How many non-HP RPN calculators do you own?


We all know that HP is synonumous with RPN, but the great guru DB has shown us the path to the alternative universe of non-HP RPN calculators. I was oblivious to this fact until only several years ago when I first came across the Spyropoulos site and then DB's wonderful collection of non-HP RPN calculators. Since then I've been ravenously collecting these wonderful machines, and today my collection increased its size to 17 when my most expensive acquisition to date, an immaculate OMRON 12SR, arrived in the mail. The quality of this Japanese made calculator rivals anything that HP ever made and was absolutely state of art with features like hyperbolic functions even before HP introduced them in the HP-32e. Definitely not a pocket calculator, it has nearly the size and heft of an HP-19c. Though not quite as clicky as the keyboard buttons on an HP, they have an excellent feel and action. Finally, the power system is very versatile in that it can be operated using AA alkaline cells, AA NiCad cells or a AC adapter; the only caveat being that any alkaline cells must be removed if the AC adapter is used. Unlike HP, the AC adapter uses a universal plug and there is no special battery pack. The same convenience applies to its Corvus 500 sibling. Anyways, lots to like in these old calcs.

I'm curious as to how many of you also possess and collect non-HP RPN calcs and how many you have. I am pleased to report that all 17 of mine are fully operational, although some needed repair, mostly internal battery transplants. I think my favorite is the Soviet Elektronika MK61, which is programmable. It even comes with it's circuit schematic, so presumably owner repairs can be made or it can be hacked. And of course, the keyboard is in cyrillic and the manual is in Russian.



I have only one non-HP RPN 'Frankencalc', something that was made with two different calculators. I´ll let the picture speak for itself.

Click to enlarge

What called my attention in this model is that the T-register contents is not duplicated when the stack registers contents drop after a two-number function (+, -, etc).

In time: I know that many of you know which Commodore is it. Any info?


Luiz (Brazil)


Based on DB's site, it looks like a Commodore 4921


Hi, Michael.

It is a 4921, indeed.


Luiz (Brazil)


Frankencalc? You Betcha. I'm going to have nightmares.

Luiz; is your country where bad calculators go when they die? Brazil- the land of beautiful women and tormented electronics.


Well, it seems that the bad calculators come here to 'reilectronate' when they die, though...

About the beautiful Brazilian women... I've seen many. In the magazines, TV, movies... none closer enough to make sure they are real. <*^(

Edited: 25 June 2010, 3:38 p.m.


Do you count the 8, 10, 10 solar, 30 as HP?

Do you count the 33S. 35S?


A TI SR-10 (since it was my first calculator) an SR-50 (since it was touch-and-go between it and an HP for my second) and an HP-01.

I think any more non-HP additions would be because of a unique display (e.g. Nixie or incandescent).

Edited: 25 June 2010, 9:47 p.m.


They are not RPN, through. I'm only interested in RPN calculators that are not HP.


Oops - for some reason I just saw "non-HP" and completely blew by the RPN part. An effect of the haze of nostalgia it generated, I guess :-)

I do have a Sinclair which uses postfix notation, but I'm not sure it's really RPN (see Katie's post below - I tend to agree).

Edited: 26 June 2010, 2:47 p.m.


I also have a Sinclair Scientific, and by my definition it qualifies as RPN (see post below), although it's extremely crude.


an Aurora FN1000 (I own 8 of them.....aspergers syndrome??) It is the calc I carry in shirt pocket every day.....made in China and absolutely good quality (sorry keypad though). Got them when they were $20-$25.


"....aspergers syndrome??".....................or Alzheimer's!

but who am i to point at someone who bought a couple of extra calcers? A rough count of my psychosis returns 64 non-hp RPNs, and to add insult to injury; 8 of them have no differences from others in the collection except the name on the badge. Help me Dr. Mike.

Wapner's on at five.

Edited: 25 June 2010, 12:47 a.m.


Oh, sure: turn to a shrink with a calculator addiction for guidance!


A rough count of my psychosis returns 64 non-hp RPNs

I'd be willing to relieve you of the burden if you'd like. I'll even pay the shipping. I'm just that kind of considerate guy.


Only one, an Elektronika MK61.


Same here.

- Pauli


Only one, a National Semiconductor Mathematician 4510.


i have two Soviet Elektronika MK-61 RPN calculators. they are the only non-HP calculators in my stable.

i would love to own an Omron SR-12

Edited: 25 June 2010, 10:20 a.m.


Wow - considering that you use it by looking up two numbers first then moving a dual curser to find the result, would my circular slide rule qualify?

Jim (hey, it got me through college) Horn


I have two Novus 650 (one non-working and one almost non-workinhg)
Both novus 650 have the LED flex ribbon cables broken which I have tried to solder them together.

I also have one Victor V12 (an HP 12C clone)

Edited: 26 June 2010, 7:16 a.m.


I'd like to ask a question...

What's the definition of an RPN calculator? Some of the calculators in db's collection aren't quite RPN in my opinion (sorry, db), they just use an alternate method to do basic arithmetic.

I'd say that a "real" RPN calculator has to have at least a 3-level stack and an ENTER key that pushes up the stack. I'd like to add the SWAP key into this too, but that's really just a convenience not a necessity, same for other stack manipulation functions.

I think that the Commodore Minuteman 6/6X is an interesting case to consider in defining an RPN calculator. The 6 and 6X are in fact exactly the same calculator with the 6X having an ENTER key which is simply a 2nd key that performs the exact same function as the "+=" key does. (That's right, the ENTER key does addition on this calculator.)

I'm curious what other people here think about this definition.



I'd say that a "real" RPN calculator has to have at least a 3-level stack and an ENTER key that pushes up the stack. I'd like to add the SWAP key into this too, but that's really just a convenience not a necessity, same for other stack manipulation functions.


