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Calculator blog

Musings and comments about our common interest


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Cable VI - what you can do with it!


IMG_0346_en_tamano_medianoI have already had 3 customers that have purchased a HP12c (NOT HP15c CE !!) + the programming cable + HP16c overlay. 

You can guess what the plan is: to get a much cheaper HP16c in addition to their HP15c CE (which all three purchased before, with the newest firmware already fixing the DEC bug in HP16c.

Also, around 300 customers have purchased the cable to fix their firmware. While this is the original reason, these customers will profit of the cable in ways that we can only imagine. So far, when you rewrite the calculator firmware, you need to reset it and all programs and data previously introduced are lost.

However, the Atmel processor has other USB modes that can be used to back up the user memory, and restore it if needed. This opens the possibility for multiple configurations, and even HP15c "application pacs"

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Cable V

Finally we received the cables (just 300 units of the 1000 ordered- the rest after Chinese new year), and we are shipping them to customers already. Hold on- yours is about to be sent if it hasn't yet.

On the other hand, we have created some pages with information on the use of the cable. We have just focused on the current use, but there will be more! So I would also consider obtaining a 3D printed back door - to avoid having to carry a screwdriver with you!

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cable V !!

We finally have a DHL delivery date for the cables ! We are receiving them this Thursday. We have already prepared many packages and documents to be able to send as many as we can immediately.

The cables have CE certification. I will post it here when I get the original copies of it.


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cable IV

Here are some pictures of the complete cable:



Hopefully sent to customers by the end of the week.

There will be some sort of compensation for the delay to all.

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Battery door for the back door hp19bII

IMG_0131The HP19bII is a fantastic calculator with one of the most complete function sets. It was the workhorse of the MBA I did at the beginning of the nineties. Most of my companions were using this calculator.

However, several years later most units had been damaged at the same point: the side door. Nearly all units after a long time become brittle and break over time.

Hewlett-Packard recognized this problem and redesigned the calculator, moving the battery door to the back. There were other minor changes, with a darker brown body and black keys instead of the grey of the past version. This model is much more demanded nowadays - because it lasts.

I bought one in a local second hand site, but it was missing the back door. So I designed one for it. I thought that there would be exactly one unit produced ever - this was not a piece that breaks, and it was difficult to lose too. So color me surprised when a customer asked me where such back door could be found. "It is here", I said. 

So I take the opportunity to offer it to you all. However, I still think that there will not be a lot of orders - rationale is the same as it was when I designed it.

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Cable III

Here is a picture of the complete cable:


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Possible improvements to HP15c firmware

overlay_-_1What could be improved on the HP15c CE by changing the firmware?

  • Back up: in my opinion, the ability to write down user memory (including memory disposition) would be fantastic. not only you save your hard-entered programs, but you could store different profiles (EE, algebra, finance, others...) and change them "on the fly"
  • Memory is now far less important - we have enough! there are some methods to bring it to 224 registers, but what's the point?
  • Debouncing time adjustment - a little like screen contrast?
  • Any other that you can imagine?
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Why HP15c CE "Enhanced" ?


We are offering theHP15c CE "Enhanced" at a 10€ premium over the standard price of the HP15c CE.

What makes it "enhanced"? 4 main items, in my opinion.

  • We implement the latest firmware with a correction for the HP16c hidden mode, so that it is not affected by the DEC bug.
  • We include a HP16c overlay so you can use the HP16c mode with ease.
  • We include a programming cable so that you can change the firmware when new versions appear with more functions or solutions for yet unknown bugs. (In particular we are working on a back up feature - memory is big enough to save it just in case. Alternatively, you can have several profiles (finance, electrical, construction, algebra) that can be changed with the back up feature)
  • My preferred: a proper QC done by me. I open the calculator, see whether the bezel has any spot, and test the keyboard. In particular, I test all keys for recording properly using the self test feature, for any hard key (is fair to say that this test does not test for rebounds, but our new firmware also changes debouncing to 100ms from 80ms in the original machine)


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Reprogramming cable latest news

WhatsApp_Image_2024-01-11_at_10.23.28We have received news that the first batch of programming cables will be shipped late next week - this means typically one week more of delay compared to the latest Jan-15th schedule we published. We apologize for the delay and expect that all changes made on the design result in a better, sturdier cable. 

The head had to be redesigned some times so that a proper connection with the calculator could be made. The pins are sturdier than the original model. There was also a late change on the switchbox, to make it more resistant. 

The tests performed succeeded in reprogramming a unit with the latest firmware, and recovering the firmware from the machine too.

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The function key program trick for HP15c

Now that we have so many program steps in the new HP15c CE, we need to orgaize the programs. In many cases, you need to input several variables to run the program (say TVM, with 5 variables: n, i, PV, PMT and FV; but there may be several other programs)

It consists in using one label to store the value and solve for that variable when pressing R/S. An example:

STO 1 Stores the value in X in R1
R/S            Stops. Here you can press another function key or R/S to solve for A
... some code here to solve for this variable
Same with other labels of the first row, each using with another register to store the value: 2, 3, 4, 5.
Now, this is typically used for a main program, say TVM; but you can use these labels to enter the variables in registers 1, 2, 3, 4, 5, and then press GSB another label to run other program with solve. In my current set up, I use the "solver loader" in 4 different programs (not all variables - but for sure the first three). When you enter it once in a program in your calculator, you can use it to fill in R1 to R5 in any other program (if these are the ones you set it for). Just make sure of not pressing R/S but the initial label of whatever program you want to run. 
A nice trick that saves programming steps and makes an easier number entry for your programs!
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