HP entry methods--please enlighten me


Hello all.

Yes, as a loyal HP addict, I know about RPN & RPL. What I'm curious about are the algebraic/AOS variations and what their official names are.

HP-20S and 27S--seems to employ the AOS from TI SR-56, 52, 58/59.

Now here's where I'd like clarification as to the name of the following entry methods:

HP-28C to the current 50G have the single-line expression enclosed by single quotes.

HP-48 to 50G have the equation mode where you have full four-direction access to the entire screen to enter the expression as it's written on paper. On that topic, what is CAS?

Now, the 33s and 35s--both have the same single-line entry method but, I presume there's a different name for the 33s variation because, rather than inactivity, the 33s evaluates the expression as it's being entered and leaves the unprocessed portions of the expression until the preceding portions can be evaluated

So, I would think there are two different names for those entry systems. And what would the names be?

Thanks in advance you for your insights.

Edited: 25 Mar 2012, 3:00 p.m.


AOS, the classical hierarchical entry system introduced by the SR-52, you already know. Functions with two operators are infix, single argument functions are postfix. The precursors SR-50 and SR-51 had only a two level priority scheme and no parenthesis. I do not remember if this was already called AOS be TI.

The TI-88 adds a variation of the scheme which made the single argument functions prefix but does not include a line editor. The evaluation of subexpressions is carried out when they are ready to processed, not with the final "=".

EOS (Equation Operation System) is the TI name for an (editable) entry line which is evaluated not before the ENTER key is hit. The same is named D.A.L (Direct Algebraic Logic) or V.P.A.N (?) by Sharp and Casio. HP just speaks about algebraic expressions in the RPL line of calculators.

The 2 dimensional entry of equations is named Equation Writer by HP, Math Input (as opposed to Line Input) by Casio and Text Book Entry by other manufacturers. I don't know what TI names this.

A CAS is a Computer Algebra System that knows about symbolic math with algebraic expressions.


Me thinks and seems to remember that the SR-50/51 logic system was simply called Arithmetic Logic. And yes, you're right, it is V.P.A.M. which stands for Visually Perfect Algebraic Mode.

Edited: 25 Mar 2012, 3:34 p.m.


The HP 71B in calculator mode adds an interesting scheme which is similar to the TI-88 in that what can be evaluated will be but allows backspacing as kind of undo which is immediately reflected in the display.


There aren't different names.

20s is quite different from 27s.

20s has a SWAP and a LAST function. It carries two operating registers. The "INPUT" allows two number regression statistics to be easily entered.

This SWAP makes it possible to correct an inverse or reciprocal on the fly, e.g. 9 / 5 SWAP = 0.555555.

It also allows two independent calcs to be carried out, e.g. 4+5=20 SWAP 6*90=540
SWAP will allow you to toggle. Get LAST in there and you mess it up. LAST is a bit different from LASTX in RPN.

LAST takes the last answer into the current calc, e.g. 4+5=20; 9 X 9 + LAST = 101.

Note that LAST contenst survive when turning machine off, but the "x" register clears to zero.

27S has a LAST but not a SWAP. It has a STACK with 4 levels. You cannot change the order of the stack (darn!) but you can recall rotate it and use whatever happens to be on the level above you (LAST). Yo ucan either CLEAR the current line and get to what is on the upper line, or you can just start a calc, with what you want to use as LAST sitting on the lower line.

27S performs intermediate calculations as you go along and puts that result on the line but does not add if there is a pending operation--but shows the addend on the line. The 21S carries out intermediates as well, holding pending, but does not show the addend. It is more "classic Ti" to my feeling (SR40? I think that's the one I knew as a kid the best).

33s has its own flavor. 35s has the fully "INFIX" version.

20s, 27s and 33s all have Postfix for most or all one number functions (sin cos log etc) but infix for arithmetic. Two number functions vary: some machines utilize the "input" and others (33s) do not. y^x is usually infix with y^x being the separator.

The reason RPN seems so "LOGICAL" is that because it has been defacto HP only, there has been just about only one way it works (well, there are lots of gotchas in old 70s machines!). "Algebraics" come in many flavors and yo have to know each machine...

Edited: 25 Mar 2012, 3:40 p.m.


The variation in the 33s reminds me of the ENTER vs = citations against TI's AOS. Namely, as a processed result is displayed, the user has no idea what portion of the calculation that result pertains to. BUT, in the 33s' advantage, since the entry line is visible, it's as if you're watching the calculation disseminate on the spot rather than having to deal with only digits in the display to cue you where and how the calculation is proceeding. In other words, the 33s shows the equation in its process and progress, something which the TIs of their day could not. And, what you see on the 33s screen is what you had to keep in mind while using an SR-56, 52 or other AOS successor.

And yes, I'm sorry for the 20/27 mixup. I've used both and I should remember their differences.

Edited: 25 Mar 2012, 4:12 p.m.


I just stumbled upon my father's SR40 manual! So my memory was pretty good, after looking over Joerg's DATAMAth--that's the Ti I knew the best. It was in the years before 1978 that he got it and that seems to check out. I do remember that he had another one with I think a silver face earlier, that didn't do anything other than arithmetic.

The SR40 was fun to watch doing long calculations -- I think we pushed exponents into it or something and it would make a sort of "thinking" pattern with the RED LEDSzzzzz!


RE: "thinking patterns and RED LEDzzzzzzzzzzzzzzzzz"

We were so easily amused back then. I remember spelling my last name on a new TI at Barry Herrera's house one night in about '75. Everyone was moderately impressed. Then i (sort of) spelled my first name too. They were blown away. I was a fookin nerd legend. Must have had something to do with what we were smoking.

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