I'm curious what other people here think about this definition.

Katie --

I'd say that Exchange (SWAP) and Roll Down are absolutely essential in order for RPN to be useful. RPN relies upon placement of mathematical arguments in the correct postions; Exchange and Roll Down allow the stack to be arranged in any order.

Roll Up, however, is convenient but not essential with a fixed stack if Roll Down is present. Roll Up is available but not directly accessible on the HP-41 and HP-42S; it is unavailable on the HP-32S and HP-10C.

-- KS

Edited: 26 June 2010, 7:38 p.m.


Hi Katie,

To me it's a matter of semantics. I have a NOVUS 650, which has a 2-level stack, no "=" key and a key labelled "ENT +". The first time a number is keyed and the ENT + is pushed, the number is raised in the stack. If a second number is keyed and ENT + is pushed, then the two numbers are added, however, the stack does not drop and the first number remains as constant in the stack. If I continue to depress the ENT + key, or -, or X, the number in the stack continues to be added, or subtracted, or multiplied. I consider this to be an RPN calculator, simply because it is postfix, where every time an arithmetic button is pushed it results in the execution of the arithmetic operation. However, what this calculator lacks is the ability to perform chained operations, which is equivalent to an algebraic calculator without parentheses. So, although it is very crude and limited, I still think it meets the basic definition of RPN.

BTW - This calculator lacks decimal arithmentic as well, and division results in truncation, so 9/6 = 1.

Edited: 26 June 2010, 2:46 p.m.


I tend to associate "RPN" with HP's particular implementation of post-fix notation, but I may be way off the formal definitions of these terms.

The most stripped-down non-algebraic (i.e. in-fix) interface I've seen is on the old Sinclair Scientific - it doesn't have a multi-level stack, roll operators or even an ENTER key! (Edit: very similar to the calculator Michael mentions above) I usually call it a "post-fix notation" calculator rather than RPN.

Edited: 26 June 2010, 2:58 p.m.


There's a concept in formal logic where one carries a proposition over literally and far beyond it's intended meaning. Like most of what i learned in college; i forget what it's called. Anyway:

To be an Real Hairy Chested Manly RPN; a calculator must equal or exceed HP's perfect implementation of postfix notation. It must have rechargeable nicads, a Norm friendly red LED screen, two shift keys, 1/100th the calculator forensics error of a 41, and a FIVE level stack with a replicating top register. It's just got to. The Heathkit 1401 is the only real RPN.

But really; i only get all-inclusive so i can say that there have been over 180 RPN (& RPL) calculators marketed so far. I don't mind being in a minority, and i know i'm unique (just like everyone else), but i don't want my calculator logic to be weird. Or an endangered species.


Actually, the only really good implementation was in the HP RPL calcs, because they have virtually unlimited stack levels. I can remember more than one frustrating instance at work where I carelessly wrote long chained calculations with more than 3 pending operations, and somewhere along the line lost results out the back of the t register. So even the HP's much heralded 4-level stack is quite limited. Even today, I use my HP-50g for all serious computing instead of my HP-35s.


Hi Katie; No lesser an authority than Dave Hicks has said that like you, he doesn't count a calculator with only a two level stack as true RPN. So i'm going against the big guns here.

IMHO, i count anything that uses postfix notation for all arithmetic functions as RPN. Adding machine logic calcs like most of the early units based on TI ROMs, and others, can use postfix in addition and subtraction but need an equals key in multiplication and division so they are not RPN to me. The Commodores insist on the user clearing the stack before beginning any calculating, so using the + or - key is literally just combining the first number with zero as it puts it into the x & y registers and gets ready to accept a second entry. One strokes clear, 2, +, 3, X, and gets 6. Then she can just continue on like on an hp or hit clear again to start a new chain. That's not too much of a stretch for me to call that RPN. And disqualification of two level stacks means that the scientific programmable MC 1103 doesn't count, How can that be good?

Everything i can think of uses postfix for one number functions like trig and log. So as an aside; i think RPN is the most consistent entry system. It's the only one that is always postfix.


None - unless you count the Sinclair Scientific... Rest of the family have various Casio models but I don't get on with them.

Mike T.


Yes, as per my above posts, I consider any postfix calc such as the Sinclair Scientific to be RPN. I have one of those beasts in my collection and use it occasionally as a 4-banger, since the trig functions work only in radians mode and pi isn't even present. Also, logs are base 10 only. Pretty useless for technical work. I also used to own a Sinclair ZX81, but it died many decades ago. Learned Basic programming on it.


Ola, Michael;

I also learned (Sinclair) BASIC with a ZX-81 'clone', named 'NE-Z80'. I had it till a decade ago, but the 'flat keyboard' no longer worked and now I have some extra components for teaching digital electronics to my students... Needless to say that I totally regret not making any effort to repair it (it´s been about 12 years now) because it is worth showing our students what kind of tool we had some 30 years back in time... At least I have the HP's family to show them.


Luiz (Brazil)


Likewise. I learned programming on the "next generation" Sinclair, the Timex-Sinclair 1000. After 2-3 years w/the TS-1000, I moved on to the Atari 8-bit.

BTW, w/r/t finding Sinclair parts, you can still buy the kits new. Many places online sell the kits; most are in the UK or elsewhere in the EU. The one below is based in N. America.

Edited: 2 July 2010, 11:30 a.m.


I learned programming on the "next generation" Sinclair, the Timex-Sinclair 1000. After 2-3 years w/the TS-1000, I moved on to the Atari 8-bit.

2-3 years! I gave up on that thing after 3 days!

Moved on to a PCjr!

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