HP 12C - 12c Platinum -- VAT amount, NET, GROSS

[Dr_Who2]

thx 2old2randr and AndiGer for the reply.

@ 2old2randr:
Thx for taking your time to participate.
Just to avoid any confusion:

I. My HP-12CP
1.
With my HP-12CP off, if I press and hold "g" and "Enter" and then "ON", then release ON, then "g" and then the "Enter" key, right in that order, I receive the following on the display:

1.L 2.C 3.H 4.CPU

[I guess, what I read as "H" is supposed to be a "K", because hitting 3 enables a test of all keys with every digit on the display lit and going partially off, as I press every key]

However, pressing 2 reveals
ChE - - 564Ah

pressing "Enter reveals
VEr 02

pressing "Enter" again reveals
C 2004 hP

pressing "Enter" the last time, reveals the initial start ( 1.L 2.C 3. H 4.CPU )

pressing "4" reveals
3939790 h2

Not much more, I can say about this device.

2.
Removing the battery lid on this HP-12CP , there are 2 CR2032 batteries. Between them and slightly above their compartment, there is a square hole internally covered with a black plastic strip; probably covering an interface for ROM updates. I will not remove that to find out as of now.

II.
@2old2randr: What happens if you press "+" right after PV? By the stack content you gave, my guess is, it calculates -196 ( -200 + 4 = ) since 4 is saved in Y-Stack in your model, as you described.

III. HP -12CP doing math
I checked with my HP-12CP again using your description and in that order.
My results, regarding the individual stack content, remains the same as before.

1.
Just to be sure, I checked after each TVM keypress the content of the x,y,z,t stacks:

So, following 1,2,3, each separated with "Enter" and finally pressing 4,
stack wise I can see the following:
t: 1
z: 2
y: 3
x: 4

Now keying in "0 PMT" results in
t: 2
z: 3
y: 4
x: 0

Then keying in "1 n" results in
t: 3
z: 4
y: 0
x: 1

Then keying in "19 i" results in
t: 4
z: 0
y: 1
x: 19

Then keying in "238 FV" results in
t: 0
z: 1
y: 19
x: 238

Now, if I press "PV", I get the following result
t: 1 (n value)
z: 19 (i value)
y: 238 (FV value)
x: -200 (calculated PV result)

2.
Note: From what I estimate, every hit on one of the TVM keys stores that information in its according financial memory and at the same time, elevates the x-Stack, as if you have hit "Enter", hence the given value is elevated to the y-stack and so forth.

For this pressing "+" right after PV, results in ( x + Y = | -200 + 238 = ) 38

III. HP-12C (3 Batteries) same calculation
Now, I did the same procedure with the HP-12C.

a) 1,2,3, each followed by ENTER and keying 4 as last key shows the stacks content as described. No worries here.
b) 0 PMT, 1 n, 19 i shows also no difference to the HP-12CP stack content
c) 238 FV and stacks are still as expected
d) pressing "PV" shows a difference, the stack content reads
t: 0 (PMT value)
z: 1 (n value)
y: 19 (i value)
x: -200 (calculated PV result)

To me, it looks like on this model/ROM-version, pressing PV

1. does store to the financial memory and is to the x-stack while
2. pressing PV will not elevate the former FV value to y-stack and
3. overwrites the x-stack (former FV value) with the calculated PV value.

For this, pressing "+" will not give you the VAT amount as the HP-12CP does.

@ AndiGer
Thank you for the comment. I guess, 2old2randr tried to figure out the stack content, hence filled them up with values, so just to check. I read his description regarding keying in TVM values as you indicated, so without "Enter".

IV.
@ all
Over all, I think this is an important difference between the two devices I have.
Even more, if one wants to program something, that relies on TVM/stack-content.

Since I am currently not aware, what other financial calculations and keys behave differently between the HP-12C and HP-12CP, older programs for the HP-12C might not be suitable or flawed for the HP-12CP and vice versa.

Kalevipoeds program is not affected, since he does not rely on the TVM keys and what stack is filled with what pre given value. Tony´s approach lesser so, even though for the HP-12CP it works just as fine and requiers no self written program.

It might sound questionable to pursue this issue, but since both devices have CFP® and CFA® Certification Exams, GARPFRM™ Exam, some people might look up solving common, financial tasks like the given, fairly small problem.
They might end up unsatisfied with the usability of the device, if a given or suggested solution does not work for them on their device.

Looking at the HP-12C, it would have not been a bad idea by HP (40 or so years ago), to implement ("g" +) as add Tax and ("g" -) as deduct Tax to ease this common task.
For the HP 12CP it could have been ("f" STO) to add tax and ("f" RCL) to deduct tax.

kind regards