator (semi RPN?)
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I recently found a interesting calculator. It is covered with sheet aluminum and has a green LED display it reads "Unisonic 890" on the housing. Upon putting in 4 AA batteries and turning this calculator on I noticed something unusual. I type [9] [X] [9] [=] and it reads "81", If i type [9] [/] [3] [=] it reads "3", thats normal. But typing [6] [-] [3] [=] it reads "-3"

If I type [6] [=] [3] [-] it reads "3" I only works correctly if I put in the multiplication and division algebraically and input the addition and subtraction using RPN.

What the heck? Was this calculator designed to do this? or is it broken? If it is "broken" I want to "brake" it more so the multiplication and division is RPN too!

ARUID

From what you tell us, it seems like this calculator uses "adding machine logic" like the HP-10A did many moons ago. In the HP-10A, the += key (the same key for both functions) was used to either complete a pending multiplication or division, or to keep accumulating numbers (adding them).

If the calculator you have follows that logic, then the = key must be accumulating, and that's why 6 - 3 = gives a -3: you're subtracting 6 (6 -) and then adding 3 (3 =), giving a net result of -6 + 3 = -3.

On the other hand, perhaps the calculator is just broken. 8^)

i think ernie is right. a lot of early to mid 70's calcers used exactly that type of logic. some had keys marked += and -= and most of these had no = key. a real weird one was the sears M8. you could switch the = key to be a % key. and they said rpn is strange.