I still have my original HP-15C from 1987, along with two SwissMicros DM-15L (I had also purchased the official HP reedition HP-15C LE, but one was stolen from work in a burglary, and I resold the other one at a tidy profit, the DM-15L is much better). I also have the official HP-15C iPhone app. And a HP-48GX, HP-33S, HP-35S, DM-42L.

To be frank, it's the only one of my HP calculators I can remember how to program, it's extremely simple, unlike newer models.

HP reedited the 15C manual for the limited edition:

http://www.hp.com/ctg/Manual/c03030589.pdf

I still have my original hp-15C and the owner's handbook with a print date of 2/84

The HP codes aren't too bad because they correspond to the location of the keys on the keyboard, i.e. y^x is 21 because it's the second row and first column of the keyboard. If you want something really confusing try programming a Soviet Elekronika calculator, they actually work fairly similarly to the HPs but the codes displayed on the screen when inputting a program are in hexadecimal displayed with weird symbols because you can't actually show A-F on a seven segment display.

Ah, I forgot all the fun I had squaring EGGOGs.[1] I'm working on fully documenting the MK-52 in English but I've been sidetracked with school.

[1]: for the uninitiated, when you'd do anything meaningful on the MK-52 (or 61 or one of the other ones in the series) you'd get an "ErrOr". the "ErrOr", in cyrillic, looks like "EGGOG" because "r" and the cyrillic g look similar, so a lot of people call them EGGOGs. sometimes you could square them and do other things to mess even farther with the calc. in my research I've found that there are more undocumented functions than documented, a lot have to do with the EGGOG manipulation and isolating them and entering that into a program. wild stuff

I've typically seen A-F rendered like so:

   _         _        _   _
  |_|  |_   |    _|  |_  |_
  | |  |_|  |_  |_|  |_  |

(Variations would be a double-story lower case 'a' and lower case 'c'.)

But if they cloned the 7447 decoder they'd look like

                   _     
     _    _   |_| |_   |_ 
    |_    _|       _   |_

with 15 blank.

> you can't actually show A-F on a seven segment display.

Yes you can - I used a 7 segment display with hex numbers (a 6800 Dev kit of some kind) - A, b (lower case), C, d (lower case again) E, F are all straightforward.

See https://en.m.wikipedia.org/wiki/Seven-segment_display

> Wow that sounds a lot more complicated than the one I had, HP 42S,

Yeah... the 42S derives from the earlier 41 series, and the 41 series was the first to use alphanumeric symbolic names for programming commands. Prior to that, there was always a very direct correlation between the button you pressed on the keyboard and the way the command was encoded in memory. (Which made sense given how resource limited the earlier calculators were.)

HP took this evolution one a step further with the HP28C, which was released a few years past the 41's. The 28C introduced a garbage collected heap and a tagged type representation for objects that gave you something essentially like a small RPN Lisp running on a handheld.

I consider HP-28S a work of art. The programming language can be described as an effective mixture of Forth, Lisp and even APL and J. The memory was also very efficiently used, the HP-28S had whole 32K! (not a little for that time).

And all that with 4-bit CPU (!) which worked for a very long time on very small batteries.

https://en.wikipedia.org/wiki/HP-28_series

> I consider HP-28S a work of art.

I'd extend that to the whole HP calculator lineup through about 1994 or 95.

There are a lot of aspects I could single out, but William Kahan's involvement in their numerical algorithms is a particular standout for me. They wanted high quality numerics out of battery powered 1970's era micro-electronics, so they brought Dr. Kahan in to design the algorithms necessary for the task.

(Dr. Kahan is also noteworthy for his involvement in IEEE-754, which is still the standard for floating point today.)

Serious, serious engineering went into these calculators.

What would you say is the final model that "has it"? Besides my vintage HPs (HP15C, 2xHP16C, HP45) I also have the 50g and the modern 35S. For me, the 50g's lack of a proper ENTER key ruins it. I'm sort of on the lookout for a nice 48 series to replace it for that reason. The 35S at least has a proper ENTER.

> What would you say is the final model that "has it"?

For me, at least, probably the combination of the 48GX and 42S. Both represent the end of the line of HP's traditional hardware engineering, which is such a significant part of using these devices. The software gaps between the 48GX and the later 49 can be made up by installing software into a 48, if that turns out to matter for you.

One of the more significant new parts of the 49 software is the "Meta Kernel", which was externally developed by Jean-Yves Avenard as a 48 extension. IIRC, Avenard ultimately wound up working for HP on their calculators, which I believe is how MetaKernal wound up as part of the core 49G ROM. IIRC, this was also a bit of a reconstitution of HP's calculator engineering effort, as the previous team had been disbanded a few years prior. (ie; William Wickes, who was heavily involved in the 41 and 28/48, wound up at some point working on video conferencing systems for HP.)

The reason I'm also including the 42S is that it's a lot more traditional in the way that it operates. If the 28/48/49/50 series feels like a small handheld computer with a custom programming language, the 42 feels more like an actual calculator (with a lot of capabilities).

Thanks for your reply. I’m certainly going to keep a lookout for both.

I used an HP41 for many years but it eventually died and I didn't really use an engineering calculator enough any longer to justify either repairing it or picking one up on eBay. I really liked the ergonomics and, although I have an emulator on my iPhone, it's not the same without the physical keys.

I did pick up a 28 at one point but never really bonded with it.

> I did pick up a 28 at one point but never really bonded with it.

IMO, the 28 and follow ons lost a lot of the focus of the earlier (more 'primitive') devices. They were more cumbersome to use, and whatever marginal improvement they offered in functionality didn't matter because everybody started to use PC's, laptops and the like.