Calculator blog

Musings and comments about our common interest


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New HP15c batch arrived!

We are shipping now all the pending orders of HP15c, since the boat units of HP15c have arrived yesterday!! Hectic days again.


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Repurposing the new HP Voyager calculators ?


IMG_0251_en_tamano_medianoThe new HP Voyager (HP12c, HP15c Collector's Edition)calculators have a connector in the battery bay that can be used to change its firmware. Theconnector is that 6-pin set just below the batteries. The previous version had the same connector, and it needed a special cable to do programming with it; however, the previous version had a TTL-level adaptor chip, and is not pin-compatible with the current connector - which runs a USB protocal (albeit with a completely different connector!).

I have tried hard to find the connector to buy or produce such cable, but after looking everywhere for it, I've been unable to get one, or even to order one for production. Now, apparently, there were close to 1000 units of the previous version produced by HP for the 2011 production run. These can be used, but they need to be changed to use the new pin mapping required for the USB connection. I will describe the cable changes in a future blog issue.

there could be another solution, but that involves more effort: you need to open the machine and solder a USB connector in a place designed for it. Only the nano-connector applies!

Why all this fuzz? because using it, you can alter the original firmware to get more memory ! there are instructions of what positions to change in the program memory to increase the memory, and also the palces to tamper to avoid problems with the MEM screen and program limits. You can find them in . Be careful with what you do because you can brick the calculator !

PS: if anyone want more precisions, please contact us at

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Another speed comparison for the HP15c Collector's Edition


Let's try another speed comparison for the HP15c Collector's Edition, this time with a financial workhorse for financial duties: the HP12c (original version)

We will compare the time to calculate the interest rate in a long Time Value of Money calculation. The HP12c using its native TVM functions; the HP15c, using the Time Value of Money program in the Advanced Functions Manual (shifted to another place in the program memory - with so much memory I have entered several other programs too in lower memory positions)

The data for the problem:

  • 360 periods (n)
  • 480.000€ initial value (PV)
  • -2050€ payments (PMT)
  • 0 final/future value (FV)
  • Solve for i

There is no general analytical solution for i : there is no alternative for iterative solving ! The HP12c uses an optimized machine-code algorithm, but it has a slow processor, dating from 1981 and running at 880 khz (give or take) - 38% faster than the HP41c. On the other hand, the HP15c is running a keystroke program, using labels and the HP15c's Solve function called from within the program, running the keystroked TVM formula in a loop.

The results:

The newest HP12c is faster than either - but has none of the other features of the HP15c.

So, having entered programs for %T, NPV/IRR (using solve too) and TPV, and despite how comfortable feel the incr%, T% and % of the HP12c, I will take just one calculator with me! Still a lot of memory to solve equation systems oe enter other programs. There is more concern about the label codes left than memory!



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Important announcement regarding HP15c CE deliveries

We have learned some things about the coming production and deliveries of HP15c Collector's Edition:

  • Production is scheduled to take place around the 10th of July.
  • Air freight deliveries will typically occur during week 3rd or 4th of July.
  • So far only The Calculator Store has ordered Air freight (at a cost!) : 600 units, of which 490 are already sold. We will begin to ship by the end of July.
  • The rest will be shipped by boat, and will arrive not before the end of August/beginning of September. 
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Calculate digits of e


IMG_0561Several years ago Valentín Albillo posted a document, "Long Live to the HP15c", where he lauded the HP15c, describing how wonderfully put together it was. Also, to demo the calculator possibilities, he created a program that calculated digits of e, using the expansion e = 1 + 1/1! + 1/2! + 1/3! + 1/4! + ..., and using matrices.

The method he used was able to get up to 208 digits of e in the available memory of the HP15c. He himself said the program was not optimized for performance - indeed, he introduced a Pause displaying the digit being calculated, so you can imagine how slow it could be.

The program gets 8 decimals of e per register used; the input to the program is the number of registers to be used. The time to get 208 decimals is 62'43'' with the original HP15c. The program as such could not be run on the HP15c LE, since it had the Pause bug, and this program uses PSE once per iteration!

I have run the program and found that for the HP15c CE the biggest time consumer was the pause step. (it did work well - the PSE bug has been erradicated in this firmware!). I removed it and the previous RCL I, and got 21 seconds for the 208 decimals - quite an improvement from the original.

The maximum digits in the original HP15c were 208. The program uses 2 matrices with the same size, in this case 26x1. Therefore it is 52 registers used. The program occupies 10 registers, and we have left 3: we have occupied 65 registers.

Valentín told me that to go further we need to get just 7 digits per register. So far we have lived with 8 since there were 2 max digits for carry - so 8+2 = 10 which is the mantissa size in the HP15c. However, for more than 26 registers the carry digits are 3 - and we need now to plan for 7 digits + 3 carry = 10 mantissa digits per register. So step 11 needs to be changed from 8 to 7. Now we could only fit 26*7 = 182 decimals in the original machine - but now we have additional memory.

With this new program we can get up to 42*2 + 10 + 3 = 97 registers used, with 42*7 = 294 digits of e !! 

The speed is still very good: less than 48 seconds to calculate all of 294 digits ! 

Another example of the power of the new HP15c Collector's edition !!

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Selling the HP15c CE to non-fanatics

Captura_de_pantalla_2023-04-26_a_las_9.15.21We created this website because we are calculator fanatics - much like you. And we see the beauty of the HP15c. In some cases we've been longing for the calculator for a long time. In any case, no one needs to give us a reason to get one.

However, it is difficult to "sell" the HP15c to non-fanatics. As example, to my children. Both are technically-minded: one has just finished sophomore year in engineering, and the other is in international baccalaureat and intending to get into engineering too. Both are users of the HP Prime, and swear by it. It gives them an edge compared with their peers that use Casios or Texas. 

However, it is difficult for me to convince them to get a HP15c - when it becomes available within a year. 

The first obstacle comes with RPN. No matter how I insist, they prefer the text entry method of the prime - with a clear view of the equation being introduced. They have never enabled the RPN mode in their Primes.

The second obstacle is the small screen. You need to know what you are doing at every time, You need to know what you have in the four stack levels. Also you have no alphanumerics, and you can't get any graphics. What is the point, also, to edit a matrix in such a cumbersome interface? Can't touch the HP Prime when editing matrices, programs, etc.

Calculating with the HP15c has become second nature for most of us - and that gives us incredible speed when using these machines - well above in many cases than when using the HP Prime or the HP50g - simply because we know the machine better and RPN helps us. But this speed does not apply to our children. 

So, I have not been able to convince them yet. At this point, they cannot see any instance where they could not have a Prime with them. I can tell them that in my professional life I have seen people with their 48 on their tables - but for any of them I have seen 4 or 5 with HP17bII or HP12c (gold or platinum) and 40 people using 4-bangers. There is just some times when you cannot take a Prime with you - but there will always have a place in your jacket for a Voyager.

Me? I like the landscape format of the Voyagers, and I am now having a good time using the HP15c for really everything now. They will come onboard sooner or later.

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Just in...

Here is what we have received this morning - aHP15c CE test unit.

Let's do a proper unboxing tonigh, shall we?





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Difference between standard case and leather case for the HP15c calculator

We have received many questions on the leather vs PU cases for the new HP15c calculator. 

The first comment that comes to mind is that it is not "either/or". When you select the Leather version, you get the leather case plus the case the HP15c comes with. So if and when one of the two breaks down, you still have the other.

The second: here is how the HP15c CE PU case looks like:


The leather case is a true Ubrique cowhide leather: (the one sold with the HP15c is the black model). However, it can be available in 3 colors: Burgundy, black and brown. The one that looks ok with the HP15c is the black one!


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HP15c Collector's Edition speed tests


I mentioned in yesterday's blog that I intended to do some speed comparisons between the HP15c Collector's Edition and othe HP15c models - however, I am missing a HP15c LE since the one I have is bricked from a previous attempt at reprogramming it with a firmware with more memory. Also my Mac cannot handle a RS232 interface properly (at least I have not managed to do it - I did with older verisons of Mac OS) which is required for the LE to be reprogrammed.

However, Gene Wright has managed to do it with all three models - you can see his findings here:

(I have to say that I replicated perfectly the measures for both the original machine and the HP15c Collector's Edition prototype I have. You can find the original programs in HPmuseum)


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